Air Compressor, Welder, and Heavy Equipment Circuits in Denver
Compressors and welders are sized by different rules, and using the wrong one leaves a circuit that nuisance trips or runs hot.
A rotary screw or reciprocating compressor is a motor load. Conductors get sized at 125 percent of the full-load current from the table value or the nameplate, overload protection comes off the nameplate, and the branch-circuit short-circuit device is allowed to be sized well above the running current under NEC 430.52 so locked-rotor inrush does not trip it on every start. A disconnect goes within sight of the unit, and starting method matters on larger machines, since across-the-line starting on a big motor can pull voltage down enough to bother everything else on the feeder.
Welders follow NEC Article 630 instead, and the rule that surprises people is duty cycle. Table 630.11(A) lets conductors be sized to the rated primary current multiplied by a factor tied to duty cycle, so a machine at 50 percent duty needs conductors rated for well under its nameplate primary current, while a 100 percent duty machine gets no reduction. Overcurrent protection is permitted up to 200 percent of rated primary current under 630.12, which is why a welder receptacle often carries a breaker that looks oversized next to the wire. Where several welders share a feeder, 630.11(B) demand factors apply because they rarely all strike an arc at once.
What size circuit does a welder need?
Start with the rated primary current on the nameplate, not the output amperage. Multiply it by the Table 630.11(A) factor for the welder type and duty cycle to get the conductor ampacity, then protect the circuit at up to 200 percent of rated primary current per 630.12, using the next standard size where the calculated value is not standard. That is why a 50 percent duty machine can land on smaller wire than its nameplate primary current suggests.
Why does an air compressor trip its breaker on startup?
Inrush is the usual culprit. A motor draws several times full-load current for the first moments of starting, and if the branch-circuit device was sized to running amps instead of the allowances in NEC 430.52, it trips before the motor gets up to speed. Other common causes are an unloader valve that is not venting head pressure so the motor starts against load, voltage drop on a long undersized run, and a failing start capacitor or contactor on single-phase units.
Can two welders share one circuit?
Each welder normally gets its own branch circuit and receptacle sized to that machine. Diversity is applied at the feeder, where NEC 630.11(B) allows demand factors for multiple welders on the assumption that they are not all drawing rated current at the same instant. Pushing two machines onto one branch circuit gives up that separation and tends to show up as dropped arcs and tripped breakers when both operators strike at once.
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