Column: More than you ever wanted to know about multi-wired branch circuits
“Randy, in your report you mentioned the home has some multi-wired circuits and recommended an electrician. We had an electrician check on this. He told us we have to turn off two breakers to shut off all power to a circuit, but said this is common and is not a concern. We just wanted you to know this so you don’t report on this in the future.”
This has actually come up a few times recently, and I don’t think I’ve ever written about it in this column. Here is my comment from that report, which will help explain what multi-wired branch circuits are:
“There are numerous multi-wired branch circuits (also known as ‘shared neutral’ or ‘common neutral’ wiring). This was common at one time, and is still used in some circumstances. Some brand new homes have a multi-wire branch circuit for the outlet below the sink – one breaker for the disposer and a different breaker for the dishwasher but only one neutral wire. This wiring is safe in normal use. However, turning off a breaker does not ensure there will not be power in the neutral wire for that circuit, since the neutral wire also serves another circuit (breaker). Usually the breakers are physically connected, like a 240 volt breaker, when there is a shared neutral wire to ensure all power to that circuit is turned off. You should consult with an electrician regarding this wiring.”
So the concern with shared neutral wiring is if a breaker trips off, or is turned off, there is still power to that circuit. Simply put, the hot (black) wire is power going to the outlet, the neutral (white) wire is the power coming back. With AC (alternating current) electricity, you can have one neutral wire serve two hot wires, since only one at a time will be sending power back to the panel.
The electrician told you that you have to turn off two breakers to shut off a circuit, but are you positive you know which two breakers? And what if you have a friend install a ceiling fan, will he know you have to turn off two breakers on that circuit? If not, he may get a free hairdo when he starts wiring that fan.
Multi-wired wiring is only visible if you remove the deadfront (inner cover) on the panel. And even if someone removes the deadfront, they may not notice the shared neutrals. Sometimes you can spot shared neutral wiring because there are red wires on the 120 volt breakers. But sometimes there are no red wires, and the only way to notice this is either by counting the neutral (white) wires, or checking where the branch wiring enters the panel.
After receiving that letter, I wanted to confirm my comments. I called a master electrician that teaches at our home inspector classes. He said with shared neutral wiring the breakers must either be a dual pole (aka common trip, for 240 volt circuits) or have a “breaker handle tie,” which is a metal “clip” that connects the handles on two adjacent breakers.
I also called the Prescott Building Department and left a message with an inspector. My voice messages get emailed to me (makes it easy to copy and paste), here is his reply:
“Good afternoon Randy, This is the city of Prescott building department returning your call concerning the shared neutral question. It is allowed to share the neutrals. A common trip is not required, but a breaker tie handle is required so that if one breaker trips it does not necessarily trip the other one, but if you turn them off the breaker tie handle would turn both of them off. That can be accomplished by using either a two pole breaker or a breaker handle tie. So I hope that answers your question.”
They both said the same thing. Understand that a dual pole/common trip breaker is used for 240 volt circuits, like a range or water heater. It is designed to trip off both sides (“poles”) if either circuit trips off. This ensures that all power to the appliance is shut off if either 120 volt circuit trips. The breaker handles are also physically connected with a metal clip called “bridging.” This is to ensure all power is off if someone turns off the breaker (rather than it tripping).
Connecting two 120 volt breakers with a “breaker tie,” as required for shared neutral wiring, is not quite the same. A breaker handle does not have to move when a breaker trips. So if a 120 volt breaker trips off, a 120 volt breaker that is connected to it with a breaker tie will not trip off. This can make a tripped breaker hard to find because the handle is not in the “tripped” position.
So with a dual pole 240 volt breaker both handles will move if either side is tripped. And with two 120 volt breakers with a breaker tie, if one side trips the handles do not move, making it hard to locate a tripped breaker.
I inspected a brand new home recently with two multi-wire circuits. One was for the outlet under the sink (separate breakers for the disposer and dishwasher), and one was for the two 120 volt outlets in the laundry (separate breakers for the washer and gas dryer). Both of these had 240 volt common trip breakers. This is more convenient because the breaker will completely turn off if either side is tripped or turned off.
I will continue to alert my clients to multi-wire circuits if the breakers are not connected. And especially if they are in the general wiring, e.g. bedroom outlets, rather than for a washer/dryer or disposer/dishwasher. Like other comments in a home inspection report, the client may choose to not make any improvements, but at least they are aware of the situation.
Randy West owns Professional Building Consultants in Prescott. He is state-certified and has performed more than 7,000 home inspections in the Prescott area. West serves on the Home Inspector Rules and Standards Committee for the Arizona Board of Technical Registration. Contact him at firstname.lastname@example.org or visit http://inspectprescott.com.