Multiwire branch circuits back in the day

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mopowr steve

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NW Ohio
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Electrical contractor
I'm trying to remember what was allowed as far as how many branch circuits could land on the same neutral.
Here's the situation, have 4 separate existing #12 circuits 20amp on a single phase service ( assuming 2 on phase A and the other 2 on phase B) with a single #10 for neutral.
Something tells me that it used to be allowed but I question whether those circuits should have been on 15amp breakers not 20's. I'd say the original work was in the 60's
The job scope is to extend existing circuits to a new panel location not sure if I can do it without pulling in more neutrals to existing building recepts/lights?
 

480sparky

Senior Member
Location
Iowegia
I'm trying to remember what was allowed as far as how many branch circuits could land on the same neutral.
Here's the situation, have 4 separate existing #12 circuits 20amp on a single phase service ( assuming 2 on phase A and the other 2 on phase B) with a single #10 for neutral.
Something tells me that it used to be allowed but I question whether those circuits should have been on 15amp breakers not 20's. I'd say the original work was in the 60's
The job scope is to extend existing circuits to a new panel location not sure if I can do it without pulling in more neutrals to existing building recepts/lights?

It was either the '08 or '11 that put a stop to that practice.
 

infinity

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That code change was written by one of moderators and was accepted in the 2011 NEC. Common neutrals are still permissible under 225.7(B).
 

don_resqcapt19

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Illinois
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retired electrician
That code change was written by one of moderators and was accepted in the 2011 NEC. Common neutrals are still permissible under 225.7(B).
Also under 215.4.

The only problem now is that the term "common neutral" is not defined in the NEC and when the code change was made in 2011, many thought that the neutral that is used with a multi-wire branch circuit was a common neutral.

A definition of common neutral was also submitted, but was rejected.
 

mopowr steve

Senior Member
Location
NW Ohio
Occupation
Electrical contractor
So now the question is , If the existing install was compliant (somebody with a 1960's code book look this up) can I extend this method described or will I have to pull in multiple neutrals?

I see that Art. 225.7 relates to lighting equipment outside. And 215.4 refers to common neutral to feeders. Was there an article that related to indoor installations for receptacle circuits and lighting circuits? As I remember finding this in a shopping center years ago.
 

GoldDigger

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Placerville, CA, USA
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My take would be that you can extend existing circuits that share the single neutral but are not all one MWBC.
But you cannot pull a new circuit from the panel (or maybe even extend an existing circuit) without providing it with its own independent neutral.
In the case of extending an existing circuit, you can do that by connecting to the shared neutral at the box where the extension starts, but the new wire runs must have independent neutrals from that point on.
JMO and looking for other opinions.
Some might argue that extending a circuit would require pulling a new neutral back to the panel.
May be an AHJ thing if there is no clear interpretation.

If the extension requires you to install an AFCI or GFCI breaker, then the shared neutral will prevent them from working and you will have to repull.
 

don_resqcapt19

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Illinois
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So now the question is , If the existing install was compliant (somebody with a 1960's code book look this up) can I extend this method described or will I have to pull in multiple neutrals?

I see that Art. 225.7 relates to lighting equipment outside. And 215.4 refers to common neutral to feeders. Was there an article that related to indoor installations for receptacle circuits and lighting circuits? As I remember finding this in a shopping center years ago.
There were no other code sections that specifically permitted the use of a common neutral. There were experts that said you could use one, other that those two specific locations, but others that said you couldn't.

Both of those sections use the words "shall be permitted". The code panels intended those two specific permissions to act as a prohibition to the use of common neutrals in other areas. In my opinion the language just does notwork that way. Telling me that I can do one specific thing, in no way restricts me me from doing something else...in this example those two specific permissions, in no way prevented me from using a common neutral in other locations.

The term "shall be permitted" is intended, per the NEC Style Manual, to act as an exception. The problem here is that there was no rule that these two specific permissions can act as an exception to. With the 2011 code change there is a rule that these permissions can act as an exception to.

In my opinion the code, prior to 2011, did not prohibit the use of common neutrals in any location.
 

mopowr steve

Senior Member
Location
NW Ohio
Occupation
Electrical contractor
The whole scenario is that an existing exhibit building at a fairgrounds has a new addition put on. The plans called for existing service panel to be relocated to wall of new addition and to extend existing building circuits to new panel location.

So from a contractors point of view since that's as far as the scope of work goes hopefully extending circuits as originally installed (so long as concideration is given to loads imposed) would be OK. What do you think?

Now from an electricians standpoint I fully understand many reasons why I should change it. Which would involve practically rewiring the entire existing building something not on the plans which may set off needing another permit or addendum to current permit and a additional work order.

Looking at commentary around the 225.7 article leads me to believe I'm on the right track about possible current load on the neutral as in my OP. In that If I can extend with a single#10 neutral that the ungrounded conductors would have to be on 15amp breakers? Agree?
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
I don't understand why you are suggesting that the breakers must be 15 amp. You have #12 ungrounded conductors, and you do not have enough current-carrying conductors in the same conduit to cause the #12s to be derated below 20 amps. Can you explain your thought process on this one?
 

charlie b

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Lockport, IL
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Some might argue that extending a circuit would require pulling a new neutral back to the panel.
That's not the issue. The existing neutral is not moving anywhere. It is the panel that is moving. So you will have to pull neutrals (and hot conductors) for all the circuits back to the panel's new location from the panel's old location. If you are doing that anyway, I think you should pull at least one additional neutral wire. This will allow you to establish two MWBCs from the four hot conductors and the two neutrals. The difficulty is, what happens when those wires are spliced at the panel's old location? Do you (or must you) continue the separate neutral all the way to the loads? I think we would all agree that that would be the best thing to do. But is it practicable? Only you can tell us that. And is it required? Perhaps only the AHJ can tell us that.

 

don_resqcapt19

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Location
Illinois
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retired electrician
I don't understand why you are suggesting that the breakers must be 15 amp. You have #12 ungrounded conductors, and you do not have enough current-carrying conductors in the same conduit to cause the #12s to be derated below 20 amps. Can you explain your thought process on this one?
He has a common #10 neutral that serves two circuits with 20 amp breakers that are on the same phase or leg of the supply system and he has a potential 40 amp load on the neutral.
 

mopowr steve

Senior Member
Location
NW Ohio
Occupation
Electrical contractor
I don't understand why you are suggesting that the breakers must be 15 amp. You have #12 ungrounded conductors, and you do not have enough current-carrying conductors in the same conduit to cause the #12s to be derated below 20 amps. Can you explain your thought process on this one?

The reason why is because the maximum current that the common neutral could be subjected to would be the sum of the ungrounded conductors OCPD's. In this case 2 of them would land on the same phase so therefore 15a + 15a = 30a max allowed on #10 conductors(so to speak) instead of 20a + 20a = 40a. Which would require a #8 common neutral.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Remember that the ampacity of 'small conductors' is generally greater than the maximum permitted OCPD protecting those conductors.

The maximum current that this neutral can potentially see is 40A, but generally #10 conductors must be protected by no more than 30A breakers.

In the case of this installation, the #10 is being protected by 20A breakers, so the question is: what is the ampacity of the #10? I think that would depend upon temperature rating of the insulation and the number of conductors sharing a conduit.

-Jon
 

George Stolz

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Windsor, CO NEC: 2017
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Both of those sections use the words "shall be permitted". The code panels intended those two specific permissions to act as a prohibition to the use of common neutrals in other areas. In my opinion the language just does notwork that way. Telling me that I can do one specific thing, in no way restricts me me from doing something else...in this example those two specific permissions, in no way prevented me from using a common neutral in other locations.

The term "shall be permitted" is intended, per the NEC Style Manual, to act as an exception. The problem here is that there was no rule that these two specific permissions can act as an exception to. With the 2011 code change there is a rule that these permissions can act as an exception to.

240.50(A) features this exact problem in the 2014 NEC. What does that section prohibit?
 

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
Obviously this has been done in the past and some may still do this. To me it is stepping over dollars to pick up a penny. They saved one conductor back in the day, WOW !

Maybe with old TW it was a big deal back then. ?
 

kwired

Electron manager
Location
NE Nebraska
Obviously this has been done in the past and some may still do this. To me it is stepping over dollars to pick up a penny. They saved one conductor back in the day, WOW !

Maybe with old TW it was a big deal back then. ?
Whether it was right or not is one issue, but assuming it was permissible - you possibly save more then just one conductor, you also potentially reduce the number of current carrying conductors which may keep you from needing to increase conductor sizes as well as with less conductors you possibly can use a smaller raceway.
 

kwired

Electron manager
Location
NE Nebraska
In the case of this installation, the #10 is being protected by 20A breakers,
It may be depending on how you see the meaning of protection - but that conductor is not necessarily being limited to carrying a maximum of 20 amps by any particular device and if two 20 amp circuits on same leg of supply are sharing the same neutral return path that neutral conductor can carry 40 amps without tripping any protective device.
 

don_resqcapt19

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Location
Illinois
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retired electrician
In two of the three code sections that permit a common neutral, no guidance is given as to the sizing of that conductor but 227.7(B) says this:
(B) Common Neutral. The ampacity of the neutral conductor shall not be less than the maximum net calculated load current between the neutral conductor and all ungrounded conductors connected to any one phase of the circuit.
The rule above may actually permit the #10 neutral, if we knew the loads as is says the neutral is sized by the load and not by the OCPDs. With unknown loads it would make some sense to size the neutral for the circuit in question at 40 amps.
 
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