Neutral Sizing on a Retrofitted Subpanel
A contractor is adding a 120/240 V single-phase subpanel fed from a 200 A main panel with a 4-wire feeder. The subpanel will supply a 9 kW electric range (nameplate 40 A, 240 V), a 4.5 kW dryer (240 V), six 20 A small-appliance branch circuits, and a 3-ton heat pump with a 25 A minimum circuit ampacity and 40 A maximum overcurrent device. The feeder conductors are 2/0 copper THHN with an equipment grounding conductor. The installer proposes to downsize the feeder neutral to 6 AWG copper, arguing that the neutral only carries the unbalanced load of the 120 V circuits. The service is 120/240 V, and the calculated load on the subpanel is 150 A.
1. Is the proposed 6 AWG neutral acceptable? Show your reasoning.
Read the worked answer
The 240 V loads (range, dryer, heat pump) contribute no neutral current at balanced operation, so the neutral load is driven by the 120 V small-appliance circuits. Six 20 A circuits at 120 V are not all loaded simultaneously; the calculated neutral load is the maximum unbalanced load between the ungrounded and neutral conductors.
Key code point: the neutral must be sized to carry the maximum unbalanced load (NEC 220.61), and it may not be smaller than the required minimum. For a feeder with a 2/0 copper ungrounded conductor, the neutral may be reduced only where the calculated unbalanced load permits - but the neutral must still be sized for that load, and 6 AWG copper (65 A at 75°C) is far larger than the small-appliance unbalanced load would require. The real issue is whether the neutral can be smaller than the ungrounded conductor at all: yes, under 220.61 and 250.24(C)/310.15 considerations, provided the calculated unbalanced load is served.
So the 6 AWG neutral is electrically adequate for the unbalanced 120 V load, but the installer must verify: (1) the neutral is not required to be full-size by any local amendment, (2) the neutral terminates on the neutral bar and is not used as a grounding conductor, and (3) the calculated unbalanced load does not exceed 65 A. With six 20 A circuits, the calculated unbalanced load is well under 65 A, so 6 AWG is acceptable. The answer is not a simple yes - it depends on documenting the unbalanced load calculation.
2. The heat pump nameplate shows 25 A MCA and 40 A maximum overcurrent protection. What size branch-circuit conductors and overcurrent device should be used, and why does the 40 A device not require 40 A conductors?
Read the worked answer
Select conductors with ampacity at least equal to the minimum circuit ampacity (MCA) of 25 A, after any adjustment factors. A 10 AWG copper conductor at 75°C is 35 A, which exceeds 25 A, so 10 AWG is acceptable for the branch circuit (assuming no derating).
The overcurrent device is sized to the nameplate maximum of 40 A, not to the conductor ampacity. This is permitted because the equipment nameplate MCA and maximum overcurrent protection are determined by the manufacturer under the listing standard (UL 1995 for heat pumps), and NEC 440.22 permits the branch-circuit short-circuit and ground-fault protective device to be sized per the nameplate. The 40 A device protects the equipment; the 10 AWG conductors are protected against overload by the equipment's internal overload protection and against short circuits by the 40 A device, which is acceptable because the conductors are part of a listed assembly with the MCA stamped on the nameplate.
Important: the 40 A breaker is not a general-purpose 40 A branch circuit - it is the specific maximum permitted for this listed unit. Do not upsize conductors to 8 AWG unless voltage drop or derating requires it.
3. The feeder is 2/0 copper THHN in a raceway with an ambient temperature of 40°C in the attic portion. What correction applies, and does it change the feeder ampacity?
Read the worked answer
2/0 copper THHN is rated 90°C, so its base ampacity is 195 A (90°C column). At 40°C ambient, the correction factor from the 90°C column is 0.91, giving 195 × 0.91 = 177 A. If the termination is rated 75°C, the 75°C ampacity of 2/0 copper is 175 A, and the corrected 90°C value (177 A) exceeds it, so the 75°C termination limit governs at 175 A.
The calculated load is 150 A, so 175 A ampacity is adequate. However, the installer must confirm the overcurrent device does not exceed the corrected ampacity: a 200 A main breaker would exceed 175 A, so either the feeder must be upsized (e.g., 3/0 copper) or the load calculation must be revisited. The 40°C attic ambient is a real derating condition under NEC 310.15(B)(2)(a); it cannot be ignored just because the run is short. The judgement call is recognizing that the 90°C insulation rating does not automatically give 195 A at the terminations.