10 hp at 480 V: Nameplate, Table FLC, and the Overload That Keeps Tripping
A 10 hp, 480 V, 3-phase, design B compressor motor drives a shop air system. Nameplate: 13.0 A at 480 V, service factor 1.15, rated for 40 C ambient. The branch circuit is 14 AWG THHN in EMT with three current-carrying conductors, terminated on 75 C lugs, protected by a 35 A inverse-time breaker, with a thermal overload relay in the starter set to 14 A. The compressor room runs near 40 C in summer. The relay has tripped several times in warm weather, and the maintenance tech proposes installing a larger heater element and, if that does not work, replacing the 35 A breaker with a 50 A breaker. Tabulated full-load current for a 10 hp, 460 V motor is 14 A.
1. State which current governs the conductors, which governs the short-circuit protection, and which governs the overload, and explain why they differ.
Read the worked answer
| What is being sized | Current used | Rule |
|---|---|---|
| Branch-circuit conductors | Table value, 14 A x 125% = 17.5 A | 430.22, 430.6(A)(1) |
| Short-circuit / ground-fault protection | Table value, 14 A x 250% = 35 A | 430.52(C)(1) |
| Overload protection | Nameplate, 13.0 A x 125% = 16.25 A (SF >= 1.15) | 430.32(A)(1) |
Why the split. 430.6(A)(1) says the tabulated full-load currents are used to size conductors and short-circuit protection, not the nameplate value. Those two items must survive motor starting and must carry the motor's normal current; the table values are conservative, generic, and deliberately usable without reading a nameplate in the field.
The overload is different in kind. It is the device that protects the motor from overheating, so it must track how much heat that specific motor actually makes. That is a function of its nameplate current, its service factor, and its temperature rise - information the table cannot know. A 10 hp motor with a 1.15 service factor can run continuously at 115% of nameplate, so 430.32(A)(1) allows the overload to be set as high as 125% of nameplate current.
Mixing the two up is the most common motor error in the field, and it is exactly what the tech is about to do.
2. Check whether the 14 AWG conductor and the 35 A breaker are acceptable in this environment.
Read the worked answer
Conductors - 14 AWG passes by 0.1 A, which is not a comfortable place to be.
- Required ampacity: 125% x 14 A = 17.5 A (430.22).
- 14 AWG Cu THHN at 75 C: 20 A.
- Ambient correction for 40 C from the 75 C column: 0.88 -> 20 x 0.88 = 17.6 A.
17.6 A >= 17.5 A: compliant, with essentially zero margin. Now look at what eats that margin: if the terminations are actually 60/75 C and the lug is 60 C, 14 AWG is 15 A and the circuit fails outright. Add a fourth current-carrying conductor, a higher ambient, or a rooftop or boiler-room condition, and it fails. 240.4(D)'s 15 A limit for 14 AWG does not apply here because 240.4(G) exempts motor branch-circuit conductors. Recommendation: 12 AWG. It removes the argument entirely for negligible cost and gives a defensible derated ampacity.
Breaker - correct as installed.
430.52(C)(1) permits an inverse-time breaker at up to 250% of the tabulated FLC: 2.50 x 14 A = 35 A, which is also a standard rating under 240.6. There is no justification for a 50 A breaker: 50 / 14 = 357%, far past the 250% ceiling, and it would leave the motor conductors and the starter unprotected against a real fault. If the breaker were tripping on inrush, 430.52(C)(1) Exception No. 1 would allow the next standard size above 35 A - but this breaker is not the thing that is tripping.
3. The relay set at 14 A keeps tripping in hot weather. Give a technically sound plan and state the maximum setting you may use.
Read the worked answer
Maximum permitted setting: 125% x 13.0 A = 16.25 A (430.32(A)(1), service factor 1.15 or greater). Select the heater element or set the electronic relay so that the actual trip point does not exceed 16.25 A. Note that 430.32 establishes a maximum, not a minimum: a 14 A setting is legal, but it is only about 108% of nameplate, and on a 40 C day with the motor at thermal equilibrium that is very likely to nuisance-trip.
Do this in order:
- Measure first. Clamp the actual running current at operating temperature and the actual supply voltage at the starter. If the motor is drawing more than 13.0 A for the load it is being asked to carry, there is a mechanical or voltage problem, not an overload-setting problem - dirty filters, a failing bearing, low voltage, a stuck unloader, or a compressor cycling too fast. Raising the setting would then simply hide a fault until the winding fails.
- Check the ambient correction. A thermal overload in an enclosure in an unconditioned 40 C room does not see the same ambient as the motor. Verify the relay is ambient-compensated and apply the manufacturer's correction for the enclosure temperature. This is often the real answer when the setting 'used to work' and the weather changed.
- Verify motor cooling. A design B motor rated 40 C is fine at nameplate, but airflow, a clogged fan shroud, or a motor mounted in a hot corner raises the effective winding temperature.
- Then, if the measured current is at or below nameplate and the trip still occurs on a properly corrected relay, raise the setting toward 16.25 A - never past it.
- Never increase over 16.25 A without 430.32(C). That section permits a higher value only where the motor's thermal protection and the manufacturer's data demonstrate the motor will not be damaged, and it requires documentation.
And absolutely do not change the 35 A short-circuit protection. An overload trip is telling you the motor is being asked to do too much work or is not being cooled. A bigger breaker does not change the heat in the winding; it only removes the protection against a short circuit. The tech's second proposal replaces a nuisance trip with a fire.