Nuisance Tripping on a 5 HP Single-Phase Compressor

Motor full-load current: Table 430.248 for a 5 HP, 230 V single-phase motor gives FLC = 28 A (use the table value, not the nameplate, for conductor and branch-circuit protection sizing).

Conductors (430.22): 28 A × 125% = 35 A required ampacity.

• 10 AWG Cu THWN-2 at 75 °C = 35 A - it just makes it in a 30 °C ambient.
• But the raceway runs through a 40 °C mechanical room. Applying the 310.15(B)(2)(a) correction factor of 0.88 for the 75 °C column: 35 A × 0.88 = 30.8 A, which is below the required 35 A. The 10 AWG is therefore undersized; 8 AWG Cu (50 A × 0.88 = 44 A) is required.

Short-circuit and ground-fault protection (430.52(C)(1), Table 430.52): An inverse-time breaker may be sized at 250% of FLC = 28 × 2.5 = 70 A. The 30 A breaker installed is only 107% of FLC - roughly one-third of what the code permits and far too small to ride through a 112 A inrush.

Diagnosis: The breaker is tripping because it is the wrong device for the duty, not because the motor is bad. Replacing it with a 70 A inverse-time breaker sized per 430.52 is the code-correct fix. The overload relay is a separate issue and does not protect against short circuits.

Rule (430.32(A)(1)): For a motor with a marked service factor of 1.15 or greater, overload protection is set at no more than 125% of the nameplate full-load current. This is a maximum, not a target.

• 26.0 A × 1.25 = 32.5 A maximum.
• The installed 30 A setting is only 115% of nameplate, and the motor is running in a 40 °C ambient where its actual winding temperature and running current will be at the top of the range. A 30 A overload on a 26 A motor with a 1.15 SF will nuisance trip during long, hot run cycles or after a hard start.

Correct approach:

1. Set the overload to the maximum permitted value from the manufacturer's selection table for this motor - typically the 32.5 A (or next standard device) point.
2. Measure actual running current under load. If running current is well under 26 A, the overload can be set lower for better protection; if it approaches 26–28 A, the compressor may be over-loaded or the supply voltage low.
3. 430.32 permits the overload to be increased beyond 125% only where the manufacturer's data or a documented engineering evaluation supports it - do not simply crank it up.

Key distinction to explain to the owner: The overload relay protects the motor winding from sustained overcurrent. The branch breaker protects the conductors from short circuit and ground fault. Setting both at 30 A confuses the two functions.

Response: Increasing the breaker is only legitimate up to the limits in 430.52 - and only if the conductor ampacity is also correct. Oversizing a breaker past the code maximum leaves the conductors unprotected against overload and creates a fire hazard, and the inspector will red-tag it.

Permitted maximums for this motor (FLC = 28 A):

• Standard rating: 250% = 70 A. That is the normal design point and is almost certainly enough here, since inrush is 112 A (160% of a 70 A breaker) - well within a thermal-magnetic breaker's instantaneous trip range.
• If the motor still will not start: 430.52(C)(1) Exception No. 1 allows the next higher standard rating, and Exception No. 2 allows up to 400% of FLC (112 A) where the need is demonstrated. Beyond 400%, the protection must be selected under engineering supervision with documented evidence.

Before going above 250%, investigate the real cause:

• Voltage drop on the 60 ft run - low voltage lengthens the start and raises current.
• Breaker located in a hot mechanical room - a breaker's thermal calibration derates at elevated ambient; consider a breaker rated for the environment or a different enclosure location.
• A tired or wrong-size run capacitor causing hard starting.
• Unloader valve not functioning, so the compressor starts against head pressure.

Document the decision and the measured starting current; an inspector or the owner's insurer may ask for it.