Multiwire Branch Circuit?

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new_ee

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I have a question about 210.4 (2002).

If I have a 480V 3 phase system and 480V lighting fixtures is the following setup considered a multiwire branch circuit and code compliant?

I have a 3 pole breaker that feeds (3) 480V lighting circuits. The circuits are fed evenly across the three phases (with line to line circuits across two phases) to get the 480V. So the circuit has a conductor from each of the three poles of the 3 pole breaker and a single EGC. There is no neutral.

I guess I'm confused about 210.4(C) which requires multiwire branch circuits to only supply line to neutral loads. However, exception no. 2 seems to allow the line to line connections that I have.
 
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Why are you using 480V for lighting fixtures? It can be done, but it's dangerous for maintenance personel who might think it's a lower voltage.

Why not 277V?
 
new_ee said:
I have a question about 210.4 (2002).

There is no neutral.

I guess I'm confused about 210.4(C) which requires multiwire branch circuits to only supply line to neutral loads. However, exception no. 2 seems to allow the line to line connections that I have.
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I do not think you have one of these

Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.
 
By definition, a ?multiwire branch circuit? has a neutral. So your proposed installation does not comprise a MWBC. But that is not important. What you have described is an acceptable installation. I have used it before, for parking lot lighting I seem to recall.
 
new_ee said:
I have a question about 210.4 (2002).


I have a 3 pole breaker that feeds (3) 480V lighting circuits. The circuits are fed evenly across the three phases (with line to line circuits across two phases) to get the 480V. So the circuit has a conductor from each of the three poles of the 3 pole breaker and a single EGC. There is no neutral.

.
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What is the voltage to ground and have you considered 210.6??
 
cpal said:
What is the voltage to ground and have you considered 210.6??
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fixtures will be mounted higher than 22ft above fixed ground (even though the earth is in constant rotation).
 
charlie b said:
By definition, a “multiwire branch circuit” has a neutral. So your proposed installation does not comprise a MWBC. But that is not important. What you have described is an acceptable installation. I have used it before, for parking lot lighting I seem to recall.
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well thats what my dilemma is. i dont see anything wrong with it on the surface, but i want to make sure theres nothing that specifically outlaws it in the code. does this setup have a name or even a trade "nickname".

im also a little confused as to how to figure the load on each phase. say AB has 5000VA AC has 5000VA and BC has 3000VA. I will size the conductors the same, but do I have to add the current seen on AB and AC or BC and AB or AC and BC? Or since the phases are out of phase the maximum current seen across AB will not occur at the same time at the maximum current across AC?
 
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rr said:
Why are you using 480V for lighting fixtures? It can be done, but it's dangerous for maintenance personel who might think it's a lower voltage.

Why not 277V?
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I think to minimize voltage drop and thus minimize cost.
 
new_ee said:
I'm also a little confused as to how to figure the load on each phase. say AB has 5000VA AC has 5000VA and BC has 3000VA. I will size the conductors the same, but do I have to add the current seen on AB and AC or BC and AB or AC and BC?
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Step One is to try to balance the load as best you can. Step Two is to add the load (13,000 VA in your case), and treat it as if it were balanced. That is, divide 13,000 by 480 and divide again by 1.732, and you get 15.6 amps. Step Three is to size the conductors on the basis of 15.6 amps.
 
new_ee said:
Well that?s what my dilemma is.
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I think the solution to your dilemma is to recognize the purpose of a MWBC. The idea is to allow us to run more than one phase (ungrounded) conductor from the panel, but to use only one neutral (grounded) conductor to serve them all. This has nothing to do with using a single Equipment Grounding Conductor to serve all the loads, or using a separate EGC for each load.

Bottom line: You don?t need a neutral, and you don?t have a neutral, so you can?t call it a MWBC, but you don?t need to call it a MWBC.
 
We did it in the glass plant I worked in. All our high bay lights were wired this way. We had everything, some were 120,277 and 480. Depending on how long you work, you get to see all the new stuff thats out there. I worked 37 years so I got to see alot of it.
 
which voltage drop equation would i use? single or 3 phase? i would think single because the condutors are seeing the 15.6 A through two phases.


edit note: i stand corrected
 
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It is not technically correct to speak of ?current across two phases.? Current goes through a wire; voltage is imposed across two points.

This is a three phase system, so you use the three phase version of all calculations.
 
new_ee said:
im also a little confused as to how to figure the load on each phase. say AB has 5000VA AC has 5000VA and BC has 3000VA.
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Charlie B also described a load formula for unbalanced circuits, which use phase angles that don't cancil; (ie) 2 of 3 phases. But, even with different loading across those phases, I believe the balanced formula still applies to you, since your phase angles are expected to cancel evenly.
 
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ramsy said:
Charlie B also described a load formula for unbalanced circuits, which use phase angles that don't cancil; (ie) 2 of 3 phases. But, even with different loading across those phases, I believe the balanced formula still applies to you, since your phase angles are expected to cancel evenly.
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That's not how I see it.

I get, using my vector diagram technique:
  • A = 18.0A @ 0?
  • B = 14.6A @ -128.2?
  • C = 14.6A @ -231.8?

So where is it that hosts linkable images for free... since we no longer can attach them???
 
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