15 HP Conveyor Motor Tripping Its Overload
A 15 HP, 230 V, three-phase conveyor motor has a nameplate FLA of 42 A, service factor 1.15, and a 40°C ambient rating. The branch circuit is 8 AWG THHN copper in EMT with three current-carrying conductors, protected by a 60 A inverse-time breaker. The starter has a thermal overload relay set at 45 A. After a motor replacement, the conveyor trips the overload after 10–15 minutes under normal load. The maintenance technician wants to increase the overload setting to 60 A and the breaker to 100 A. The motor terminations are rated 75°C. The electrician is asked to determine whether the conductors, overload, and short-circuit protection are correctly sized.
1. What is the minimum branch-circuit conductor size, and is the existing 8 AWG compliant?
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For a 15 HP, 230 V, three-phase motor, NEC Table 430.250 gives a full-load current of 42 A. The motor branch-circuit conductors are sized at 125% of the table FLA under NEC 430.22.
- Required ampacity: 42 A × 1.25 = 52.5 A
At 75°C, 8 AWG copper has an ampacity of 50 A, which is less than 52.5 A. The 8 AWG THHN might have a 90°C ampacity of 55 A, but the terminations are rated 75°C, and the 40°C ambient also requires correction. Even the 90°C value corrected for 40°C ambient is 55 A × 0.91 = 50.05 A, still below 52.5 A.
A 6 AWG copper conductor at 75°C has an ampacity of 65 A, which exceeds 52.5 A and is compliant. The existing 8 AWG branch-circuit conductors are therefore undersized for the motor branch circuit.
2. What is the maximum allowable overload relay setting, and is 45 A causing the tripping?
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The overload relay is sized from the motor nameplate current, not from the Table 430.250 value. The motor has a service factor of 1.15, so NEC 430.32(A)(1) permits an overload setting of up to 125% of nameplate FLA.
- Maximum overload setting: 42 A × 1.25 = 52.5 A
A 45 A setting is only 107% of nameplate FLA. It is below the maximum permitted value and may be too low for a motor that runs near its nameplate current, especially with voltage unbalance or a slightly higher actual load. That could explain nuisance tripping after 10–15 minutes.
The technician's proposal to set the overload at 60 A is not acceptable: 60 A exceeds 52.5 A and would fail to protect the motor from overload. A setting of 50 A or 52.5 A would be within the code maximum, but the actual running current should first be measured. If the motor is drawing well below 42 A and still tripping, the overload relay, voltage unbalance, or mechanical load should be investigated rather than simply raising the setting.
3. What is the maximum standard inverse-time breaker rating and the minimum disconnect rating?
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Short-circuit and ground-fault protection
NEC 430.52(C)(1) permits an inverse-time breaker sized at up to 250% of the motor FLA for a standard motor.
- 42 A × 2.50 = 105 A
- The maximum standard rating not exceeding 105 A is 100 A.
A 110 A breaker is the next standard size and may be permitted only under the specific allowances in 430.52(C)(1) Exceptions, such as a documented need to start the motor. The existing 60 A breaker is below the maximum and is permitted as short-circuit protection, but it may nuisance trip on starting. The technician should not increase it to 100 A unless starting current or tripping data shows the need; the breaker is not the overload protection.
Disconnect
NEC 430.110 requires the disconnecting means to have an ampere rating of at least 115% of the motor FLA.
- 42 A × 1.15 = 48.3 A
- The next standard disconnect size is 60 A.
A 60 A disconnect is the minimum compliant rating. The branch-circuit conductors, overload relay, short-circuit protection, and disconnect all must be checked separately.