Transformer OCPD Calculator

Protected on both the primary and the secondary, a 45 kVA 480 to 208Y/120 three-phase transformer takes a maximum 125 amp primary device and a maximum 175 amp secondary device. The primary rated current is 54.1 amps, and the primary and secondary row of NEC Table 450.3(B) allows 250 percent of that, which is 135.3 amps. That is a maximum with no Note 1 marker on it, so it rounds DOWN to the next standard rating at or below it, giving 125 amps. The secondary rated current is 124.9 amps, and the 125 percent cell gives 156.1 amps. That cell does carry the Note 1 marker, so where the result is not a standard rating the next higher standard rating is permitted, giving 175 amps.

Down. This is the error that matters most on this table. Note 1 is the only note that grants a round-up, and its own text scopes it twice over: where 125 percent of the current does not correspond to a standard rating of a fuse or a nonadjustable circuit breaker, a higher rating not exceeding the next higher standard rating is permitted. Both limits bind. On an adjustable-trip breaker the setting is not a standard rating to miss, so Note 1 never engages and the cell percentage stays a hard maximum. Its superscript appears on the 125 percent cells and nowhere else. The 250 percent cells carry marker 3 instead, and Note 3 is the coordinated thermal overload permission of six times and four times rated current. It says nothing at all about rounding. Under a table titled Maximum Rating or Setting of Overcurrent Protection, a cell Note 1 does not reach is a ceiling, and rounding a ceiling up oversizes the device and defeats the protection the table exists to require. On the 45 kVA example the ceiling is 135.3 amps: rounding down gives 125 amps, and rounding up would give a device above the code maximum.

Table 450.3(B) has a primary only row, and satisfying it protects the transformer. That is not the whole question, because it does not protect the secondary conductors. NEC 240.4(F) states that single-phase transformers other than 2-wire, and multiphase transformers other than 3-wire delta-delta, do not have their secondary conductors protected by the primary device. Conductors fed by a single-phase transformer with a 2-wire single-voltage secondary, or by a three-phase delta-delta transformer with a 3-wire single-voltage secondary, may be protected by the primary device, provided that protection also does not exceed the secondary conductor ampacity multiplied by the secondary-to-primary voltage ratio. 240.21(C)(1) carries the same rule. So on a 480 to 208Y/120 wye secondary, which is the most common dry-type step-down in a commercial building, the primary device cannot be counted as the secondary conductors protection, so those conductors need their own protection under 240.21(C). Primary-only protection of the transformer itself is still permitted by Table 450.3(B).

Table 450.3(B) does not use one percentage for every transformer. On the primary only row, a primary of 9 amps or more takes 125 percent, a primary under 9 amps takes 167 percent, and a primary under 2 amps takes 300 percent. Small control transformers land in those columns constantly. A 1 kVA 480 volt control transformer draws 2.08 amps on the primary, which is under 9, so the ceiling is 167 percent, or 3.48 amps. That cell has no Note 1 marker, so it rounds down, and the largest standard rating at or below 3.48 amps is a 3 amp fuse. Note that the answer is a fuse: NEC 240.6(A) gives fuses four ratings a circuit breaker cannot use, 1, 3, 6 and 601 amps, and on a transformer this small there is no standard circuit breaker small enough to satisfy the ceiling at all. On the primary and secondary row the primary stays at 250 percent across all three primary columns, including under 9 and under 2 amps.

Rated current is the kVA rating times 1000, divided by the voltage on that side, and for a three-phase transformer divided again by 1.732. Compute it separately for each side, because the two sides have different currents and Table 450.3(B) reads each against its own column. On the 45 kVA 480 to 208Y/120 example the primary is 45 times 1000 divided by 480 and by 1.732, which is 54.1 amps, and the secondary is 45 times 1000 divided by 208 and by 1.732, which is 124.9 amps. Leaving the 1.732 out is a common and expensive slip: the same 45 kVA at 480 volts single-phase draws 93.75 amps, about 73 percent higher, and every device chosen from that number comes out oversized.

Note 3 applies to the 250 percent cells, the primary side of the primary and secondary row. Where the transformer is equipped with coordinated thermal overload protection provided by the manufacturer and arranged to interrupt the primary current, the primary device may instead be rated or set at not more than six times rated current for a transformer of 6 percent impedance or less, and not more than four times rated current for a transformer over 6 percent through 10 percent impedance. Above 10 percent impedance Note 3 does not reach the transformer and the 250 percent cell stands. Note 3 grants a larger multiplier. It does not grant a round-up: those cells are still maximums and still round down.

No, and they are easy to confuse. This tool sizes the overcurrent protection for the transformer under 450.3(B). Sizing the conductors on the secondary is 240.21(C), and its ratios are taken against the device protecting the transformer primary. That is a different reference device from 240.21(B)(3), the feeder tap rule that happens to supply a transformer, whose ratios are taken against the device protecting the feeder. Same-looking arithmetic, different reference device, different conductor. Table 450.3(B) also gives maximums, so the device you actually install is often smaller: the load, the conductors, and the inrush the transformer needs to survive all push the other way.