What size primary breaker do I need for a 45 kVA transformer?

Start with the primary current, then decide whether you are protecting the primary only or both sides, because 450.3(B) gives two very different answers.

Primary full load current

For a three phase transformer, current equals kVA times 1000 divided by voltage times 1.732. For 45 kVA at 480 volts: 45,000 divided by 831.4 equals 54.1 amperes.

For a single phase 45 kVA transformer at 240 volts primary the math is 45,000 divided by 240, or 187.5 amperes. Use the actual nameplate, not the calculation, when the two disagree. Section 110.3(B) makes the listing and labeling instructions binding.

Primary only protection

Table 450.3(B) covers transformers 1000 volts nominal or less. With primary protection only, where the primary current is 9 amperes or more, the maximum rating of the primary overcurrent device is 125 percent of the primary current.

54.1 times 1.25 is 67.6 amperes. Note 1 to Table 450.3(B) permits the next higher standard rating from 240.6(A) where 125 percent does not correspond to a standard size. The next standard size above 67.6 is 70 amperes. So a 70 ampere breaker is the answer for primary only protection at 480 volts three phase.

Primary and secondary protection

If you install overcurrent protection on the secondary side rated at not more than 125 percent of the secondary current, Table 450.3(B) lets the primary device go as high as 250 percent of the primary current.

250 percent of 54.1 is 135.2 amperes, so a 125 ampere primary breaker fits under the ceiling. On the secondary side of a 480 to 208Y/120 transformer, secondary current is 45,000 divided by 208 times 1.732, or 124.9 amperes. 125 percent of that is 156.2 amperes, and Note 1 again permits the next standard size, which is 175 amperes.

The 250 percent option is what you use when the transformer has a high inrush and a 70 ampere breaker nuisance trips on energization.

Conductors on each side

Primary conductors are sized under 215.2 and Article 310, not by the breaker. A transformer is a continuous load for this purpose, so 125 percent of 54.1 is 67.6 amperes. In the 75 degrees C column of Table 310.16, 4 AWG copper at 85 amperes clears it and 6 AWG at 65 amperes does not. Use 4 AWG copper THHN or THWN-2.

Secondary conductors follow 240.21(C), which is the transformer secondary conductor rule. Section 240.21(C)(2) allows a 10 foot secondary conductor with conditions, 240.21(C)(6) allows a 25 foot secondary conductor for systems over 1000 volts, and 240.21(C)(1) covers the case where the secondary conductors are protected at their ampacity. For a 208Y/120 secondary at 124.9 amperes, 125 percent is 156.2 amperes, which lands on 1/0 copper at 150 amperes or 2/0 copper at 175 amperes depending on how you round. Note that 240.21(C) does not permit an unprotected secondary tap on a two wire to two wire transformer beyond the specific allowances listed.

The rest of the installation

Section 450.14 requires a disconnecting means within sight of the transformer, or capable of being locked in the open position with the location field marked. Section 450.21(B) sets separation and enclosure rules for dry type transformers over 112 1/2 kVA, which a 45 kVA unit is below. Section 450.9 requires ventilation and prohibits blocking the ventilating openings. Section 110.26 working space applies to the transformer enclosure.

Grounding is the piece most often missed. A 45 kVA 480 to 208Y/120 transformer is a separately derived system under Article 100, so 250.30(A) requires a system bonding jumper, a supply side bonding jumper, and a grounding electrode conductor sized by 250.66 based on the largest secondary conductor.

Field takeaway

70 ampere primary breaker for a 45 kVA 480 volt three phase transformer with primary only protection. Go to 125 amperes primary with a 175 ampere secondary device if inrush trips the 70. Primary conductors at 4 AWG copper, and treat the secondary as a separately derived system under 250.30.