{"id":10657,"date":"2026-08-29T23:43:22","date_gmt":"2026-08-29T15:43:22","guid":{"rendered":"https:\/\/q8bbzjnpm5.wpdns.site\/?p=10657"},"modified":"2026-08-29T23:43:22","modified_gmt":"2026-08-29T15:43:22","slug":"how-to-determine-if-a-transformer-meets-capacity-requirements","status":"publish","type":"post","link":"https:\/\/subian-electric.com\/fr\/news\/how-to-determine-if-a-transformer-meets-capacity-requirements\/","title":{"rendered":"Comment d\u00e9terminer si un transformateur r\u00e9pond aux exigences de capacit\u00e9"},"content":{"rendered":"<p>Dans une usine de transformation alimentaire dans le Midwest, l'un des ing\u00e9nieurs a r\u00e9cemment install\u00e9 un tout nouveau transformateur mont\u00e9 sur socle avec une capacit\u00e9 de 200 kVA destin\u00e9 \u00e0 soutenir l'expansion d'un cong\u00e9lateur. Cependant, dans un d\u00e9lai de trois semaines apr\u00e8s l'installation, le transformateur a rencontr\u00e9 des d\u00e9clenchements en raison d'une surcharge thermique pendant les heures de production et l'utilisateur s'est retrouv\u00e9 avec des factures d'\u00e9lectricit\u00e9 excessives qui ont annul\u00e9 tous les avantages anticip\u00e9s que le transformateur \u00e9tait cens\u00e9 fournir. Le probl\u00e8me ne vient pas du transformateur lui-m\u00eame mais plut\u00f4t du fait qu'avant l'installation, personne n'a v\u00e9rifi\u00e9 si le transformateur avait la capacit\u00e9 appropri\u00e9e pour la charge \u00e9lectrique, en particulier en termes de courant d'appel et de charges harmoniques, tout en tenant compte des futures expansions de charge. De telles situations se produisent quotidiennement, en fait chaque ann\u00e9e, elles ont lieu dans des environnements industriels et commerciaux ainsi que dans les services publics.<\/p>\n<p>Dans ce document, je fournirai des instructions \u00e9tape par \u00e9tape sur la d\u00e9termination des exigences de charge auxquelles un transformateur doit faire face en utilisant les informations de sa plaque signal\u00e9tique, les calculs de kVA, l'analyse du facteur de charge et des tests sur site.<\/p>\n<blockquote><p>Vous pouvez d\u00e9terminer si un transformateur peut soutenir vos besoins en capacit\u00e9 en calculant d'abord la demande r\u00e9elle : additionnez les valeurs de kVA de toutes les charges connect\u00e9es, tenez compte du facteur de diversit\u00e9 de 0,7-0,85, et enfin multipliez le nombre atteint par une marge de croissance de 10-20 %. Comparez les r\u00e9sultats de faisabilit\u00e9 avec le kVA de la plaque signal\u00e9tique du transformateur et utilisez le calcul du facteur de charge \u2014 les charges continues ne doivent pas d\u00e9passer plus de 80-90 % de la valeur nominale pour les transformateurs de type sec et 70-80 % pour ceux \u00e0 huile.<\/p><\/blockquote>\n<p style=\"text-align: center;\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-10658\" src=\"https:\/\/subian-electric.com\/wp-content\/uploads\/2026\/08\/How-To-Determine-If-A-Transformer-Meets-Capacity-Requirements.webp\" alt=\"Comment d\u00e9terminer si un transformateur r\u00e9pond aux exigences de capacit\u00e9\" width=\"1448\" height=\"1086\" \/><\/p>\n<hr \/>\n<h2 id=\"capacity-meaning\">Que signifie \u201c exigences de capacit\u00e9 \u201d pour un transformateur ?<\/h2>\n<p>La capacit\u00e9 d'un transformateur est sa puissance apparente de sortie d\u00e9clar\u00e9e en kVA ou MVA dans des conditions de refroidissement sp\u00e9cifiques. Le fabricant fixe cette valeur selon la norme IEC 60076-1 ou IEEE C57.12.00, \u00e9tant la plus haute puissance apparente de sortie de l'appareil qui peut \u00eatre utilis\u00e9e en continu sans d\u00e9passer les limites d'\u00e9l\u00e9vation de temp\u00e9rature \u2014 g\u00e9n\u00e9ralement 65 \u00b0C d'\u00e9l\u00e9vation moyenne des enroulements pour les transformateurs immerg\u00e9s dans l'huile et 100\u2013150 \u00b0C pour les types secs selon la classe d'isolation correspondante.<\/p>\n<p>En ce qui concerne le respect des \u201c exigences de capacit\u00e9 \u201d, cela signifie que le transformateur est capable de supporter la charge instantan\u00e9e accrue \u00e0 condition qu'il y ait suffisamment de marge pour trois param\u00e8tres que les ing\u00e9nieurs doivent prendre en compte : le courant d'appel caus\u00e9 par le d\u00e9marrage d'un moteur (cela pourrait atteindre 5\u20138 fois le courant nominal), les courants harmoniques apparaissant en raison des variateurs de fr\u00e9quence et les perspectives d'augmentation future de la charge. Dans quelques ann\u00e9es, le transformateur correctement calcul\u00e9 aujourd'hui pourrait facilement devenir mal dimensionn\u00e9 si une charge suppl\u00e9mentaire est ajout\u00e9e au fonctionnement de l'installation.<\/p>\n<p>Il y a deux raisons pour lesquelles les incoh\u00e9rences mentionn\u00e9es apparaissent. Tout d'abord, les acheteurs sont susceptibles de confondre kVA (puissance apparente) avec kW (puissance r\u00e9elle), perdant ainsi de vue le fait que la charge fonctionnant avec un facteur de puissance de 0,8 n\u00e9cessite 25% de kVA de plus par rapport au kW. De plus, les acheteurs peuvent additionner les valeurs nominales de chaque appareil sans appliquer le facteur de diversit\u00e9 n\u00e9cessaire, ce qui les am\u00e8ne \u00e0 surestimer la capacit\u00e9 du transformateur d'environ 30\u201350% sans avoir r\u00e9ellement besoin de payer pour des circuits qui pourraient ne pas \u00eatre utilis\u00e9s par la suite.<\/p>\n<h2 id=\"load-calculation\">\u00c9tape 1 : Calculer la charge connect\u00e9e r\u00e9elle<\/h2>\n<p>Tout d'abord, il est n\u00e9cessaire de dresser des listes de toutes les charges qui sont aliment\u00e9es par le transformateur et de les regrouper. Une bonne classification est la suivante :<\/p>\n<ul>\n<li>Charges continues \u2014 \u00e9clairage, CVC et \u00e9quipements de processus fonctionnant pendant une longue p\u00e9riode.<\/li>\n<li>Charges intermittentes \u2014 pompes, compresseurs, grues qui s'allument et s'\u00e9teignent.<\/li>\n<li>Charges moteur \u2014 intensit\u00e9 de courant \u00e0 pleine charge, courant de d\u00e9marrage et si l'\u00e9quipement utilise des d\u00e9marreurs progressifs, des variateurs de fr\u00e9quence ou non (combien de courants il utilise).<\/li>\n<li>Charges non lin\u00e9aires \u2014 variateurs de fr\u00e9quence, redresseurs et syst\u00e8mes d'alimentation sans interruption qui ont un courant harmonique.<\/li>\n<\/ul>\n<p>Maintenant, vous devez additionner les kVA de tous les types de charges. Ne calculez pas seulement les valeurs de kW pour chaque charge. Si vous n'avez que des kW et un facteur de puissance, trouvez les kVA en utilisant la formule ci-dessous. Une fois que vous avez les donn\u00e9es brutes sous forme de somme de toutes les charges, vous devez appliquer le facteur de diversit\u00e9.<\/p>\n<table>\n<thead>\n<tr>\n<th>Cat\u00e9gorie de charge<\/th>\n<th>Facteur de diversit\u00e9 typique<\/th>\n<th>Marge de demande recommand\u00e9e<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Commercial g\u00e9n\u00e9ral (vente au d\u00e9tail, bureau)<\/td>\n<td>0.60\u20130.75<\/td>\n<td>10\u201315%<\/td>\n<\/tr>\n<tr>\n<td>Usines de processus industriels<\/td>\n<td>0.70\u20130.85<\/td>\n<td>10\u201320%<\/td>\n<\/tr>\n<tr>\n<td>H\u00f4pitaux, infrastructures critiques<\/td>\n<td>0.90\u20131.00<\/td>\n<td>15\u201325%<\/td>\n<\/tr>\n<tr>\n<td>Centres de donn\u00e9es (haute densit\u00e9)<\/td>\n<td>0.95\u20131.00<\/td>\n<td>20\u201330%<\/td>\n<\/tr>\n<tr>\n<td>\u00c9coles, b\u00e2timents municipaux<\/td>\n<td>0.50\u20130.70<\/td>\n<td>10%<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Augmentez la demande diversifi\u00e9e de 10% \u00e0 20%, car cette marge garantira que l'unit\u00e9 reste dans ses limites thermiques pendant les charges de pointe estivales et \u00e9galement apr\u00e8s d'\u00e9ventuelles r\u00e9novations futures. Cette marge est la forme d'assurance la plus rentable disponible au moment de la s\u00e9lection.<\/p>\n<h2 id=\"kva-formulas\">\u00c9tape 2 : Appliquer les formules de calcul des kVA<\/h2>\n<p>Maintenant, convertissez tout en kVA afin de le comparer \u00e0 la puissance nominale sp\u00e9cifi\u00e9e. Trois \u00e9quations sont n\u00e9cessaires dans ce cas, qui sont form\u00e9es sur la base des lois de la puissance AC.<\/p>\n<p>Dans le cas d'un monophas\u00e9 : kVA = (Volts \u00d7 Amps) \u00f7 1000<\/p>\n<p>Dans le cas d'un triphas\u00e9 : kVA = (\u221a3 \u00d7 Volts \u00d7 Amps) \u00f7 1000<\/p>\n<p>Dans le cas de la conversion de kW en kVA : kVA = kW \u00f7 facteur de puissance<\/p>\n<p>Exemple de calcul : Dans le cas d'une ligne triphas\u00e9e fonctionnant \u00e0 480 V avec 240 A de courant, la demande est (1.732 \u00d7 480 \u00d7 240) \u00f7 1000 = 199,5 kVA. En tenant compte d'un facteur de puissance \u00e9gal \u00e0 0,85, cela signifie que l'\u00e9nergie r\u00e9ellement consomm\u00e9e est \u00e9gale \u00e0 170 kW.<\/p>\n<table>\n<thead>\n<tr>\n<th>Tension nominale de ligne (V)<\/th>\n<th>Courant \u00e0 pleine charge (A)<\/th>\n<th>Demande calcul\u00e9e (kVA)<\/th>\n<th>Puissance nominale standard la plus proche (kVA)<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>240 (monophas\u00e9)<\/td>\n<td>100<\/td>\n<td>24.0<\/td>\n<td>25<\/td>\n<\/tr>\n<tr>\n<td>480 (triphas\u00e9)<\/td>\n<td>120<\/td>\n<td>99.8<\/td>\n<td>112.5<\/td>\n<\/tr>\n<tr>\n<td>480 (triphas\u00e9)<\/td>\n<td>240<\/td>\n<td>199.5<\/td>\n<td>225<\/td>\n<\/tr>\n<tr>\n<td>480 (triphas\u00e9)<\/td>\n<td>480<\/td>\n<td>399.1<\/td>\n<td>500<\/td>\n<\/tr>\n<tr>\n<td>13 800 (triphas\u00e9)<\/td>\n<td>25<\/td>\n<td>597.5<\/td>\n<td>750<\/td>\n<\/tr>\n<tr>\n<td>13 800 (triphas\u00e9)<\/td>\n<td>42<\/td>\n<td>1003.8<\/td>\n<td>1250<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Toujours opter pour la norme IEC sup\u00e9rieure (25, 50, 100, 160, 250, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500 kVA) plut\u00f4t que d'essayer d'obtenir la puissance minimale disponible. Cela est d\u00fb au fait que les normes IEC sont con\u00e7ues pour s'adapter aux conceptions de transformateurs standard, aux tailles de noyaux et aux instruments de test ; des classifications sp\u00e9ciales co\u00fbteront plus cher \u2014 en fait, de 15 \u00e0 30 % de plus \u2014 et prendront plus de temps \u00e0 livrer.<\/p>\n<p style=\"text-align: center;\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-10660\" src=\"https:\/\/subian-electric.com\/wp-content\/uploads\/2026\/08\/4-key-steps.webp\" alt=\"4 key steps\" width=\"1448\" height=\"1086\" \/><\/p>\n<h2 id=\"load-factor\">\u00c9tape 3 : V\u00e9rifier le facteur de charge et le cycle de service<\/h2>\n<p>La capacit\u00e9 globale est d\u00e9termin\u00e9e non seulement par le volume maximum mais aussi par la dur\u00e9e des op\u00e9rations \u00e0 haute tension. Ainsi, le facteur de charge est d\u00e9fini comme le rapport de la consommation moyenne \u00e0 la consommation maximale \u00e0 un certain moment dans la formule ci-dessous :<\/p>\n<p>Facteur de charge = Consommation moyenne \u00f7 Consommation maximale<\/p>\n<p>Une usine qui consomme 400 kVA pendant une demi-heure le matin seulement et a une consommation moyenne de 150 kVA a un facteur de charge d'environ 0,375. Cette situation est avantageuse pour l'utilisation d'un transformateur \u00e0 faible inertie thermique, car un transformateur rempli d'huile peut supporter des surcharges temporaires de 1,3 \u00e0 1,5 fois la capacit\u00e9 nominale pendant plusieurs heures selon la norme IEC 60076-7 concernant les charges cycliques. D'autre part, un transformateur qui fonctionne sous une charge de 85% en continu souffre d'un vieillissement pr\u00e9matur\u00e9 de l'isolation.<\/p>\n<p>The insulation life according to the Arrhenius law is reduced by half with every 6-8\u00b0C increase in continuously functioning high point. For instance, when a transformer is loaded by 90% and has the rise of less than 65\u00b0C, the hot point temperature is higher than 105 C\u00b0 for quite a long time. This is why the 80% loading criterion is not just a marketing trick.<\/p>\n<h2 id=\"verification-methods\">Step 4: Verification Methods (Nameplate, Thermal, Tests)<\/h2>\n<p>After the transformer has been set up, you need to evaluate capacity at three different levels. First, you need to check the administrative aspect: you need to verify the plate rating, the level of impedance, vector group, and cooling class ONAN, ONAN\/ONAF, AN\/AF using the purchase specifications and the load study. The second aspect to check is the operational aspect where you need to check the secondary current, voltage, and temperature of the winding.<\/p>\n<p>It is important to ask the manufacturer for the factory report for new transformers. According to IEC 60076-1, routine tests should be conducted with regards to the winding resistance, ratio, idle losses, active losses, and temperature rise tests. This is the only lab-based evidence that you can rely on to prove that the transformer can operate at the rated kVA continuously without exceeding the limit of insulation temperature.<\/p>\n<table>\n<thead>\n<tr>\n<th>Verification Method<\/th>\n<th>What It Confirms<\/th>\n<th>Typical Cost \/ Effort<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Nameplate review vs. load study<\/td>\n<td>Rating, impedance, cooling class match<\/td>\n<td>No cost, 1 hour<\/td>\n<\/tr>\n<tr>\n<td>One-week demand logging<\/td>\n<td>Peak kVA vs. rating under real duty<\/td>\n<td>$200\u2013$600 (logger rental)<\/td>\n<\/tr>\n<tr>\n<td>Infrared thermography<\/td>\n<td>Hot spots at bushing, tank, and cable connections<\/td>\n<td>$300\u2013$800 per survey<\/td>\n<\/tr>\n<tr>\n<td>Dissolved gas analysis (DGA)<\/td>\n<td>Early thermal\/arcing faults in oil units<\/td>\n<td>$80\u2013$200 per sample<\/td>\n<\/tr>\n<tr>\n<td>Factory routine test report<\/td>\n<td>Ratio, losses, insulation, temp-rise proof<\/td>\n<td>Included in purchase<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>For units that seem to be overloaded, first conduct a DGA and a temperature survey, then consider purchasing anything else. Often the real issue is a bad connection or blockage of cooling fins rather than the core itself; replacing or improving these can bring back full function at a lower cost than purchasing a new transformer.<\/p>\n<h2 id=\"capacity-table\">Common Capacity Ratings and Their Load Limits<\/h2>\n<p>The table provided indicates the recommended standard ratings and the maximum sustained load for all cooling technologies under consideration, which is based on diversified needs and normal climate circumstances of 30 degrees celsius (average temperature).<\/p>\n<table>\n<thead>\n<tr>\n<th>Rated kVA<\/th>\n<th>Typical Voltage Class<\/th>\n<th>Max Sustained Load \u2014 Dry-Type (AN)<\/th>\n<th>Max Sustained Load \u2014 Oil-Immersed (ONAN)<\/th>\n<th>Typical Price Range (FOB China)<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>100<\/td>\n<td>0.4\/0.4 kV, 11\/0.4 kV<\/td>\n<td>80\u201390 kVA<\/td>\n<td>70\u201380 kVA<\/td>\n<td>$2,500\u2013$6,000<\/td>\n<\/tr>\n<tr>\n<td>250<\/td>\n<td>11\/0,4 kV<\/td>\n<td>200\u2013225 kVA<\/td>\n<td>175\u2013200 kVA<\/td>\n<td>$4,500\u2013$11,000<\/td>\n<\/tr>\n<tr>\n<td>500<\/td>\n<td>11\/0,4 kV, 33\/0,4 kV<\/td>\n<td>400\u2013450 kVA<\/td>\n<td>350\u2013400 kVA<\/td>\n<td>$8,000\u2013$18,000<\/td>\n<\/tr>\n<tr>\n<td>1000<\/td>\n<td>10\/0.4 kV, 35\/10 kV<\/td>\n<td>800\u2013900 kVA<\/td>\n<td>700\u2013800 kVA<\/td>\n<td>$15,000\u2013$32,000<\/td>\n<\/tr>\n<tr>\n<td>1600<\/td>\n<td>35\/10 kV, 33\/0.4 kV<\/td>\n<td>1,280\u20131,440 kVA<\/td>\n<td>1,120\u20131,280 kVA<\/td>\n<td>$24,000\u2013$48,000<\/td>\n<\/tr>\n<tr>\n<td>2500<\/td>\n<td>35\/10 kV<\/td>\n<td>2,000\u20132,250 kVA<\/td>\n<td>1,750\u20132,000 kVA<\/td>\n<td>$38,000\u2013$75,000<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>It must be pointed out that the assumed continuous limits of supply apply under the conditions of a clean supply and a power factor at the terminals of the transformer almost equal to one. However, if one takes into account harmonics, the situation becomes quite different. For instance, a 20-% total harmonic distortion of the current causes an increase in the effective thermal loading of 5 to 10%, and an already-running apparatus may find itself under an overload even though its kVA appears to be in normal limits.<\/p>\n<h2 id=\"specs\">Specifications That Affect Capacity<\/h2>\n<p>In addition to the kVA rating, several other nameplate specifications affect the actual capacity provided by the transformer in your application:<\/p>\n<ul>\n<li>Cooling classification (IEC 60076-2): An ONAN transformer self-cools to its full rating. An ONAN\/ONAF transformer has additional active fans, providing around 20% to 30% more than its standard rating.<\/li>\n<li>Temperature increase (K): A transformer rated for 65 K has more capacity than a transformer rated for 55 K of the same size but will subject its insulation to more wear and tear.<\/li>\n<li>Impedance voltage (uk%): It is usually around four to ten percent for distribution transformers. A higher impedance value reduces the fault current but has worse voltage drop characteristics.<\/li>\n<li>Vector group (Dyn11, Yyn0, Yd11): Needs to suit the system to prevent issues with circulating currents.<\/li>\n<li>De-rating due to altitude and temperature: As the altitude rises above 1000 m and temperature exceeds 40 \u00b0C, according to IEC 60076-1 the transformers should be de-rated (generally 0.5 % to 1 % each 100 m above 1000 m).<\/li>\n<li>Material used for windings: Copper windings have around 15% to 25% higher short-circuit capacity than aluminum windings but are also about 20% to 40% more expensive.<\/li>\n<\/ul>\n<h2 id=\"brands-prices\">Brands and Price Ranges for Capacity-Graded Units<\/h2>\n<p>When it comes to overseas products, capacity validation is of paramount importance, since the difference between performance specifications and actual performance increases when customers cannot physically examine the product at the factory. International brands charge more for their nameplate capacity, while Chinese producers provide almost the same validated capacity at much lower prices.<\/p>\n<table>\n<thead>\n<tr>\n<th>Marque<\/th>\n<th>Typical Range<\/th>\n<th>Notable Position<\/th>\n<th>Indicative Price (500 kVA)<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>ABB<\/td>\n<td>Distribution to 50+ MVA<\/td>\n<td>Global leader, strong service network<\/td>\n<td>$18,000\u2013$30,000<\/td>\n<\/tr>\n<tr>\n<td>Siemens<\/td>\n<td>Distribution to transmission<\/td>\n<td>High-efficiency amorphous-core options<\/td>\n<td>$18,000\u2013$32,000<\/td>\n<\/tr>\n<tr>\n<td>Schneider Electric<\/td>\n<td>Dry-type distribution<\/td>\n<td>Deep low-voltage integration<\/td>\n<td>$15,000\u2013$28,000<\/td>\n<\/tr>\n<tr>\n<td>Hitachi Energy<\/td>\n<td>Utility and industrial<\/td>\n<td>Grid-strength and HVDC expertise<\/td>\n<td>$17,000\u2013$30,000<\/td>\n<\/tr>\n<tr>\n<td>Eaton<\/td>\n<td>Distribution, pad-mount<\/td>\n<td>Strong North American footprint<\/td>\n<td>$14,000\u2013$26,000<\/td>\n<\/tr>\n<tr>\n<td>CG Power, TBEA<\/td>\n<td>Utility and industrial<\/td>\n<td>Large-volume manufacturers<\/td>\n<td>$10,000\u2013$18,000<\/td>\n<\/tr>\n<tr>\n<td>Jiangsu Subian Electric Power<\/td>\n<td>10 kVA\u2013100 MVA<\/td>\n<td>IEC 60076-certified, full factory test reports, copper windings standard<\/td>\n<td>$8,000\u2013$16,000<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>The prices above are only estimates, based on FOB China \/ ex-works, for the 500 kVA class unit. Variations in price will depend on the customer requirements and use cases. The international brands listed above are only a selection of many competitors in the market with years of experience. What makes Jiangsu Subian Electric Power special for someone with capacity requirements? All the units come with the IEC 60076 report for routine testing as well as copper windings as standard. Essentially, a buyer knows exactly what he\/she receives with the unit. This information makes the decision-making process easier, as the capacity requirements are presented in the report. In case of doubts regarding the parameters, the client can always check the factory report figures against the calculation in this study.<\/p>\n<p style=\"text-align: center;\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-10661\" src=\"https:\/\/subian-electric.com\/wp-content\/uploads\/2026\/08\/A-7-Point-Capacity-Verification-Checklist.webp\" alt=\"A 7-Point Capacity Verification Checklist\" width=\"1448\" height=\"1086\" \/><\/p>\n<h2 id=\"checklist\">A 7-Point Capacity Verification Checklist<\/h2>\n<ul>\n<li>You should compile a comprehensive list of all loads and group them as continuous, intermittent, motor, or nonlinear.<\/li>\n<li>You need to convert every load into kVA based on either the single-phase, three-phase, or kW-to-kVA conversion formula.<\/li>\n<li>You should take advantage of a diversity factor (which is usually somewhere in the range of 0.7 to 0.85 for industrial plants) and a growth factor of about 10 to 20 percent.<\/li>\n<li>Once you have obtained these figures, you can check them against nameplate kVA and ensure they fall within the range of standard IEC values.<\/li>\n<li>Make sure you confirm that cooling class, impedance, vector group as well as temperature rise comply with system standards.<\/li>\n<li>To confirm that everything is alright, you should request and read factory routine tests reports before you make any payment.<\/li>\n<li>After you have installed the system, monitor its demand for one week and establish that peak kVA does not exceed the prescribed level.<\/li>\n<\/ul>\n<h2 id=\"faq\">Questions Fr\u00e9quemment Pos\u00e9es<\/h2>\n<h3>How do I know if my transformer is overloaded?<\/h3>\n<p>Compare the average demand of the last 15 minutes with the rated capacity of the transformer. If the average exceeds 80% of the capacity for oil-type transformers or 90% for dry types on a constant basis, this means that the device is, in fact, overloaded. Among the signs of overloading that can be observed are the temperature of the oil above 90\u00b0C (one can use class-A paper), hot-spot readings that give above 105\u00b0C, noise in the tank, and gas composition analysis of gasses like hydrogen and ethylene. One would be able to cope with the problem in case of rebalancing the load, cooling it, or replacing the transformer with the next standard rating higher than the current one.<\/p>\n<h3>What is the difference between kW and kVA in transformer capacity?<\/h3>\n<p>kW stands for the actual power, whereas kVA stands for apparent power. kW can be calculated using the formula kVA = kW \u00f7 power factor. For instance, when the power factor is 0.8, the power of 100 kW requires transformers with 125kVA capacity. The nameplate of the transformer is always in kVA because all thermal capacity of the transformer depends on the winding. If the power factor is less than one, the transformer might not meet the requirements when calculated only with the help of kW.<\/p>\n<h3>What load factor should I use when sizing a transformer?<\/h3>\n<p>Use the real diversity factor of the loads in the facility: 0.6\u20130.75( for office buildings), 0.7\u20130.85(for industrial sites), 0.9\u20131.0 (for hospitals or data centers). Do not confuse these factors \u2014 the diversity and the load factor (average \u00f7 peak demand). If you are dealing with a new facility, use threat factor of 0.75 with a 15\u201320% growth margin. In case you are working with an existing facility and have all the utility bills for the last 12 months, you only need to compute your load factor.<\/p>\n<h3>Can a transformer run above its rated kVA temporarily?<\/h3>\n<p>Yes, it can, was allowed to do it only under strict circumstances. According to IEC 60076-7, the oil-immersed transformer with ONAN cooling system can handle up to 1.3 of its rated electrical load provided that it did not exceed 70% and heating temperature stays below 120\u00b0C. Every hour of usage of overload would shorten the lifespan of the insulation significantly \u2014 for example insulation at the temperature of 110\u00b0C would function only for a few years instead of several decades when used properly.<\/p>\n<h3>What does an extra 10% of capacity margin cost?<\/h3>\n<p>If the transformer has a capacity of 500 kVA, getting the new transformer with a capacity of 630 kVA would cost you 15\u201325% more (which means about $2,000\u20134,000). If you consider replacing the transformer after it failed, you should be prepared to pay for the new device $8,000\u201318,000 and waste up to 1\u20134 weeks to wait for it. If you can avoid switching off the load just once during 20 years, your investment in margin of 10\u201320% would have already paid off.<\/p>\n<h2 id=\"references\">R\u00e9f\u00e9rences<\/h2>\n<ul>\n<li><a href=\"https:\/\/webstore.iec.ch\/en\/publication\/200\" rel=\"nofollow noopener\" target=\"_blank\">IEC 60076-1: Power Transformers \u2014 General<\/a> \u2014 the international standard defining transformer ratings, capacity, and test requirements.<\/li>\n<li><a href=\"https:\/\/webstore.iec.ch\/en\/publication\/57088\" rel=\"nofollow noopener\" target=\"_blank\">IEC 60076-7: Power Transformers \u2014 Loading Guide for Oil-Immersed Transformers<\/a> \u2014 the authoritative guide for cyclic overload and temperature limits.<\/li>\n<li><a href=\"https:\/\/standards.ieee.org\/ieee\/C57.12.00\/6735\/\" rel=\"nofollow noopener\" target=\"_blank\">IEEE C57.12.00: General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers<\/a> \u2014 North American equivalent capacity and testing framework.<\/li>\n<li><a href=\"https:\/\/www.electrical4u.com\/transformer-load-factor\/\" rel=\"nofollow noopener\" target=\"_blank\">Electrical4U \u2014 Transformer Load Factor and Efficiency<\/a> \u2014 a clear explanation of load factor, diversity, and efficiency curves.<\/li>\n<li><a href=\"https:\/\/www.eaton.com\/content\/dam\/eaton\/products\/electrical-circuit-protection\/transformers\/transformer-sizing-guide.pdf\" rel=\"nofollow noopener\" target=\"_blank\">Eaton \u2014 Transformer Sizing Guide<\/a> \u2014 practical kVA calculation tables and sizing examples used across industry.<\/li>\n<li><a href=\"https:\/\/www.fluke.com\/en-us\/learn\/blog\/power-quality\/thermal-imaging-transformers\" rel=\"nofollow noopener\" target=\"_blank\">Fluke \u2014 Thermal Imaging of Transformers<\/a> \u2014 field guidance on infrared verification of loading and connections.<\/li>\n<li><a href=\"https:\/\/en.wikipedia.org\/wiki\/Transformer\" rel=\"nofollow noopener\" target=\"_blank\">Wikipedia \u2014 Transformer<\/a> \u2014 background on transformer theory, ratings, and cooling classifications.<\/li>\n<\/ul>\n<h2 id=\"conclusion\">Conclusion<\/h2>\n<p>Knowing if your transformer capacity is sufficient is not a matter of speculative guess about demand. The process involves four straightforward steps: determining the peak demand, calculating demand in kVA, determining the load factor, and checking nameplate data supported by factory-provided test results. Following the rule of permitting an increase of 10\u201320% and keeping the load factor below 80% for oil-filled transformers is rather cheap today and can prevent costly failures in the future.<\/p>\n<ul>\n<li>Make the calculations in kVA and use a diversity factor between 0.7 to 0.85 for mixed industrial loads.<\/li>\n<li>Check the capacity with the IEC 60076 report \u2014 that is the only proof, not the catalog.<\/li>\n<li>Provide for 10-20% margin for future needs \u2014 that is the cheapest form of insurance at the procurement stage.<\/li>\n<li>Choose the vendor based on the tested capacity and level of transparency, rather than the price per kVA.<\/li>\n<\/ul>","protected":false},"excerpt":{"rendered":"<p>In a food-processing plant in the Midwest, one of the engineers had recently installed a brand new pad-mounted transformer with a capacity of 200 kVA which was meant to support the expansion of a freezer. However, within a span of three weeks after installation, the transformer faced trip due to thermal overload during production hours [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":10658,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[77],"tags":[],"class_list":["post-10657","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-news"],"blocksy_meta":[],"acf":[],"_links":{"self":[{"href":"https:\/\/subian-electric.com\/fr\/wp-json\/wp\/v2\/posts\/10657","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/subian-electric.com\/fr\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/subian-electric.com\/fr\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/subian-electric.com\/fr\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/subian-electric.com\/fr\/wp-json\/wp\/v2\/comments?post=10657"}],"version-history":[{"count":2,"href":"https:\/\/subian-electric.com\/fr\/wp-json\/wp\/v2\/posts\/10657\/revisions"}],"predecessor-version":[{"id":10910,"href":"https:\/\/subian-electric.com\/fr\/wp-json\/wp\/v2\/posts\/10657\/revisions\/10910"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/subian-electric.com\/fr\/wp-json\/wp\/v2\/media\/10658"}],"wp:attachment":[{"href":"https:\/\/subian-electric.com\/fr\/wp-json\/wp\/v2\/media?parent=10657"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/subian-electric.com\/fr\/wp-json\/wp\/v2\/categories?post=10657"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/subian-electric.com\/fr\/wp-json\/wp\/v2\/tags?post=10657"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}