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I like ceiling fans & EMT
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Discussion Starter · #1 ·
I have an area fed by 6 20 amp circuits, from a distant panel, all into the same junction box. It's 3 runs of 12/3.

I finally took a look at this today and discovered that each run of 12/3 has two circuits on the same phase. All three phases are represented, but with three different neutrals, and each neutral could theoretically see 40 amps.

Obviously my plan is to go into the panel, and switch one leg from each run, so each run has opposing phases.

But I am curious: other than it being against code, what is the argument against simply tying all three neutrals together in the junction box?
 

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But I am curious: other than it being against code, what is the argument against simply tying all three neutrals together in the junction box?
Electrolysis, 1 of the neutrals will have a lower resistance than the others, assuming that you are talking about an armored 12/3, eventually the current imbalance will burn through the armor.
 

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I like ceiling fans & EMT
Former commercial, occasional (small) residential
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929 Posts
Discussion Starter · #3 ·
It's actually romex. Most of this building is wired in romex. I am converting it to EMT in most of the areas I've been working on.
 

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I like ceiling fans & EMT
Former commercial, occasional (small) residential
Joined
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929 Posts
Discussion Starter · #5 ·
This building went up around 1980, and was originally 3 floors of offices. 600 amp 3 phase service, 200 amp main panel and a 150 amp sub panel on each floor. EVERYTHING original to the building was romex, involving some of the thickest romex I've ever seen, and quite a few rats nests of wires. Over the years, as previous owners/tenants remodeled, there were a lot of changes made, and a lot of BX wire added. Also a lot of walls that were cut through, wire and all. Most of the work original to the building appears to have been done very well, but there were clearly some subsequent hack jobs. This shared neutral error is the first thing I've seen that's clearly original to the building and clearly . . . bad.

I'm trying to understand why anything would overload if they're all three tied together at both ends.
 

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I like ceiling fans & EMT
Former commercial, occasional (small) residential
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929 Posts
Discussion Starter · #7 ·
How great a resistance difference would it need to be for one cable to carry more than 20 amps? Especially given that there are 3 opposing phases canceling each other?

I've seen this done on 2 phase before, I didnt touch it.
 

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I like ceiling fans & EMT
Former commercial, occasional (small) residential
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929 Posts
Discussion Starter · #9 ·
As stated earlier in the thread, I'm going to do it right, but I'd like to learn why.
 

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How great a resistance difference would it need to be for one cable to carry more than 20 amps? Especially given that there are 3 opposing phases canceling each other?

I've seen this done on 2 phase before, I didnt touch it.
I'm curious, what is it about your education level that allowed you to feel comfortable leaving code violating parallel neutral/sharing neutral circuits uncorrected?

Here is the thing, you really don't have the choice to decide which code you follow and which one you ignore without accepting the consequences of your decisions. You may never have a problem, but you don't know when you might. Karma can be a b1tch.
 
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