# Can you use the same circuit breaker for single phase and three phase electricity?

The title says it all. Can you use the same circuit breaker for single phase and three phase electricity? For example, if a 20 amp circuit breaker is in a panel of a building with single phase electricity, can it be pulled out and then used as a 20 amp circuit breaker in a building that receiving three phase electricity?

• Can we assume that the nominal voltages here are no higher than 240V? Oct 21, 2017 at 1:11
• I'm in the US. To be honest, I'm just doing some exploration here and I have next to no electricity knowledge outside your run of the mill basic every day knowledge. I was thinking the voltage would be 415 on the three phase. Oct 21, 2017 at 1:12
• 415 would be a three-phase corner to corner voltage, which in the usual wye configuration means 240 corner to neutral. Typically a house is served one phase and neutral. Of course you're talking UK, India, etc. That voltage does not exist in the USA expect at factories making things for export who need to test them on EU voltages. Oct 21, 2017 at 2:07
• @StatsStudent -- that's worthy of a separate question, but the short answer is "yes" as you can always bring your own transformer to the party :) Oct 21, 2017 at 2:24
• @StatsStudent in a commercial building in the US, you might have 208 3-phase wye (120 corner-neutral), 120/240 split-phase possibly from a local transformer, or 480 wye (277 corner-neutral) which is also usable as 480 delta. Sometimes you'll have 240 delta but never wye. The voltage you are interested in is the common "rest of the world" voltage, which I find interesting. Oct 21, 2017 at 6:55

## It depends. No, really, it does.

Unlike Europe, where everything low-voltage is on the same system (415Y/230V) and breakers are pretty universally interchangeable due to the DIN rail form factor (subject only to interrupting rating and trip curve specifications), the US uses a mix of systems, even for low voltage (<1kV) utilization equipment. As a result, breakers designed for one system may or may not be usable on another, even if both panelboards are listed for the breaker type in question and the interrupting rating of the breaker in question is not exceeded, which is a serious consideration with main breakers in underground service areas or on higher power services, and even for branch breakers in industrial or other high-power applications.

Even if we set aside 480V delta (whether isolated or corner grounded) and 480Y/277V systems, as those are only used in heavy commercial and industrial work, we still have two single phase and three three phase systems to deal with:

• 120/240V split phase (the stuff we see in our homes and in light commercial buildings too)
• 120/208V "2 out of 3" split phase (used in high-rises and downtowns that have what's called "network service" from the electric utility)
• 208Y/120V three phase (the modern standard for light three phase service)
• 240/208/120V high leg or wild leg delta (an obsolete way to provide "mixed" service for single phase and three phase loads)
• 240V delta, typically corner grounded (an obsolete service that was used when only three phase loads needed to be supplied)

Given all this, we'll start with the easiest case, namely the 208Y/120V three phase Wye system that is today's standard of choice for light three phase services. You indeed can do the breaker transplant you mention in this system, as the line-to-ground voltage is still the 120V it always was, and most modern light duty circuit breakers are slash rated at 240/120V (240V line-to-line, 120V line-to-ground).

However, it gets more complicated when you get to the delta-based systems. In a high leg delta, the center tapped winding between A and C phases has its tap grounded to form a neutral point in the system, allowing split-phase loads to be fed from A and C phases in conjunction with the neutral; these can use ordinary 240/120V breakers. However, the B phase sits at 208V to ground -- this means that to put loads on the B phase, you need to use a 208, 240, or higher voltage straight rated breaker -- these are quite uncommon, and are typically only seen in the two pole variety when they are found. (Three pole breakers are straight rated for 240V use, so they aren't an issue.)

It gets worse when you go to a corner grounded (aka "grounded B-phase") 240V delta system. Oftentimes, the panels used with this will be single phase panels, with the B (grounded) phase connected to the neutral bar in lieu of being run through a busbar. However, this requires 240V, 1φ-3φ rated breakers in the panel -- only a select few types of light duty breakers (usually higher end types such as QO) have grounded B phase ratings on their 240V straight rated 2 pole breakers. As a result, breakers from a single phase system can almost never be applied on a corner grounded 240V delta system.

Yes, because there's little difference between a 3-phase panel and a split-phase panel of the same corner-neutral voltage. Apocryphally, 208V 3-phase found in Manhattan and 240V split-phase found in the rest of North America. All of it 120V corner to neutral.

The panel and breakers need to be listed for the highest neutral to corner voltage. The apocryphal mistake is putting in wild-leg delta (208 wild-leg-to-neutral) and using 120V rated breakers.

If you're dealing with 5-continent power, that is generally served to your local pole as 3-phase "wye", 220-240V each corner to neutral, and 384-416V corner to corner. Since that's basically the only power around, everything is rated for service in that environment.

If you have 60A single phase service, the next house down might have 2-phase service... and if you tell the power company you want two big on-demand heaters or a heat pump with aux heat, they're going to bring you the other two phases.