Transformer Calculations

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Before beginning to discuss the calculations involved with transformers, it is necessary to outline the most commonly available transformer sizes, in terms of KVA (kilo-volt-amps). This chart is not necessarily based on the code but is more of an industry standard held by most manufacturer's (GE, Square D, Eaton, etc.):

• 15 kva
• 30 kva
• 45 kva
• 75 kva
• 112.5 kva
• 150 kva
• 225 kva
• 300 kva

It is also important to define some terms that will be used throughout the calculations:

• connected load = total kilowatt load with no diversity taken into account.
• demand load = total kilowatt load with diversity calculated in.
• kva = kw (kilo-volt-amps = kilowatts)
• NEC = National Electrical Code (NFPA 70)
• Overcurrent Protection = Means of disconnect which prevents excess current on the conductor(s) to powered equipment.
• For more information about the primary side and secondary side of a transformer, click here.

Calculating the Transformer Size

Transformers are sized based on the total connected load on the secondary side and then selecting the next available kva size up from the connected load. Let's look at an example:

• Suppose we need to size a 480v-208v 3ø transformer. There is a 100amp panelboard on the secondary side serving power receptacles and workstations where the total connected load is 23.50kva. By reviewing the transformer chart from above, you will find that 30kva is the next available size up from 23.50kva. Therefore a 30kva transformer is required for this system.

Calculating Overcurrent Protection on the Primary Side

Overcurrent protection for a transformer on the primary side is typically a circuit breaker. In some instances where there is not a high voltage panel, there is a fused disconnect instead. These are the two most common ways to provide overcurrent protection on the primary side.

According to NEC 450.4, "each transformer 600 volts, nominal, or less shall be protected by an individual overcurrent device installed in series with each ungrounded input conductor. Such overcurrent device shall be rated or set at not more than 125 percent of the rated full-load input current of the autotransformer." Further, according to NEC Table 450.3(B), if the primary current of the transformer is less than 9 amps, an overcurrent device rated or set at not more than 167% of the primary current shall be permitted. Where the primary current is less than 2 amps, an overcurrent device rated or set at not more than 300% shall be permitted.

From my experience, the primary current is rarely 9 amps or less, much less 2 amps. For the most part you will be using 125% to determine the primary overcurrent protection.

Let's take a look at an example:

• What size circuit breaker (overcurrent protection device) is required on the primary side to protect a 75kva 480v-208v 3ø transformer?


• 75kva x 1,000 = 75,000va
• 75,000va / (480V x √3) = 90.21 amps
• (Note: 480V 3ø is calculated as 480V x √3 or 831.38)
• The current (amps) is more than 9 amps so use 125% rating.
• 90.21 amps x 1.25 = 112.76 amps


• Therefore: Use 125amp 3-pole circuit breaker (the next highest fuse/fixed-trip circuit breaker size per NEC 240.6).

**It is important to note that the overcurrent device on the primary side must be sized based on the transformer KVA rating and not sized based on the secondary load to the transformer.

Calculating Overcurrent Protection on the Secondary Side

In order to calculate the overcurrent protection on the secondary side, most of the same principles apply as with the primary calculations, with a few exceptions.

Obviously the voltgae is different. For a 480v-208v 3ø transformer, the calculations will use 208V x √3 instead of 480V x √3.

Also, according to NEC Table 450.3(B), where the secondary current of a transformer is 9 amps or more and 125% of this current does not correspond to a standard rating of a fuse or circuit breaker, the next higher standard rating shall be required. Where the secondary current is less than 9 amps, an overcurrent device rated or set at not more than 167% of the secondary current shall be permitted.

Let's take a look back at the previous example:

• What size circuit breaker (overcurrent protection device) is required on the secondary side to protect a 75kva 480v-208v 3ø transformer?


• *Note: Calculate the secondary overcurrent protection based on the size of the transformer, not the total connected load.
• 75kva x 1,000 = 75,000va
• 75,000va / (208V x √3) = 208.18 amps
• (Note: 208V 3ø is calculated as 208V x √3 or 360.27)
• The current (amps) is more than 9 amps so use 125% rating.
• 208.18 amps x 1.25 = 260.24 amps


• Therefore: Use 300amp 3-pole circuit breaker (per NEC 240.6).

Click here for charts showing overcurrent protection on both the primary and secondary side for single phase and three phase systems.

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