What is the primary current of a 45 kVA transformer at 480 volts three phase?

54.1 amps. Divide 45,000 volt amperes by 480 volts times 1.732.

What size wire feeds a 45 kVA 480 volt transformer?

4 AWG copper at the 75 degree C column, which carries 85 amps against a required 67.7 amps at 125 percent of the 54.1 amp rated primary current.

What size breaker protects a 45 kVA 480 volt transformer primary?

70 amps where there is no secondary protection, from the 125 percent limit in NEC 450.3(B) rounded to the next standard size. Up to 125 amps is permitted where the secondary is protected at no more than 125 percent of rated secondary current.

What is the minimum wire size to feed a 45 kVA 480 volt transformer?

4 AWG copper is the practical minimum for the primary of a 45 kVA transformer on a 480 volt three phase system. The rated primary current is 45,000 divided by 480 times 1.732, which is 54.1 amps. Sizing the conductors at 125 percent of that gives 67.7 amps, and 4 AWG copper at the 75 degree C column of Table 310.16 carries 85 amps. The next size down, 6 AWG at 65 amps, falls short of 67.7.

Primary overcurrent protection

Section 450.3(B) governs the transformer, not the conductors. For a transformer with a primary current of 9 amps or more and no secondary protection, the primary device is limited to 125 percent of rated primary current, which is 67.7 amps. The note to Table 450.3(B) permits the next higher standard rating when 125 percent does not correspond to a standard size, so 70 amps is the maximum primary breaker.

Where the transformer has secondary protection sized at no more than 125 percent of rated secondary current, the primary device is permitted up to 250 percent of rated primary current, which allows a 125 amp primary breaker. That option exists mainly to keep the primary device from nuisance tripping on inrush, since a dry type transformer can draw 8 to 12 times rated current for the first few cycles.

Secondary conductors

A 45 kVA transformer feeding a 120/208 volt three phase panel delivers 45,000 divided by 208 times 1.732, which is 124.9 amps. At 125 percent that is 156 amps, so the secondary conductors need 2/0 copper at 175 amps in the 75 degree column, feeding a 150 amp or 175 amp secondary main. Section 240.21(C) is what allows the secondary conductors to be tapped without protection at the transformer, and each of its subsections carries its own length and ampacity conditions, with 240.21(C)(2) covering the common 10 foot secondary conductor rule and 240.21(C)(6) the 25 foot rule.

Grounding and bonding

The secondary of a separately derived system requires a system bonding jumper under 250.30(A)(1) sized from Table 250.102(C)(1), and a grounding electrode conductor under 250.30(A)(5). The equipment grounding conductor for the primary feeder comes from Table 250.122 based on the 70 amp primary device, which calls for 8 AWG copper. If the primary conductors are upsized for voltage drop, 250.122(B) requires the equipment grounding conductor to be increased proportionally.

Field takeaway

4 AWG copper on the primary with a 70 amp breaker is the baseline for a 45 kVA 480 volt transformer. Verify the transformer nameplate before ordering wire, since impedance and taps vary and some units list a different rated primary current than the nominal calculation.