15 hp Motor: Three Different Amps for One Circuit
A 15 hp, 230 V, three-phase, design B motor with a nameplate FLA of 34 A and a marked service factor of 1.15 drives a conveyor. It is supplied from a 100 A molded-case circuit breaker in a panel 80 ft away, with copper THHN conductors in EMT, terminations rated 75°C, and a combination starter with thermal overloads and a non-fused disconnect at the motor. The conveyor occasionally trips the 100 A breaker during acceleration, but the overloads never trip. A millwright suggests, "Just put a 150 A breaker in it - the wire is already big enough." You need to check the conductor, overload, and short-circuit protection sizing before anyone touches that panel.
1. Size the branch-circuit conductors and the disconnecting means, and explain why the nameplate FLA is not used for the conductor sizing.
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Conductors - 430.22(A). Motor branch-circuit conductors are sized at 125% of the motor full-load current taken from Table 430.250, not from the nameplate.
- Table 430.250, 15 hp at 230 V = 42 A
- 42 A × 1.25 = 52.5 A
- #6 Cu THHN at 75°C column of Table 310.16 = 65 A → #6 Cu works.
Why the table rather than the nameplate? The branch-circuit conductors must carry starting current and normal running current for a motor whose actual efficiency may be better than the table value; the Code deliberately uses the conservative table value so that the same motor circuit is adequately sized regardless of the specific motor installed later. The nameplate FLA is used for the overload sizing, because that device must match the actual motor's thermal capacity.
Disconnecting means - 430.110(A). The disconnect must have an ampere rating of at least 115% of the nameplate FLA: 34 A × 1.15 = 39.1 A → a 60 A non-fused disconnect is compliant (a 30 A switch would not be).
The disconnect must also be within sight of the motor and readily accessible per 430.102(B).
2. Size the overload protection and explain the difference between what the overloads protect and what the breaker protects.
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Overload sizing - 430.32(A). Separate overload devices are set at 125% of nameplate FLA:
- 34 A × 1.25 = 42.5 A → select the overload range/heaters so the setting does not exceed 42.5 A (use the next standard size only if 42.5 A is not obtainable, per 430.32).
- If the 125% setting will not carry the load, 430.32 permits increasing toward a maximum of 140% (34 A × 1.40 = 47.6 A) where the motor has a service factor of 1.15 or greater - which this one does.
Division of labor:
- The overload device protects the motor, the conductors, and the controller against sustained overcurrent from overload, phase loss, or a bad bearing. It is sized to the motor's thermal limit and is intentionally close to running current.
- The branch-circuit short-circuit and ground-fault protective device protects the circuit conductors and equipment against short circuits and grounds. It is sized generously (up to 250% of table FLA for an inverse-time breaker) so it will ride through starting inrush without tripping.
The reported symptom - breaker trips on acceleration, overloads do not - is consistent with an inrush problem, not an overload problem. That is a clue about the breaker sizing, but it does not justify touching the overload settings.
3. Evaluate the short-circuit protection. Is the 100 A breaker correct, is 150 A permissible, and what should you check before increasing it?
Read the worked answer
Maximum standard size - Table 430.52. For a 15 hp, 230 V motor, an inverse-time breaker is permitted at 250% of Table 430.250 FLA:
- 42 A × 2.50 = 105 A → 240.6 permits the next higher standard rating, 110 A.
So a 100 A breaker is actually within the 250% allowance, and is the reason it is marginal rather than clearly wrong. Stepping up to 110 A is the normal first move.
Exception No. 2 to 430.52(C)(1). Where the calculated value is not sufficient for starting, the breaker may be increased up to a maximum of 400% of FLA: 42 A × 4.00 = 168 A. A 150 A breaker is 357% of 42 A, so it is inside the ceiling - meaning it is not automatically illegal. But "not automatically illegal" is not the same as "do it." Conditions that apply:
- The overloads must still protect the motor and conductors; increasing the breaker does not license increasing conductor size rules.
- The equipment (starter, disconnect, conductors, terminations) must be rated for the available fault current and for the breaker's let-through.
- The increase must be documented and acceptable to the AHJ; it is not a casual substitution.
What to check before increasing anything:
- Is the trip actually inrush? Verify with a clamp meter and a recording, and confirm the breaker's trip curve and ambient temperature inside the panel.
- Is the supply adequate? 80 ft of #6 Cu at 42 A gives roughly 4.5 V drop (~2%), which is acceptable - but check the actual voltage at the motor terminals during acceleration; low voltage prolongs acceleration and causes nuisance trips.
- Is the breaker itself failing, or is there a loose lug / damaged breaker? A thermal scan and torque check on a de-energized panel answers this.
- Was the motor changed, or is the conveyor mechanically binding at start?
The correct answer to the millwright is: a 150 A breaker may be permitted under the 400% allowance, but only after the real cause is identified and the increase is documented - not as a first response to a trip.