Branch circuit wiring design

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ohm718

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Hi and a happy new year to all.
I am a electrician in NYC and I have run in to the following.
The Forman on the job is having us run 10/4 MC cable home runs to a pull box on the floor were he is having us run 12/2 MC cable to the outlet boxes. I believe that is not to code but I cant seem to find the pertinent article.
Thanks for any help. :)
 
What is the size of the Over Current Protection?

There is nothing wrong with this if the OCP is sized per the lowest conductors, and there are even some exceptions to this.

Roger
 
winnie said:
Dumb question here, but how many circuits are being run in the 10/4 MC, and how many phases is the supply?

-Jon
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Jon if I was betting man I would say one multiwire branch circuit containing A, B, C, & N.

10/4 MC will typically have 5 insulted conductors, Green, White, Black, Red & Blue. (other colors available)

Edit; that is insulated not insulted:roll:
 
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Right, that is what I am getting at. Let me rephrase what I was asking:

ohm718,
In the installation that you are asking about, how many circuits are being carried by the 10/4 MC, and how many phases is the supply?

iwire, you use 3 phase MWBCs all the time and know how they are supposed to be used, and so are betting that this is being done correctly. I figure that something doesn't sit right with the original poster, and so am betting on something not being run correctly, eg. three circuits being run in a single phase situation....

[Though, arguably, in a single phase situation, if protected by 20A breakers, the maximum unbalance current on the shared neutral conductor would be 40A, which is the conductor ampacity...if you have 90C terminations...I could see how one could _legally_ install 4 20A circuits on a single 10/5 cable *grin*]

-Jon
 
winnie said:
iwire, you use 3 phase MWBCs all the time and know how they are supposed to be used, and so are betting that this is being done correctly. I figure that something doesn't sit right with the original poster, and so am betting on something not being run correctly, eg. three circuits being run in a single phase situation....
Click to expand...

That certainly may be true.

I think the root of his issue was the dropping of conductor size after leaving the OCPD.

[Though, arguably, in a single phase situation, if protected by 20A breakers, the maximum unbalance current on the shared neutral conductor would be 40A, which is the conductor ampacity...if you have 90C terminations...I could see how one could _legally_ install 4 20A circuits on a single 10/5 cable *grin*]

-Jon
Click to expand...

:D

Yeah...I was thinking that as well but figured we should get the OP answered before going that route. :)


Heck for all we know the cable may contain two single phase 208 circuits.
 
ohm718 said:
Hi and a happy new year to all.
I am a electrician in NYC and I have run in to the following.
The Forman on the job is having us run 10/4 MC cable home runs to a pull box on the floor were he is having us run 12/2 MC cable to the outlet boxes. I believe that is not to code but I cant seem to find the pertinent article.
Thanks for any help. :)
Click to expand...


Your installation is code compliant as long as the circuits are 15 or 20 amps. It seems that your concern is the use of two different size conductors for the same circuit. This is not prohibited, and is a good design if voltage drop is an issue.
 
well there are 3 phases A ,B, C, N &G in the cable and the panel with 20 amp OCPD feeding the branch circuit.
It has been my understanding that in branch circuit wiring that the wire size had to remain the same through out the circuit? From the panel to the point of use.
As we in NYC have just started using the NEC could you tell me were I would look to see if this is permitted or prohibited? Thanks
 
ohm718 said:
well there are 3 phases A ,B, C, N &G in the cable and the panel with 20 amp OCPD feeding the branch circuit.
It has been my understanding that in branch circuit wiring that the wire size had to remain the same through out the circuit? From the panel to the point of use.
As we in NYC have just started using the NEC could you tell me were I would look to see if this is permitted or prohibited? Thanks
Click to expand...


You won't find an NEC requirement for the circuit conductors to be the same size for it's entire length because there isn't one. NYC is under the 2002 NEC with amendments and under the NEC different conductor sizes are permitted.
 
ohm718 said:
It has been my understanding that in branch circuit wiring that the wire size had to remain the same through out the circuit? From the panel to the point of use.
Click to expand...
Ohm, allow me to explain: yes, the wire must remain the same size (as in not smaller) when that size is the minimum allowed to be protected by the OCPD. There is no prohibition to going larger for whatever reason (except for conduit fill, etc.)

The concern here would be that someone may see a #10 and figure they can up-size the 20a breaker to a 30a. You should permanently mark this wire with a note notifying of the smaller conductor and/or maximum breaker to attach to.

What you cannot do is reduce a conductor size below the OCPD feeding it, directly or downstream. So, as long as the circuit is treated as if it were the smaller conductor size throughout its length, everything is acceptable.
 
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