Rooftop HVAC Branch Circuit and Overload Protection
A 208 V, three-phase, 10 hp packaged rooftop HVAC unit has a nameplate marked FLA 30 A, MCA 36.5 A, MOP 50 A, compressor RLA 22 A, outdoor fan FLA 3 A, and a 500 W crankcase heater. The unit is supplied by THHN copper conductors in EMT from a 208Y/120 V panel. Terminals are rated 75°C, and the rooftop ambient is 40°C. The electrician proposes a 40 A inverse-time breaker, 8 AWG copper conductors, a 30 A overload relay, and a 60 A non-fused disconnect within sight. The owner wants the smallest breaker that will hold starting current. The unit nameplate is the only motor data available; no separate motor tables are to be used. The conductors are not bundled with other circuits.
1. Evaluate the 8 AWG copper conductor proposal for the branch circuit. Show the ampacity checks.
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The nameplate minimum circuit ampacity is 36.5 A, so the branch-circuit conductors must have an ampacity of at least 36.5 A. 8 AWG copper THHN has a 90°C ampacity of 55 A. Correct for the 40°C ambient using the 90°C correction factor of 0.91: 55 × 0.91 = 50 A. The terminals are rated 75°C, so the conductor cannot exceed the 75°C column value of 50 A. The final ampacity is 50 A, which exceeds 36.5 A.
8 AWG copper is therefore acceptable for the branch circuit. The 500 W crankcase heater is small and is already reflected in the nameplate MCA; the compressor and fan currents are not added again when the nameplate MCA is used.
2. Is the 30 A overload relay acceptable? What is the maximum permitted overload protection?
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For a motor-compressor, overload protection is based on the compressor rated-load current, not the total unit FLA. The compressor RLA is 22 A. At 125%, the maximum selection is 22 × 1.25 = 27.5 A. A 30 A overload relay is above 125% and is not automatically acceptable.
NEC 440.52 permits an increase above 125% but not exceeding 140% where the 125% device is not sufficient to carry the starting current. 140% of 22 A = 30.8 A, so a 30 A overload can be acceptable only if the 27.5 A selection cannot carry the starting current. The electrician should first select an overload that does not exceed 27.5 A, then document why a 30 A setting is needed if it is used.
3. Is the 40 A breaker permitted with a nameplate MOP of 50 A, and is the 60 A disconnect acceptable?
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The nameplate MOP of 50 A is a maximum, not a required size. A 40 A inverse-time breaker is permitted if it carries the compressor starting current without tripping and provides the required short-circuit and ground-fault protection. If nuisance tripping occurs, the breaker may be increased, but it must not exceed the 50 A MOP. The 40 A breaker does not have to equal the MCA of 36.5 A.
The disconnect must be within sight of the unit and rated at least 115% of the unit FLA: 30 × 1.15 = 34.5 A. A 60 A non-fused disconnect exceeds that minimum and is acceptable. The disconnect does not provide overload protection; the overload relay protects the compressor against overload.