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[Nathan Riddle]

Okay, new question.

What about tieing in all 3 legs of the distro into split phase power?

A - X @100A
B - Y @100A
A - Z @100A
N - N (theoretically only 100A should be on this leg plus power factor = 130A)
G - G

[Tim McCulloch]

Okay, new question.

What about tieing in all 3 legs of the distro into split phase power?

A - X @100A
B - Y @100A
A - Z @100A
N - N (theoretically only 100A should be on this leg plus power factor = 130A)
G - G

I'm pretty sure this isn't kosher and yes, it's a neutral issue.

[Mark Cadwallader]

Okay, new question.

What about tieing in all 3 legs of the distro into split phase power?

A - X @100A
B - Y @100A
A - Z @100A
N - N (theoretically only 100A should be on this leg plus power factor = 130A)
G - G

Same question, but would if be okay (or acceptable) if I had a 3 phase distro with dual neutral cams?  (As sometimes used by old school lighting folks.)

[Frank Koenig]

I'm pretty sure this isn't kosher and yes, it's a neutral issue.

Maybe I'm not understanding but if the neutral is sized in accordance with the upstream overcurrent protection of the 125/250 V source this would appear to be OK from a neutral current standpoint. I find it useful to draw the actual circuit diagram is situations like this.

                             /-------100A------X
L1-----100 A-------o                                               
                             \ ------100 A-----Z

N------------------o-------------------N

L2-----100 A-------o-------100 A-------Y

The combined load on X and Z cannot exceed 100 A since it will trip the source breaker, nor can the neutral current exceed 100 A under any conditions of loading. A load across X and Z will, of course, get nothing. A load across X, Y and Z expecting 3-phase will be very unhappy.

--Frank

[Nathan Riddle]

Maybe I'm not understanding but if the neutral is sized in accordance with the upstream overcurrent protection of the 125/250 V source this would appear to be OK from a neutral current standpoint. I find it useful to draw the actual circuit diagram is situations like this.

                             /-------100A------X
L1-----100 A-------o                                               
                             \ ------100 A-----Z

N-------------------o--------------------N

L2-----100 A-------o-------100 A-------Y

The combined load on X and Z cannot exceed 100 A since it will trip the source breaker, nor can the neutral current exceed 100 A under any conditions of loading. A load across X and Z will, of course, get nothing. A load across X, Y and Z expecting 3-phase will be very unhappy.

--Frank

If done this way, sure. That was my original post.

I'm saying say 3x 100A breaker is in the 2P panel.
You'll draw 200A from L1 and 100A from L2.

                             /-------(100A)------X
L1-----(400 A)-------o                                               
                             \ ------(100 A)-----Z

N------------------o-------------------N

L2-----(400 A)-------o-------(100 A)-------Y

(parentheses are breakers)

You could, if the system wasn't balanced, easily overload the neutral. If X and Z have full loading and Y has zero load then neutral would experience a 200A draw. Then throwing harmonics and powerfactor into the equation the neutral could be overloaded even more.

But in Mark's idea, a double neutral would keep you protected.

[Frank Koenig]

If done this way, sure. That was my original post.

I'm saying say 3x 100A breaker is in the 2P panel.
You'll draw 200A from L1 and 100A from L2.

                             /-------(100A)------X
L1-----(400 A)-------o                                               
                             \ ------(100 A)-----Z

N------------------o-------------------N

L2-----(400 A)-------o-------(100 A)-------Y

(parentheses are breakers)

You could, if the system wasn't balanced, easily overload the neutral. If X and Z have full loading and Y has zero load then neutral would experience a 200A draw. Then throwing harmonics and powerfactor into the equation the neutral could be overloaded even more.

But in Mark's idea, a double neutral would keep you protected.

Ah, got it. If the neutral in the distro is only good for 100 A there is a problem as it could see up to 200 A in this configuration. Helps to draw the picture. -F

[Tim McCulloch]

Double neutral would have to extend all the way back to the breaker box or company switch, and a company switch would have to be provisioned for a double neutral (2x feed from breaker box).

If the load from the Z leg was split equally between X and Y (for which Leprecon makes switch gear and distros, and they're not the only ones...), the yes, it can be done.  If it is not possible to split the Z leg loads, then no, it's probably not to Code.

We have on venue we regularly work in that has single phase power for audio.  We maintain separate distros, rack-packs, and SOOW 4 wire cable package for this venue but we're negotiating with them to install 3 phase service so all our big boy gear will not have to be sorted and segregated from the rest.

[Jonathan Johnson]

If done this way, sure. That was my original post.

I'm saying say 3x 100A breaker is in the 2P panel.
You'll draw 200A from L1 and 100A from L2.

                             /-------(100A)------X
L1-----(400 A)-------o                                               
                             \ ------(100 A)-----Z

N------------------o-------------------N

L2-----(400 A)-------o-------(100 A)-------Y

(parentheses are breakers)

You could, if the system wasn't balanced, easily overload the neutral. If X and Z have full loading and Y has zero load then neutral would experience a 200A draw. Then throwing harmonics and powerfactor into the equation the neutral could be overloaded even more.

But in Mark's idea, a double neutral would keep you protected.

I don't think a double neutral will be acceptable if the two neutrals are tied together at the distro. Referencing NEC 366.20, there are specific requirements for parallel conductors. It seems to imply that parallel conductors must be installed in raceway (conduit) and there is no mention of temporary or portable cordsets, or receptacle-and-plug connections. (I don't have the full text of 366.20 handy; I referred to this page.)

If the two neutrals are NOT tied together at the distro, and you bank X+Z then you might need three, not two neutrals: one banked with each "phase" of the distro. Each neutral would then carry the full current of its respective "phase" and you would lose any benefit of cancellation. (You couldn't have one neutral for both X+Z, since you'd still risk overloading it, even though you've removed Y from using the common neutral.)

Another possibility is to tie the hots for X+Z as described, then bank X with a dedicated neutral and Y+Z with a shared neutral. X won't overload its neutral, and Y+Z will have the benefit of cancellation.


N1-------------------o---------------------N---\
                                               (~)
                      /-------(100A)-------X---/
L1-----(400 A)-------o                                               
                      \ ------(100 A)------Z---\
                                               (~)
N2-------------------o---------------------N---<
                                               (~)
L2-----(400 A)-------o-------(100 A)-------Y---/



This is simply an academic exercise and has not been validated as an acceptable practice. I'll bet you'll get a cross-eyed look from the inspector.

----

But that is distracting from another issue: that your feeder is rated for only 100A but "protected" by a 400A breaker or fuse upstream, since the 100A overcurrent protection in the distro is downstream of the feeder. Sure, the code allows for a length of "undersized" conductor if it feeds a downstream overcurrent device -- but your feeder probably exceeds that length limit (possibly 10 or 25 feet, depending on circumstances). And, again, it looks like it must be in a raceway. (But don't quote me on that, I am claiming no authority on the matter. Do your own research.) Section 240.21.

So then, while you might not overload the wiring due to the overcurrent protection in the distro, the feeder conductors may not be adequately protected by the upstream overcurrent device. That protection has more to do with protecting from physical abuse that causes overload (short circuit) than from equipment loading.

The only way I see around this issue is to have a portable "company switch" that sits between the source panel and the feeder and has appropriate overcurrent protection for the feeder rating. And that portable switch probably needs to be fed with cabling (if there's no raceway) rated for the upstream overcurrent protection.

And you still need to account for neutral loading.

[Mark Cadwallader]

I've only seen pictures/product listings showing a double neutral on a distro, so I don't know how they are/might be wired.  Maybe it was Whirlwind that showed that option. I'll have to look for it/them.

My hypothetical use case would be using 4/0 feeder cable, so would that work?  Or would an oversized single neutral be sufficient (from a load standpoint, if not from a Code perspective)?

I apologize for any thread-jacking I'm guilty of, Nathan.

[Nathan Riddle]

I've only seen pictures/product listings showing a double neutral on a distro, so I don't know how they are/might be wired.  Maybe it was Whirlwind that showed that option. I'll have to look for it/them.

My hypothetical use case would be using 4/0 feeder cable, so would that work?  Or would an oversized single neutral be sufficient (from a load standpoint, if not from a Code perspective)?

I apologize for any thread-jacking I'm guilty of, Nathan.

Haha no worries, this is actually the direction I was thinking it would go so as far as I'm concerned, this mental masturbation is enjoyable

I don't think a double neutral will be acceptable if the two neutrals are tied together at the distro. Referencing NEC 366.20, there are specific requirements for parallel conductors. It seems to imply that parallel conductors must be installed in raceway (conduit) and there is no mention of temporary or portable cordsets, or receptacle-and-plug connections. (I don't have the full text of 366.20 handy; I referred to this page.)

If the two neutrals are NOT tied together at the distro, and you bank X+Z then you might need three, not two neutrals: one banked with each "phase" of the distro. Each neutral would then carry the full current of its respective "phase" and you would lose any benefit of cancellation. (You couldn't have one neutral for both X+Z, since you'd still risk overloading it, even though you've removed Y from using the common neutral.)

Another possibility is to tie the hots for X+Z as described, then bank X with a dedicated neutral and Y+Z with a shared neutral. X won't overload its neutral, and Y+Z will have the benefit of cancellation.


N1-------------------o---------------------N---\
                                               (~)
                      /-------(100A)-------X---/
L1-----(400 A)-------o                                               
                      \ ------(100 A)------Z---\
                                               (~)
N2-------------------o---------------------N---<
                                               (~)
L2-----(400 A)-------o-------(100 A)-------Y---/



This is simply an academic exercise and has not been validated as an acceptable practice. I'll bet you'll get a cross-eyed look from the inspector.

Besides the NEC requirements, I'm not sure I understand why the neutrals shouldn't be tied together?

But that is distracting from another issue: that your feeder is rated for only 100A but "protected" by a 400A breaker or fuse upstream, since the 100A overcurrent protection in the distro is downstream of the feeder. Sure, the code allows for a length of "undersized" conductor if it feeds a downstream overcurrent device -- but your feeder probably exceeds that length limit (possibly 10 or 25 feet, depending on circumstances). And, again, it looks like it must be in a raceway. (But don't quote me on that, I am claiming no authority on the matter. Do your own research.) Section 240.21.

FWIW, I always saw the 125A feeder being protected by 100A breakers in the drawing. The 400A breakers were the upstream panel or something similar.

So then, while you might not overload the wiring due to the overcurrent protection in the distro, the feeder conductors may not be adequately protected by the upstream overcurrent device. That protection has more to do with protecting from physical abuse that causes overload (short circuit) than from equipment loading.

The only way I see around this issue is to have a portable "company switch" that sits between the source panel and the feeder and has appropriate overcurrent protection for the feeder rating. And that portable switch probably needs to be fed with cabling (if there's no raceway) rated for the upstream overcurrent protection.

And you still need to account for neutral loading.

Agree with all this, if the 400A load is going into 100A feeder.

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