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Inspecting Multiwire Branch Circuits

We are seeing more of these types of circuits. I think it may be due to the high price of copper. The NEC (National Electric Code) defines a Multiwire Branch Circuit as a branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system. Sounds confusing, but really it is quite simple and an efficient way to supply branch circuits. First, we will determine if a multiwire branch circuit is present; how it should be wired, and finally the proper way to inspect it:

Multiwire branch circuits do the work of four conductors using three. Basically, electric service is supplied for two branch circuits utilizing 3 conductors (plus ground). The conductors normally travel to a duplex receptacle and are split into two branch circuits. The break-off tab in the receptacle is removed to accommodate this. Each circuit is protected by a circuit breaker; however they are sharing a neutral and ground. When a home inspector is looking in the main service panel, it may be difficult to determine if a multiwire branch circuit is present. Older installations consisted of two 120 volt single pole adjacent circuit breakers; one with a red conductor and one with a black. It looked like a 240 volt circuit without the handle tie or connector.

Currently the NEC requires that:

  • All multiwire branch circuits must be connected to a double pole or two joined single pole circuit breakers. This is to reduce the risk of shock if one side of the circuit remains “live”.
  • Multiwire branch circuits must originate from the same service panel.
  • The over current protection must not exceed the conductor rating. (This is standard practice when inspecting the connections in a main service panel on all over current protection devices during a home inspection)
  • The two circuit breakers of a multiwire branch circuit MUST be connected on opposite “legs” of the hot lug or bus. This will ensure that the neutral is not overloaded creating a fire hazard. In most modern panels, this would mean the circuit breakers are adjacent to each other. The conductors should never originate from opposite sides of the panel. If they are connected on opposite sides of the panel, it may be difficult to determine if they are on opposite lugs, and it would be impossible to disconnect both simultaneously as required.
  • The multiwire branch circuit should be labeled in the panel
  • The ungrounded (hot) and grounded circuit conductors of every multiwire branch circuit should be grouped together by cable ties or by another safe method in at least one location within the service panel or point of origination. Doing this would ensure a multiwire branch circuit is not confused with a 240 volt branch circuit.

Sometimes looking in a service panel is like looking at a bowl of spaghetti. Our job is difficult when we are attempting to evaluate it. It may be impossible to determine if the branch circuit is a multiwire circuit or a 240 volt circuit. Here are some guidelines.

  • Ensure the over current protection (breaker) does not exceed the wire conductor capacity.
  • Ensure the neutral and grounded conductors are properly connected.
  • If there is no 240 volt appliances or receptacles, it may be a multiwire branch circuit.

As long as the wiring is safe, we can recommend that the circuits are properly traced and labeled. An electrician can then cable tie the multiwire branch circuits as noted above. If you are still unsure because proper wiring methods as outlined here were not followed, it would be wise to have a qualified electrician evaluate the service panel and make necessary updates.

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