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Publication Title | Apotential safety problem exists when a stand-alone 120-volt inverter is connected to a house or other structure wired for 120/240-volt alternating current.

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Text | Apotential safety problem exists when a stand-alone 120-volt inverter is connected to a house or other structure wired for 120/240-volt alternating current. | 001



Code Corner

SAFETY

ALERT

John Wiles

Apotential safety problem exists when a stand-alone 120-volt inverter is connected to a house or other structure wired for 120/240-volt alternating current. All PV Dealer/Installers and individuals who have installed such systems should review the following information carefully and take corrective actions where necessary.

Stand-alone PV and PV/Hybrid systems are frequently connected to a building, structure, or house that has been previously completely wired for 120/240-volts ac and has a standard service entrance and load center.

These structures may employ one or more circuits that the National Electrical Code ® (NEC ®) identifies as a multiwire branch circuit. See Section 100 in the NEC, “Branch Circuit, Multiwire” for a complete definition. These circuits take a three-conductor-plus-ground feeder from the 120/240-volt load center and run it to the loads in the structure where two separate 120-volt branch circuits are split out. Each branch circuit uses one of the 120-volt hot, ungrounded conductors from the 120/240-volt feeder and the common neutral conductor.

In a utility-connected system or a PV system with a 120/240-volt stacked pair of inverters, the 120/240-volt power consists of two 120-volt lines that are 180 degrees out of phase. The currents in the common neutral in the multiwire branch circuit are limited to the difference in currents from any unbalanced load. If the loads on each of the separate branch circuits were equal, then the currents in the common neutral would be zero.

A neutral conductor overload may arise when a single 120-volt inverter is tied to both of the hot input conductors on the 120/240-volt load center. This is a common practice for stand-alone PV homes and I do it in my house. At this point, the two hot 120-volt

conductors are being delivered voltage from the single 120-volt inverter and that voltage is in phase on both conductors. In the multiwire branch circuits, the return currents from each of the separate branch circuits add in the common neutral. A sketch of the multiwire branch circuit is presented below. Additional information can be found in the NEC in Sections 100, 210-4, 240-20(b), and 300-13(b), and in the NEC Handbook.

Each branch circuit is protected by a circuit breaker in the ungrounded conductor in the load center. The neutral conductor is usually the same size as the ungrounded conductors and can be overloaded with the in-phase return currents. The circuit breakers will pass current up to the ampacity of the protected conductors, but when both branch circuits are loaded more than 50%, the unprotected, common-neutral conductor is overloaded and may be carrying up to twice the currents that it was rated for.

A definite fire and safety hazard exists. All existing stand-alone PV installations using single inverters tied to both ungrounded conductors at the service entrance should be examined for multiwire branch circuits.

The NEC requires that multiwire branch circuits in some, but not all, cases have the two circuit breakers tied together with a common handle (or use a two-pole circuit breaker) so that both circuits are dead at the same time under fault conditions and for servicing. This common-handle, side-by-side circuit breaker rated at 15 or 20 amps may be one indication that multiwire branch circuits have been used. Common handle circuit breakers rated at 30 amps and higher are usually dedicated to 240-volt circuits for ranges, hot water heaters, dryers, and the like.

Examination of the wiring in the load center may show a three-wire cable (14 or 12 AWG conductors) with a bare equipment grounding conductor leaving the load center. This may be connected to a multiwire branch circuit. The circuit breakers connected to this cable and the outputs of this cable should be traced to determine the presence or absence of a multiwire branch circuit.

The multiwire circuits must be disconnected or rewired as separate circuits (“home runs”) from the load center. Another option is to limit the output of the inverter with a circuit breaker rated at the ampacity of the neutral conductor (usually 15 amps).

With 4000 watt (33-amp) inverters, a 15-amp circuit breaker on the output will certainly limit the output but won’t be very popular (only half power output).

A copy of a draft proposal for the 1999 NEC is presented below that addresses this problem.

Image | Apotential safety problem exists when a stand-alone 120-volt inverter is connected to a house or other structure wired for 120/240-volt alternating current.



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