3 phase multiwire branch circuits

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brad.tanke

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Hey everybody. Had a question about running about running some circuits at work, looking for some verification. We need 4 120VAC 20 amp circuits over for a new machine. The circuits will come out of a 208 3 phase panel. So, each circuit needs a dedicated neutral, correct? One hot of each phase and it's dedicated neutral? Then the 3 circuits of the A, B, and C phases can share one grounding conductor, while the remaining circuit will need its own grounding conductor?

Thanks!
 
You have a few options here.

One is an ungrounded conductor and a grounded conductor for each 120 volt circuit. If all pulled in a common raceway only one equipment grounding conductor is needed, if ran in four separate methods - each will need it's own EGC.

MWBC's are another option. Three ungrounded conductors (A, B and C) can share one neutral as it will only carry imbalance current. The fourth circuit would have to have a second neutral to go with it. Again if all in same raceway only one EGC is necessary.
 
Thanks for the quick feedback! Is either method regarding the grounded conductors preferred over the other? Other than the obvious material saving and filling of the conduit? All 4 circuits are in the same 3/4" emt conduit so then the one equipment ground is appropriate.

Do you have a code reference I can read more into?

Thanks again
 
brad.tanke said:
Thanks for the quick feedback! Is either method regarding the grounded conductors preferred over the other? Other than the obvious material saving and filling of the conduit? All 4 circuits are in the same 3/4" emt conduit so then the one equipment ground is appropriate.

Do you have a code reference I can read more into?

Thanks again
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EMT itself is allowed to be the EGC, sometimes a wire EGC is specified though.

There is an art 100 definition Branch circuit - multiwire and there is 210.4.

Some basic electrical theory knowledge goes a long way in helping to understand them also.
 
brad.tanke said:
Thanks for the quick feedback! Is either method regarding the grounded conductors preferred over the other? Other than the obvious material saving and filling of the conduit? All 4 circuits are in the same 3/4" emt conduit so then the one equipment ground is appropriate.

Do you have a code reference I can read more into?

Thanks again
Click to expand...
I run MWBC every chance I get. The advantages are lower voltage drop, less material, less derating, and a feeling of satisfaction of a well done job. The only disadvantage is arguing with those that do not understand MWBC and have an irrational fear of them.

EMT is an acceptable equipment ground on its own and does a better job than any wire you can stuff into it. If you insist on having your soul darkened by joining the Cult of the Green Wire then one EGC per pipe is all you need. 250.120 or there abouts has requirements for EGCs.
 
ActionDave said:
I run MWBC every chance I get. The advantages are lower voltage drop, less material, less derating, and a feeling of satisfaction of a well done job. The only disadvantage is arguing with those that do not understand MWBC and have an irrational fear of them.

EMT is an acceptable equipment ground on its own and does a better job than any wire you can stuff into it. If you insist on having your soul darkened by joining the Cult of the Green Wire then one EGC per pipe is all you need. 250.120 or there abouts has requirements for EGCs.
Click to expand...


The other disadvantage nowadays is that the breakers have to be tied together.
 
brad.tanke said:
Hey everybody. Had a question about running about running some circuits at work, looking for some verification. We need 4 120VAC 20 amp circuits over for a new machine. The circuits will come out of a 208 3 phase panel. So, each circuit needs a dedicated neutral, correct? One hot of each phase and it's dedicated neutral? Then the 3 circuits of the A, B, and C phases can share one grounding conductor, while the remaining circuit will need its own grounding conductor?

Thanks!
Click to expand...

What type of Machine?

Does the manufacture of the machine give any specific power requirements other than 4 - 120V 20 amp circuits?
Example: Four 120V 20 amp Dedicated circuits.


.
 
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Jamesco said:
What type of Machine?

Does the manufacture of the machine give any specific power requirements other than 4 - 120V 20 amp circuits?
Example: Four 120V 20 amp Dedicated circuits.


.
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Even if it is, a dedicated circuit is not going to affect whether a neutral can be shared or not.


JAP>
 
jap said:
Even if it is, a dedicated circuit is not going to affect whether a neutral can be shared or not.


JAP>
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That depends on your understanding of a dedicated circuit and that of an equipment manufacture's.
 
Seeing as how "Dedicated Circuit" is not a code defined the example would need even more information than what was posted below :

The example would need to read:

Four 120v 20 amp Dedicated circuits with separate or no shared neutrals.

Otherwise it would just be a fist fight.

JAP>
 
jap said:
Seeing as how "Dedicated Circuit" is not a code defined the example would need even more information than what was posted below :

The example would need to read:

Four 120v 20 amp Dedicated circuits with separate or no shared neutrals.

Otherwise it would just be a fist fight.

JAP>
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I think what you are thinking of a separate circuit. Not a dedicated circuit.

Yes, a separate circuit can be part of a multiwire branch circuit.
 
Jamesco said:
I think what you are thinking of a separate circuit. Not a dedicated circuit.

Yes, a separate circuit can be part of a multiwire branch circuit.
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I need you to site me the code section that states a dedicated circuit cannot share a neutral.

JAP>
 
jap said:
I need you to site me the code section that states a dedicated circuit cannot share a neutral.

JAP>
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You need?

I never said anything about NEC code requirements for a dedicated circuit.
Just a reminder. NEC is bare minimum electrical safety standards.
Article 90.1(A)(B)


Post #10
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Originally Posted by jap

Even if it is, a dedicated circuit is not going to affect whether a neutral can be shared or not.


JAP>
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My response:
That depends on your understanding of a dedicated circuit and that of an equipment manufacture's.
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"Equipment Manufacture's"


>>

I don't know how many years you been in the Trade, but you have never installed a dedicated circuit as required by a manufacture of a piece of equipment that required a dedicated branch circuit?

By dedicated, I mean, a branch circuit from an electrical panel with a, dedicated, Hot conductor, neutral conductor, and insulated equipment grounding conductor. (NO shared neutral) I even had a few manufactures spec the dedicated branch circuit must be installed in it's own conduit. The dedicated branch circuit cannot share a conduit with other branch circuits. Do you know why?

Can you find any of the above in NEC? It must be in there somewhere if the manufacture says that's what required for his equipment. WHY does it have to be in NEC? Is anything that the manufacture required violate NEC?

So circle back to my post #7, where I responded to the OP.

Jamesco said:
What type of Machine?

Does the manufacture of the machine give any specific power requirements other than 4 - 120V 20 amp circuits?
Example: Four 120V 20 amp Dedicated circuits.
Click to expand...

Best regards,
Jim
 
Jamesco said:
I never said anything about NEC code requirements for a dedicated circuit. I don't know how many years you been in the Trade, but you have never installed a dedicated circuit as required by a manufacture of a piece of equipment that required a dedicated branch circuit?

Jamesco said:


Best regards,
Jim
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If you want to pull a dedicated neutral for a dedicated circuit you're certainly allowed to do so, but, this an NEC Code Forum , so, if you can't back up a statement with an NEC rule that requires it, it's probably not enfoceable.

I don't know how many years you've been in the trade either, but, your definition of a dedicated circuit is simply your opinion and is not in the NEC as such and is not going to hold water either.

This topic has been debated time and time again with no resolve and it won't be resolved in this thread either.


JAP>

JAP>
 
The only thing that would require you to pull a dedicated neutral for a dedicated circuit would be the rule that you have to follow the manufacturer's instructions.

If the Manufacturer's instruction said to pull 4 dedicated circuits to their machine, but, didn't say "with dedicated neutrals or no shared neutrals" anywhere in the literature, there would be nothing that would require dedicated neutrals or no shared neutral because a dedicated circuit is not defined.

It's simply someone's interpretation.


JAP>
 
Consider separate neutrals for each circuit if you may have to shut one off, but keep the others on.
You can still run all 4 in one pipe. Sharing a neutral can result in 208 on the circuit if the neutral is separated under load.
 
kgn742003 said:
Consider separate neutrals for each circuit if you may have to shut one off, but keep the others on.
You can still run all 4 in one pipe. Sharing a neutral can result in 208 on the circuit if the neutral is separated under load.
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Can also do that if the feeder or service neutral gets separated even though you ran a neutral for each ungrounded conductor with the branch circuits.
 
What type of "machine"?

What type of "machine"?

You have to consider harmonic current if said machine is a large computer or uses VFDs or large switch mode power supplies like for DC motors. It is well known in technical installations that harmonic currents can easily exceed a properly rated neutral. A neutral per phase certainly prevents this. Another allowed option is to oversize the neutral 200 or 300%.

I would consult with the machine manufacture to see if separate neutrals are advised.
 
Andy Delle said:
You have to consider harmonic current if said machine is a large computer or uses VFDs or large switch mode power supplies like for DC motors. It is well known in technical installations that harmonic currents can easily exceed a properly rated neutral. A neutral per phase certainly prevents this. Another allowed option is to oversize the neutral 200 or 300%.

I would consult with the machine manufacture to see if separate neutrals are advised.
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All mostly very true.

Still doesn't get away from the fact that a dedicated circuit does not require it's own neutral or cant share a neutral, unless, as you say , the machine manufacturer specifically requires it.

That would be a good conversation to have prior to the install.

JAP>
 
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