Shop wiring question

TomH in PA

Well-known Member
When I wired my shop it crossed my mind that I could save a few feet of wire by doing the following:

I have a 220 volt outlet near the sub-panel. Also, running through metal conduit from the sub-panel to a junction box near the ceiling, the left side of the shop has a black and a white wire, right side has a red and white wire (red carrying the secondary live). It occurred to me that a single white (neutral) running from the sub-panel to the junction box and tied to both white wires inside the box would be enough, since it could never carry more amperage then either the left or right side of the shop was using; if I ever had tools running on both sides of the shop the hots would be out of phase and (in my theory) would actually reduce the load in the single white returning from the junction box to the sub-panel.

I didn't actually do it, ended up running two white wires down to the sub-panel. But I wonder how/why the idea was dumb. I suppose the biggest bad would be if someone were to move the red wire in the sub-panel over to the primary hot feed and potentially double the amperage on the single white.
 
That's the way our garage was when we bought it, and it had been wired by an electrical contractor, but not meaning it was right! My understanding is like you said that if the two legs are on opposite phase, they can share a neutral. and have a bare wire for ground. When I added on my workshop with a new 100 amp service I pulled up the old wire, as I understand it's improper to feed a building from 2 sources. I would imagine John T will have a more thorough explanation!
 
In commercial wiring they often insist on not sharing neutrals and having a neutral for every line. As long as each leg is A and B the neutral current will cancel , true . It must be paid attention to by anyone tying in a 3 wire circuit to a panel to be on A and B not somehow staggering the breakers to just one phase, thus overloading the neutral.
 
A three-wire circuit like you describe is not that uncommon, and as you describe will cut voltage drop in half if you have equal heavy loads on each side. Electricians don't seem to care for them and (in my experience) prefer to run a pair of two-wire circuits instead.

I ran a three-wire circuit in my kitchen; the refrigerator is on one side and the microwave and warming drawer are on the other.

I seem to recall John T saying that both sides of a three-wire circuit need to be on a single double-pole breaker, which makes sense.
 
John T was correct, a multi wire branch circuit now requires all ungrounded conductors to be in one breaker assembly, that was a few code cycles ago, single phase three wire multi wire branch circuit is a two pole breaker, if you do a 4 wire three phase branch circuit you have a three pole breaker
 
You can save a lot of money on labor doing it that way. Especially if you are using Romex or MC cable. Long home runs is where it is really beneficial.
 
Using a double breaker makes sense. I hadn't thought about it, even though I did use a double on the two shop circuits because I only had a few slots available in the sub-panel and wanted to keep the lights and an outdoor GFI circuit separate.
 
I have several circuits that are wired that way but I used 3 wire Romex ran in the wall so I would always know witch circuits are sharing the neutral. I suppose a little electrical tape to make a bundle to show witch circuits are shared would work fine too.

I was just informed by an electrician friend that that is no longer allowed by code. He claimed it went out about 10 years ago. So if your in a situation that you need to deal with the code officials keep that in mind and maybe there is more details to the story that I don't know.
 
(quoted from post at 18:51:24 01/21/17) That's the way our garage was when we bought it, and it had been wired by an electrical contractor, but not meaning it was right! My understanding is like you said that if the two legs are on opposite phase, they can share a neutral. and have a bare wire for ground. When I added on my workshop with a new 100 amp service I pulled up the old wire, as I understand it's improper to feed a building from 2 sources. I would imagine John T will have a more thorough explanation!

Sharing a neutral with two opposing lines with single phase is kosher. As is sharing a single neutral with three different phases with three phase power.
There ain't no phase one and phase two with single phase power . It's line 1 and line 2.
 
"There ain't no phase one and phase two with single phase power . It's line 1 and line 2. "

Single phase, doh. Thanks for the correction.
 
What are referred to as "multi wire branch circuits" with two (120/240 single phase three wire) or more (such as say 208 Y 120 volt Three Phase Four Wire) ungrounded conductors sharing a single common grounded current return conductor (Neutral) is permissible and I did it often ONLY IF THE OVERCURRENT PROTECTION DEVICE (Circuit Breaker) IS ONE COMMON TIED BREAKER (2 pole for 120/240, 3 pole for 208 Y 120) . HOWEVER in applications with a lot of inductive loads and transformer power supplies etc. IT IS NOT ADVISED due to the presence of harmonic currents. Another HOWEVER if you wire up convenience receptacles that way in a residential branch circuit Billy Bob and Bubba may not be aware 240 VAC is present inside so while I specified multi wire branch circuits in a commercial application I was NOIT a fan of them in a residence.

John T
 

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