Overloaded Subpanel in a Woodworking Shop
A 120/240 V single-phase woodworking shop is fed from a 200 A service. A 125 A subpanel in the shop area supplies a 5 hp table saw (nameplate 24 A at 240 V), a 3 hp dust collector (17 A at 240 V), a 3 hp planer (16 A at 240 V), six 20 A duplex receptacles on two 20 A circuits, and 4 kW of baseboard heating. The owner wants to add a 7.5 hp compressor. The subpanel feeder is 3-conductor No. 1/0 aluminum in a 60-foot run inside a warm attic, protected by a 125 A breaker. During operation the feeder neutral shows discoloration at both terminations, and the panel bus has a faint scorch mark near the main lug. The owner reports the main breaker has tripped twice in the past month when the saw and dust collector start together.
1. Evaluate whether the existing 125 A feeder and subpanel can carry the added compressor load, showing your load calculation and stating what must change.
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Start with the connected load. Motor currents at 240 V: saw 24 A, collector 17 A, planer 16 A, and the proposed 7.5 hp compressor is roughly 40 A at 240 V (verify against the nameplate and the applicable motor table). Receptacles: two 20 A circuits at 1.5 kVA each = 3 kVA. Heat: 4 kW. Convert to kVA at 240 V: saw 5.76, collector 4.08, planer 3.84, compressor 9.6. Total connected = 30.28 kVA, or about 126 A at 240 V before any demand factors. Applying the largest-motor 25 percent adder for the compressor gives roughly 10 A more, pushing the calculated load to about 136 A. That exceeds the 125 A feeder and breaker, so the compressor cannot be added as-is. Options: upgrade the feeder to a size and breaker that carry the calculated load with the continuous-load margin, or interlock the compressor so it cannot run with the saw and collector. The attic run and aluminum terminations must be re-evaluated for ampacity derating, and the discolored neutral and scorched lug indicate a loose or overheating connection that must be repaired before any load is added.
2. The scorch mark and discolored neutral terminations are visible. What is the correct sequence of actions, and what does this evidence suggest about the original installation?
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Treat the panel as unsafe until de-energized and inspected. Sequence: lock out and tag the upstream breaker, verify absence of voltage, then inspect and torque all terminations to the manufacturer's specification using a calibrated torque tool. The discoloration at both neutral terminations plus a scorched main lug points to high-resistance connections, likely from under-torqued aluminum conductors, missing anti-oxidant compound where required, or a neutral sized or terminated improperly. Aluminum expands and creeps under thermal cycling, so loose terminations heat and oxidize, raising resistance further. The tripping main under motor starting is consistent with either an overloaded feeder or a degraded connection causing voltage drop and higher starting current. Document findings, replace any heat-damaged conductor and hardware, and re-verify with a thermal scan under load. Do not simply reset the breaker and add load; the evidence indicates the feeder was marginal or improperly terminated from the start.