3 phase 4 wire panel wiring

[Navitech]

I need to install an autotransformer lighting controller for a 120 VAC, 208 VAC and 277 VAC lighting circuits. Each controller will handle a 16 amp measured load which means I can combine circuits to a max of 16 amps. In each panel there is a common neutral for 3 hot feeds. I was told that on a three phase 4 wire breaker panel you must combine same phases. Phase A to phase A, B to B, C to C. For example if breaker 1 is phase A, 3 is phase B, 5 is phase C you cannot combine these because they are using the same neutral. Why not? NEC sect 220-22 states that the neutral feeder must be rated for the highest unbalanced load. What is the problem with combining phases? I can't find anything on the subject.
If breaker 1,3,5 each have 9 amps then the load on the common neutral is 0.
If breaker 1 is 4 amps, 3 is 8 amps and 5 is 15 amps...why can't you combine 1 & 3 which is 12 amps when the neutral is rated for the highest un balanced load of 15 amps anyway?

 

[advidana]

If you combine any of phases together, you will have a dead short and maybe an explosion.

Forget the panel- think of the electrical system first from the perpective of the transformer that serves the panel board. If you connect any of the phases together- you are shorting secondary windings between phases, therefore a bolted fault. The neutral is the center winding of the phases. If you connect any of the phases to neutral- also a short.

Panel boards were invented as a convent place to to connect all the protection for wires connected to these transformers. They do not produce electricity like dc batteries.

caution:Autotransfomers boost connections are tricky, I suggest you get some direct assistance on this project with someone with autotranformer background.

 

[Navitech]

advidana,
Let me clarify:
Breaker 1 phase A is 4 amps.
Breaker 3 phase B is 8 amps.
Breaker 5 phase C is 15 amps.
These are 20 amp breakers.
All I want to do is remove the wires from 1 and 3, tie together and use breaker 1 to supply these two circuits. Breaker 3 is then blank. What is wrong with that? There is a common neutral for Breaker 1,3,5. In the ceiling I suppose the neutral is daisy chained somehow with 1 wire returning to the panel. I was told you need to be careful to not overload the common neutral when combining circuits. I'm trying to draw this out and cannot see any problems as long as you use the 16 amp rule and be sure the neutral is big enough. They say combine like phases...Breaker 1 phase A to breaker 7 phase A for example. But once I remove a wire from a breaker the circuit doesn't care what phase is supplying it. If I send the wire from breaker 1 phase A to breaker 9 phase B...breaker 9 is now supplying these two circuits with phase B. It's the same electrically as combining breaker 1 phase A with breaker 7 phase A.
Thank you

 

[rbulsara]

adviana:

I believe Navitech is talking about combining the neutrals, although he/she could have worded the question more accurately.

Navitech:

Assuming that you are talking about neutrals, your thinking technincally is not incorrect, but it is 'wise' to keep neutrals of each circuit separate. For example, what if somebody wants to add a couple of lights to a circuit in the future and you execeed the amapacity of a 'common' neutral. You are not saving much by not running a separate neutral and designing a poor system.

Meeting code is minimum saftey requirements, take pride in designing a good system rather than having abiltiy to meet the code to a bare bone or even challanging it.

(A motor wired to run backwards would also meet the Code as long as wiring is sized to Code and is grounded, but of no use! Would you throw a code book at the Owner then and tell him that you met the code so he has no right to complain).

By the way NEC 220-22 is for 'feeders and services' and not for the branch circuits you are talking about.

Also make sure that you have adjusted currents to the primary (supply) voltage of the autotransformers.

 

[advidana]

That is different than what you first wrote. If you were designing this load for the first time and you connected the a single phase loads of 4 amps and 8 amps to cb circuit 1 then it would be notbe any different that if your loads on circuit 3 is transfer to circuit 1 only if they are are truly single phase loads. Then it would be ok. But if the loads are part of a 3 phase load like a motor or 3 phase transformer, or other equipment, or part of a single phase 208 volt load, then no.

 

[advidana]

My first reply was a little rough. But some people on this
tread has questions about electrical problems and they don't know the difference between an ohm or a volt.
But if they are asking questions they need help, so I do the best I can.

I don't know why the people you have talked to do not explaining thing more clearly. The problem you are addressing is real common during rework.

 

[Navitech]

Avidana,
Thanks...no problem. I think I just sorted it out. The person that told me about combining circuits of like phases was referring to a 208 VAC circuit. That is when you need to combine like phases as you mentioned. I also have a 120VAC and 277VAC lighting circuits. In these cases (correct me if I'm mistaken) it does not matter if you combine unlike phases. I know it's wise to keep common neutrals of each group of circuits separate. But if you don't...does it cause a problem? When you combine circuits you have a spare breaker. Then if someone later needs that spare they would need to run a neutral. Yes this is an upgrade project and I need to combine when I can to minimize equipment installation.

 

[jbartos]

Suggestion: Sometimes, it is better to draw a sketch. The neutral might be overloaded if the neutral serves two combined loads (e.g. by connecting two conductors together and connecting these two conductors to a higher rated breaker of fuse). However, the two phase conductors will then be also overloaded. Normally, if you connect two phase conductors and protect them by one of those conductors protective device, the circuit protection shall be adequate and neutrals having the same sizes should not be overloaded. Also, neutral might be overloaded if different phase conductors share one neutral of size of each phase conductor. This is especially the case if there are harmonics originated from nonlinear loads.

 

[advidana]

It ok to transfer loads if you know what you are doing but look out for harmonics on lighting circuits. Check your neutral currents after you have done your rework to see that they are within NEC requires . Use a true RMS meter if possible .Otherwise with a regulator clamp on meter allow for less current than allow by the code. Maybe 50 percent if all phases are balanced or pull a larger neutral wire.

 

[margaret695]

Navitech: Is the existing wiring colour coded in relation to the phases?

If so, some electrician in the future could possible be confused if you combine two circuits that were originally on phase A and phase B into one circuit on phase A. Especially in the case if the wiring was done with circuits 1, 3 and 5 sharing the same neutral in the same conduit.

Also you should check if the circuit numbers were labelled at the lighting fixtures, or junction boxes, etc., and change them to suit the new arrangement.

Margaret