Class B Trip on a Commercial Kitchen Circuit
A restaurant kitchen has a 208 V three-phase panel feeding six 20 A circuits for counter receptacles, a 40 A range, and a 30 A water heater. The receptacle circuits are protected by Class B breakers. The manager reports that the breaker for the circuit serving the mixer and the adjacent counter receptacle trips after about 10 minutes of use, but only when the mixer runs and the nearby refrigerator compressor cycles. A second breaker for a different circuit trips immediately when a new 1800 W griddle is plugged in. The panel schedule is handwritten and two circuits are labeled only as 'kitchen'. A voltage reading at one receptacle shows 118 V to ground and 208 V between the two ungrounded legs. The neutral bar has two white conductors under one lug.
1. Distinguish the two tripping patterns and explain what each indicates about the circuit.
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The first breaker trips after about 10 minutes and only when two motor loads coincide. That pattern suggests a thermal or sustained overload, not a short circuit. The mixer plus refrigerator starting current may exceed the breaker's rating long enough to trip the thermal element. It could also be a shared neutral or a multiwire branch circuit where the two circuits land on the same phase, so the neutral carries the sum rather than the difference. Check whether the two circuits are on opposite legs; if they are on the same leg, the neutral is overloaded. The second breaker trips immediately with a new 1800 W griddle. At 208 V, 1800 W draws about 8.7 A, well within 20 A, so an immediate trip points to a fault: a shorted griddle cord, a receptacle wired with neutral and hot reversed, or a ground fault. Isolate by testing the griddle on a known-good circuit, then megger the branch. The two white conductors under one lug is a separate defect and must be corrected regardless.
2. Given the voltage readings and panel condition, what must be corrected and how do you confirm the multiwire circuit is correct?
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The 118 V to ground and 208 V between ungrounded legs is normal for a 120/208 V system, so the supply is not the problem. The doubled neutral termination is a defect: only one conductor per lug unless listed for two, and neutrals must be identified and terminated individually. Correct the panel schedule so every circuit is identified. For the suspected multiwire branch circuit, confirm the two ungrounded conductors are on opposite phases by measuring 208 V between them; if they read 0 V, they are on the same phase and the neutral is overloaded. Confirm the neutral is not shared with a circuit on a different phase pair in a way that violates the requirement for a common disconnect or handle tie. Verify by load-banking each circuit and measuring neutral current; on a balanced multiwire circuit the neutral current should be the difference of the two line currents, not the sum.