branch circuit design

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Ben Trueblood

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In commercial apps in 120/208 V setting, when am I not allowed to share a neutral between my ungrounded conductors in a conduit. Can I share a neutral if I have receptacle and lighting loads?
 
There is nothing in the NEC that prohibits receptacle and lighting loads from sharing a neutral in a multiwire branch circuit. This would really be a design choice.

Chris
 
Stand alone

Stand alone

2008 NEC...[690.10(C)]WARNING Single 120-Volt Supply. Do not connect MWBC's! PV inverter to lighting and receptacles.
 
I do not own a 2008 codebook yet. No areas near me are using 2008 at this time. Is it just residential that is affected by the neutral sharing where you must use a multipole breaker if you share?
 
woodduder said:
I do not own a 2008 codebook yet. No areas near me are using 2008 at this time. Is it just residential that is affected by the neutral sharing where you must use a multipole breaker if you share?
Click to expand...
simulantenous disconnect applies to ALL multiwire branch circuits
 
Ben Trueblood said:
In commercial apps in 120/208 V setting, when am I not allowed to share a neutral between my ungrounded conductors in a conduit. (?) Can I share a neutral if I have receptacle and lighting loads?
Click to expand...

You can not share neutrals when the ungrounded conductors are of the same phase or leg, as this would double up the amperage carried on the neutral instead of canceling it out.

Example: (Simplified)
?A = 10A
?B = 10A
?C = 10A
Neutral current = 0

?A = 10A
?A = 10A
?A = 10A
Neutral current = 30A

Lites and recepts - sure share away - so long as any dimmed lighting does not say not to, most that would would be from panels anyway, usually not the simple wall-box type.
 
All commercial jobs i ever did already has panel schedule. If you alter this its up to the ahj to accept or not. Do you really want tie handles between lighting and receptacles. And you also create unwanted harmonics by mixing. I would avoid if at all within reason.
 
e57 said:
You can not share neutrals when the ungrounded conductors are of the same phase or leg, as this would double up the amperage carried on the neutral instead of canceling it out.

Example: (Simplified)
?A = 10A
?B = 10A
?C = 10A
Neutral current = 0

?A = 10A
?A = 10A
?A = 10A
Neutral current = 30A

Lites and recepts - sure share away - so long as any dimmed lighting does not say not to, most that would would be from panels anyway, usually not the simple wall-box type.
Click to expand...

Provided that the grounded conductor is sized to carry the load it would be permissible to have more than one ungrounded conductor of the same phase share a grounded conductor.

This may be a poor design, but not prohibited by the NEC.

Chris
 
Jim W in Tampa said:
All commercial jobs i ever did already has panel schedule. If you alter this its up to the ahj to accept or not. Do you really want tie handles between lighting and receptacles. And you also create unwanted harmonics by mixing. I would avoid if at all within reason.
Click to expand...
The use of a common neutral does not have any effect on the circuit creating unwanted harmonics. The only issue is that is some rare cases, the harmonic currents can add and over load a common neutral.
 
e57 said:
Lites and recepts - sure share away - so long as any dimmed lighting does not say not to, most that would would be from panels anyway, usually not the simple wall-box type.
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I've heard that before, and don't understand the problem. Why would a dimmer in line with each line conductor be any different than a switch so wired?
 
LarryFine said:
I've heard that before, and don't understand the problem. Why would a dimmer in line with each line conductor be any different than a switch so wired?
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I think he is talking about dimmer panels not dimmers for device boxes.

Many dimmer panels specify two wire circuits on the load side but I see no reason for it other then they ask for it.
 
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