# 3 phase 240v lighting panel



## 480sparky (Sep 20, 2007)

If it's 3phase, one neutral is legal.


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## randomkiller (Sep 28, 2007)

Are you saying your running circuits for a/c units (acs)? If so I wouldn't run MWBC's for them, but you could.


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## Speedy Petey (Jan 10, 2007)

Curt, who is everyone?
If you are working on 3-phase you should already know the answer to this.


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## randomkiller (Sep 28, 2007)

Speedy Petey said:


> Curt, who is everyone?
> If you are working on 3-phase you should already know the answer to this.


I figured they are the other parts of "we" on the job.


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## Curt (Sep 16, 2007)

The building is sleeping rooms only and have 5000 btu units in all but one part.. All the panels but one use one neutral split between 3 individual branch circuits.. I think this is fine but 2 of the crew say all is wired wrong and the one panel with both lighting and acs are using 1 neutral for every 2 branch circuits is right. Didn't have the book for reference and look to the gurus here for some guidance..


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## 480sparky (Sep 20, 2007)

Curt said:


> The building is sleeping rooms only and have 5000 btu units in all but one part.. All the panels but one use one neutral split between 3 individual branch circuits.. I think this is fine but 2 of the crew say all is wired wrong and the one panel with both lighting and acs are using 1 neutral for every 2 branch circuits is right. Didn't have the book for reference and look to the gurus here for some guidance..


Could their 2-wire limit be because they're not used to 3-phase?

Three hots and one neutral is permitted in a 3-phase system. 210.4.


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## randomkiller (Sep 28, 2007)

Curt said:


> The building is sleeping rooms only and have 5000 btu units in all but one part.. All the panels but one use one neutral split between 3 individual branch circuits.. I think this is fine but 2 of the crew say all is wired wrong and the one panel with both lighting and acs are using 1 neutral for every 2 branch circuits is right. Didn't have the book for reference and look to the gurus here for some guidance..


You can use one neutral for two circuits as long as the wires are separated and marked and breakers common trip so one can't be independently off to back feed a neutral. One quick note, a/c or ac's read better than "acs".


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## randomkiller (Sep 28, 2007)

480sparky said:


> Could their 2-wire limit be because they're not used to 3-phase?
> 
> Three hots and one neutral is permitted in a 3-phase system. 210.4.


 
I think you hit that on the head.


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## Curt (Sep 16, 2007)

randomkiller said:


> You can use one neutral for two circuits as long as the wires are separated and marked and breakers common trip so one can't be independently off to back feed a neutral. One quick note, a/c or ac's read better than "acs".


I understand what you say.. But believe you mean this for a 240v load.. As installed each room has 1 - 120v circuit for a window a/c. 3 rooms with an a/c in each share that one neutral. Just a note.. Each unit draws 4 amps running..


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## nap (Dec 26, 2007)

Curt said:


> We are installing 110v circuits for acs. The panel has 3 120v lines and a neutral and is presently used for recpts and lighting.. There is no high leg.. There is a split view on whether 1 neutral can be used for all three legs supplying 3 separate ac circuits or if 1 neutral is required for every 2 circuits.. Everyone is going to the code books tonight but was looking for not just code but an explaination of why or why not..
> 
> Thanks all


obviously "WE" should have a code book on the job.

then; don;t forget it you are under 2008 code, you *must *use a common trip breaker for MWBC's


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## randomkiller (Sep 28, 2007)

Curt said:


> I understand what you say.. But believe you mean this for a 240v load.. As installed each room has 1 - 120v circuit for a window a/c. 3 rooms with an a/c in each share that one neutral. Just a note.. Each unit draws 4 amps running..


 
The amp draw doesn't matter for the most part, other than wire and breaker sizing. What you seem to be missing is, a Multi Wire Branch Circuit can have one neutral shared for two circuits, it is just 120v, not 240v. This is pretty basic wiring here. If what you are trying to tell us is that you have one neutral in a conduit run that is taking care of three circuits for three seperate rooms, then no that isn't allowed. There are specific rules for MWBC's that I posted for you earlier.


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## 480sparky (Sep 20, 2007)

randomkiller said:


> The amp draw doesn't matter for the most part, other than wire and breaker sizing. What you seem to be missing is, a Multi Wire Branch Circuit can have one neutral shared for two circuits, it is just 120v, not 240v. This is pretty basic wiring here. If what you are trying to tell us is that you have one neutral in a conduit run that is taking care of three circuits for three seperate rooms, then no that isn't allowed. There are specific rules for MWBC's that I posted for you earlier.


It isn't? Since when?
Where? I didn't see any.


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## nap (Dec 26, 2007)

I have to agree with 480 on this point.


I believe he was speaking to the seperation, marking, and common trip breaker. Not really sure what RK means by that though. If the '08 code is in force wherever this is, the common trip breaker is required but anything short of the '08 code does not require a common trip unless more than 1 circuit is on a yoke.

don't know what RK means by seperating and marking being required though. Marking is a good practice but I know of no requirement. and to seperating? Not sure what is meant by this at all.

one point though, backfeeding a neutral? You would not be backfeeding a neutral. You would simply still have a loaded neutral (term used around my area for a neut with a load on it) if there was any usage by the other circuit(s) of the MWBC. Maybe backfeeding is a term used in RK's area for the same thing. I consider backfeeding to be feeding a panel via a breaker (not a main breaker) 

The common neutral can pose a safety hazard if it is not known to somebody but my opinion is, if you do not understand MWBC's, then you have no business in the panel or wiring to start with.

RK, your posts almost always make sense. For some reason, this one just does not sound like you. Are we missing something?


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## wwilson174 (Apr 25, 2008)

This is obviously a 3 phase 4 wire 120/208 volt distribution system. so 3 circuits per neutral is normal. BillW


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## 480sparky (Sep 20, 2007)

wwilson174 said:


> This is obviously a 3 phase 4 wire 120/208 volt distribution system. so 3 circuits per neutral is normal. BillW


However, OP says it's 3-ph 120/*240*, with no high leg.....


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## wwilson174 (Apr 25, 2008)

He is mistaken, 3phase 4 wire with no high leg must be 120/208v. ( vectoral sums of voltages) BillW


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## nap (Dec 26, 2007)

wwilson174 said:


> He is mistaken, 3phase 4 wire must be 120/208v. ( vectoral sums of voltages) BillW


this does bring a bit a confusion to the situation and maybe a bit of clarity as well.

Ok Curt, you can;t have what you claim to have.

You got a couple choices of supply but 3 legs, 120 volts to ground plus a neut and 240 phase to phase ain't one of them.

Ya gots your grounded delta with 2 legs of 240 to ground and 1 0 volts to ground (4 wires including EGC but no 120 volt available)(probably not this because there is no 120 volt circuits available and it is pretty uncommon anyway)(and there are special considerations for breakers and marking conductors for this service)


ya gots your 3 legs to ground 120 volts but 208 phase to phase (5 wire including EGC)

and ya gots your "high leg" with 2 legs 120 to ground plus 1 leg 208 to ground and 240 phase to phase. (5 wire including EGC)

So, given you have 3 120 volt legs, it must be a 120/208 service. You can use 1/3 neut to hots.


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## wwilson174 (Apr 25, 2008)

Curt said:


> We are installing 110v circuits for acs. The panel has 3 120v lines and a neutral and is presently used for recpts and lighting.. There is no high leg.. There is a split view on whether 1 neutral can be used for all three legs supplying 3 separate ac circuits or if 1 neutral is required for every 2 circuits.. Everyone is going to the code books tonight but was looking for not just code but an explaination of why or why not..
> 
> Thanks all


Hi ,Curt I hope you are not confused by all the misinformation generated by your question. The reason that you are allowed to put three circuits on a neutral in your 3 phase panel is that the current flowing in each branch circuit is shared by 2 other phases just as the current in a single phase system circuit is shared by only one other phase.Congratulations on seeking a reason for this seeming anomaly.BillW


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## wwilson174 (Apr 25, 2008)

Curt said:


> We are installing 110v circuits for acs. The panel has 3 120v lines and a neutral and is presently used for recpts and lighting.. There is no high leg.. There is a split view on whether 1 neutral can be used for all three legs supplying 3 separate ac circuits or if 1 neutral is required for every 2 circuits.. Everyone is going to the code books tonight but was looking for not just code but an explaination of why or why not..
> 
> Thanks all


Hi, Curt, I hope you are not confused by all the misinformation generated by your question. The reason you can put 3 circuits on 1 neutral in your 120/208 panel is bcause the current in each circuit is shared by 2 other phases as opposed to a single phase system where the current is shared by only one other phase. Congratulations for seeking an answer to this apparent anomaly. BillW


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## wirenut1110 (Feb 12, 2008)

wwilson174 said:


> Hi, Curt, I hope you are not confused by all the misinformation generated by your question. The reason you can put 3 circuits on 1 neutral in your 120/208 panel is bcause the current in each circuit is shared by 2 other phases as opposed to a single phase system where the current is shared by only one other phase. Congratulations for seeking an answer to this apparent anomaly. BillW


Thanks Bill, I'm somewhat appalled about this being an electrical forum and someone asking a question and being bashed as they are. Electricians, whether genius' or not come here to seek advice from their "brothers"; it seems to me, only being here for a short time, that there's some people here are (1) threatened in their jobs, or (2) threatened in their manhood or (3) just flat out pricks. Ther just seems to be a few that are part of the staus quo and it ruins it for the rest of the people that *thought* this was a forum for people on the same team. Noone knows everything and sometimnes you just second guess yourself. Is it THAT bad. PS. I'm glaD i'm not a helper or apprenticee that comes in and tries to "look up" to those that are supposed to be my mentors.


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## wirenut1110 (Feb 12, 2008)

Also, as Bill stated, in a 3 phase system, you can share 3 circuits on one neutral and as long as they're of different phases (ABC) being balanced is a perfect situation with 0 load on the neutral however, this isn't always practible but given that you're doing a/c's, I wouldn't recommend it. If you have the time and money, pull a seperate neutral for each phase. If you decide against that, make sure there's a common trip for it (code or not)


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## nap (Dec 26, 2007)

wirenut1110 said:


> Thanks Bill, I'm somewhat appalled about this being an electrical forum and someone asking a question and being bashed as they are. Electricians, whether genius' or not come here to seek advice from their "brothers"; it seems to me, only being here for a short time, that there's some people here are (1) threatened in their jobs, or (2) threatened in their manhood or (3) just flat out pricks. Ther just seems to be a few that are part of the staus quo and it ruins it for the rest of the people that *thought* this was a forum for people on the same team. Noone knows everything and sometimnes you just second guess yourself. Is it THAT bad. PS. I'm glaD i'm not a helper or apprenticee that comes in and tries to "look up" to those that are supposed to be my mentors.


where was there any bashing? There were recommedations given. some advice and I and another guy asked for clarification of one of the most prolific posters here in an attempt to understand his statement.

When wwilson stopped by, he brought a point out that was correct and I followed with the possible scenarios asking for input from the OP as to what his situation is. Since there are a few possibilities and somewhere along the way, he was incorrect as to the service that is at the site, the correct asnswer could be different depending on what system is at the site. Is there a problem with wanting to be able to provide a correct answer?

the only bashing I see is your post, wirenut.


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## rlc3854 (Dec 30, 2007)

Nap excellent response to wirenut1110. This is a global ineternet froum and we don't always understand one another and occassionaly the post needs clarification. Curt, is this job a hotel/motel (sleeping quarters)? IMHO I think this is possibly RK's reasoning for the speration and marking as the circuits loop through each room. I have to give it to wwilson as I also believed that you couldn't have the 120/240 Volt 3 phase system without a high leg. This is the best electrical forum out there at this time, most here post at all the other great forums as well.


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## JohnJ0906 (Jan 22, 2007)

Curt, is the voltage between hot legs 208 or 240?


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## wirenut1110 (Feb 12, 2008)

I wasn't speaking to anyone in general. I was just pointing out that the fact that people come here and ask questions and get responses like, you should know this; if you knew what you were doing then why ask this question, etc. that's all. There's nothing wrong with asking for more information. I'm not trying to get in a pissing match, if someone took it that way, sorry. Just give these people a break. We're all here to learn from others and share our experiences. As I've always been told, it's not what you say but, how you say it.


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## nap (Dec 26, 2007)

> As I've always been told, it's not what you say but, how you say it.


I agree. Maybe it was how you said it:whistling2:

I will admit to be harsh from time to time but honestly, this thread has been cool. earmark this one and when I go wonko, copy and paste it on that thread.:thumbsup:

with that; back to assistance of the needy.:thumbup:


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## Curt (Sep 16, 2007)

As you all have stated voltage is 120/208 in the panel.. Just didn't pay close attention to the wiggy for an actual reading.. Just wanted to say you all really gave me greater insight and knowledge regarding this.. Points were brought up that won't find it's way into the code books ( at least in such a clear explanation ) and with my schooling long past your experience and comments gave far more than I expected.. We decided to pull more neutrals for our circuits and want to thank all for their opinions and comments..


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## randomkiller (Sep 28, 2007)

480sparky said:


> It isn't? Since when?
> Where? I didn't see any.


I was in mid brain fart there, I'm not sure exactly what I was thinking.


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## micromind (Aug 11, 2007)

If you're putting more than just a few wires in a conduit, I would not recommend two phases sharing one neutral. You'd be much better off with all three. The reason is because with two phases sharing a neutral, the current in the neutral will be about the same as the highest phase current. Neutral current in a 3 phase wye system doesn't start to decrease unless all three phases are involved. 

As stated above, I'd bet just about anything that the guys who say you can share neutrals with only two hots have little or no experience with 3 phase systems. 

A little bit of knowledge, coupled with authority, is a very dangerous thing.

Rob


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## randomkiller (Sep 28, 2007)

micromind said:


> As stated above, I'd bet just about anything that the guys who say you can share neutrals with only two hots have little or no experience with 3 phase systems.
> 
> Rob


I think when I read 120/240 I had single phase in mind even though the op had stated three phase.


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