Can you legally per NEC share neutrals

[Msalazar1963]

Can you legally per NEC share one neutral for (3) 120v or 277 branch circuits in a 3 phase Panel.
if so must you use a 3 pole breaker or can you use (3) single pole Barkers?

 

[augie47]

You can (obviously if no two are on the same phase).
Depending on the Code cycle enforced, you may need a "simultaneous disconnect means" (see 210,4). (Most often accomplished here by handle ties)
You may also need to group your conductors (again 210.4)

 

[Dennis Alwon]

Here is why you can share a neutral. The neutral current on a single phase panel for a multiwire branch circuit (2 hots and a neutral) is equal to the difference in the amp load of the cir. Thus if cir A has 10 amps and cir B has 20 amps and they share the neutral then the neutral will only see 10 amps.

This is why the neutral generally doesn't have to be larger than the circuit conductors. Here is the formula for 3 phase

 

[Dennis Alwon]

Here is a small spreadsheet where you can input values and see the resulting amps on the neutral for a 3 phase system.

https://docs.google.com/spreadsheet/ccc?key=0AqnICUJivqdmdFMyMjEyWUpfWnhCQUtaX2N3OFZ0SHc&usp=sharing

 

[Msalazar1963]

Thank you thats what I was looking for

Thank you thats what I was looking for

You can (obviously if no two are on the same phase).
Depending on the Code cycle enforced, you may need a "simultaneous disconnect means" (see 210,4). (Most often accomplished here by handle ties)
You may also need to group your conductors (again 210.4)

 

[Sierrasparky]

Welcome to the forum:thumbsup:

 

[Dennis Alwon]

Welcome to the forum:thumbsup:

X2

 

[just the cowboy]

remember wtch for loose netural

remember wtch for loose netural

Remember though if the neutral wire comes off of the panel or box you WILL get crazy voltages to any devices that are plugged in to that circuit on 120V due to them now being hooked up in series. This will fry electronics!!! Saves money but not always good, my company does not allow it due to losing 100K in lab equipment.:weeping:

 

[GoldDigger]

Here is why you can share a neutral. The neutral current on a single phase panel for a multiwire branch circuit (2 hots and a neutral) is equal to the difference in the amp load of the cir. Thus if cir A has 10 amps and cir B has 20 amps and they share the neutral then the neutral will only see 10 amps.

This is why the neutral generally doesn't have to be larger than the circuit conductors. Here is the formula for 3 phase

The only potential problem with the application of your formula to the real world is that it is based on the assumption that all loads have a PF of 1.
In some (maybe artificial, but maybe practical) cases other than those of non-linear loads and harmonic currents, the neutral current could be larger than what the formula shows.
In particular, if you have a resistive load on one phase, say A-N, and a highly inductive load on one of the other phases (B-N or C-N depending on my uncertainty about phase rotation ) then the neutral current could be greater than the formula states because the vector of the inductive load current is such that the geometric construction on which the formula is based does not apply.

 

[qcroanoke]

Can you legally per NEC share one neutral for (3) 120v or 277 branch circuits in a 3 phase Panel.
if so must you use a 3 pole breaker or can you use (3) single pole Barkers?

Yes, as stated.
Welcome to the forum.

 

[jumper]

Remember though if the neutral wire comes off of the panel or box you WILL get crazy voltages to any devices that are plugged in to that circuit on 120V due to them now being hooked up in series. This will fry electronics!!! Saves money but not always good, my company does not allow it due to losing 100K in lab equipment.:weeping:

A properly wired MWBC will not have the loads in series nor produce crazy voltages, both scenarios would be attributed to a miswire or bad/broken neutral of a MWBC.

I understand what you meant to say, but what you wrote is a bit incorrect.

 

[ActionDave]

Remember though if the neutral wire comes off of the panel or box you WILL get crazy voltages to any devices that are plugged in to that circuit on 120V due to them now being hooked up in series. This will fry electronics!!! Saves money but not always good, my company does not allow it due to losing 100K in lab equipment.:weeping:

I don't know how many MWBC's I have wired up but I do know I have never known a day without them.

I don't know how many MWBC circuits exist but I do know it is more that I can count.

They work.

There is no place in the U.S. that is not served by a MWBC at some point.
Get used to them and there is not as much to worry about as you would like to think.

 

[roger]

Remember though if the neutral wire comes off of the panel or box you WILL get crazy voltages to any devices that are plugged in to that circuit on 120V due to them now being hooked up in series. This will fry electronics!!! Saves money but not always good, my company does not allow it due to losing 100K in lab equipment.:weeping:

The building your company is in has a MWC as the service to the building.

Roger

 

[K8MHZ]

The building your company is in has a MWC as the service to the building.

Roger

I've seen old houses with just a hot and a neutral serving them. No 240 volt circuit at all, just a single 120 volt drop. This was common in cities about 90 years ago.

 

[K8MHZ]

I don't know how many MWBC's I have wired up but I do know I have never known a day without them.

I don't know how many MWBC circuits exist but I do know it is more that I can count.

They work.

There is no place in the U.S. that is not served by a MWBC at some point.
Get used to them and there is not as much to worry about as you would like to think.

Services aren't fed by branch circuits. I think you meant to say they were served by a shared neutral. So there are (or used to be) homes with no MWBC's and no way to even run one, as I posted above.

 

[kwired]

Remember though if the neutral wire comes off of the panel or box you WILL get crazy voltages to any devices that are plugged in to that circuit on 120V due to them now being hooked up in series. This will fry electronics!!! Saves money but not always good, my company does not allow it due to losing 100K in lab equipment.:weeping:

I've seen old houses with just a hot and a neutral serving them. No 240 volt circuit at all, just a single 120 volt drop. This was common in cities about 90 years ago.

I was going to say the service or feeder is likely a multiwire circuit. True there are old 120 volt only services, and some two wire separately derived systems. There will be no true MWBC's served from those kinds of supply either though.

 

[roger]

I've seen old houses with just a hot and a neutral serving them. No 240 volt circuit at all, just a single 120 volt drop. This was common in cities about 90 years ago.

True, and those of us that have been in the trade for 40 years have seen them but, to have a MWC you need more than two wires.

Roger

 

[K8MHZ]

True, and those of us that have been in the trade for 40 years have seen them but, to have a MWC you need more than two wires.

Roger

That's what I was eluding to. I probably should have combined my two posts for clarity.

FWIW, I upgraded one less than 10 years ago. Customer wanted to know why he couldn't have 240 volt circuits. I think the service was only 30 amps. The triplex came to the weatherhead and one hot was just dangling there with some tape on the end.