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09-08-2009, 08:50 PM
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#1
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Junior Member
Join Date: Sep 2009
Location: Illinois
Posts: 3
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multiwire branch circuit
In a conduit with 3-MWBC [multiwire branches circuit] (balanced 120 or 277-volt loads on 3-phase circuits serving lighting). Do I have a derating problem.
Some people say I'm wrong. Am I thinking right?
H = Hots N = Neutrals CCC = Current Carrying Conductors MWBC = # of Multi-wire Branch Circuits
3-H + 0-N = 3-CCC x 3-MWBC = 9-CCC 70% DERATING
2-H + 1-N = 3-CCC x 3-MWBC = 9-CCC 70% DERATING
1-H + 1-N = 2-CCC x 3-MWBC = 6-CCC 80% DERATING
Last edited by jjdh; 09-08-2009 at 08:57 PM.
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09-08-2009, 08:58 PM
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#2
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Moderator
Join Date: Jan 2007
Location: NY State
Posts: 3,346
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Well, if that is all you have in the pipe then you are fine, assuming #12 wire.
Three 3-phase MWBCs is nine CCCs.
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09-08-2009, 09:59 PM
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#3
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Unlimited Lic.Electrician
Join Date: Sep 2008
Location: Charlotte N.C.
Posts: 7,725
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We had this debate at work the other day. The debate was about are neutrals CCC? I think they are,but the Project Mananger says they are not.
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09-08-2009, 10:02 PM
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#4
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Seen your member
Join Date: Sep 2007
Location: Cornpatch USA
Posts: 9,935
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Quote:
Originally Posted by william1978
We had this debate at work the other day. The debate was about are neutrals CCC? I think they are,but the Project Mananger says they are not.
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Some are, some aren't.
Check out 310.15(B)(4)(a) - (c).
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09-08-2009, 10:55 PM
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#5
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Senior Member
Join Date: Oct 2007
Location: NC
Posts: 1,526
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Quote:
Originally Posted by william1978
We had this debate at work the other day. The debate was about are neutrals CCC? I think they are,but the Project Mananger says they are not.
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If the load is balanced it's not. If the load is not balanced it is. But seriously, how often is the load really balanced? As an inspector if you questioned the conduit fill and the electrician said the neutrals were balanced how would they go about proving that to you?
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09-08-2009, 11:38 PM
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#6
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Senior Member
Join Date: Sep 2009
Location: USA
Posts: 165
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Quote:
Originally Posted by gilbequick
If the load is balanced it's not. If the load is not balanced it is. But seriously, how often is the load really balanced? As an inspector if you questioned the conduit fill and the electrician said the neutrals were balanced how would they go about proving that to you?
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Unswitched lighting?
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09-09-2009, 12:20 AM
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#7
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Seen your member
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Location: Cornpatch USA
Posts: 9,935
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Quote:
Originally Posted by BadSplice
Unswitched lighting?
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Until one light goes out. 
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09-09-2009, 12:25 AM
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#8
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Senior Member
Join Date: Sep 2009
Location: USA
Posts: 165
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Quote:
Originally Posted by 480sparky
Until one light goes out. |
That won't hold up. That's like an inspector failing you because your ground rod impedance might go up if it doesn't rain as much next year.
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09-09-2009, 12:35 AM
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#9
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Seen your member
Join Date: Sep 2007
Location: Cornpatch USA
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Quote:
Originally Posted by BadSplice
That won't hold up. That's like an inspector failing you because your ground rod impedance might go up if it doesn't rain as much next year.
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So one light goes out right before the inspector shows up.
FAIL.
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09-09-2009, 12:41 AM
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#10
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Senior Member
Join Date: Sep 2009
Location: USA
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Quote:
Originally Posted by 480sparky
So one light goes out right before the inspector shows up.
FAIL.
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Never happen! 
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09-09-2009, 12:53 AM
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#11
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Seen your member
Join Date: Sep 2007
Location: Cornpatch USA
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Quote:
Originally Posted by BadSplice
Never happen! |
I'm glad you live in a perfect world where you never get a defective lamp or luminaire. 
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09-09-2009, 02:13 AM
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#12
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Not Peter D
Join Date: Apr 2009
Location: South Eastern MA
Posts: 2,882
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Quote:
Originally Posted by gilbequick
If the load is balanced it's not. If the load is not balanced it is. But seriously, how often is the load really balanced? As an inspector if you questioned the conduit fill and the electrician said the neutrals were balanced how would they go about proving that to you?
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The balancing has nothing to do with the NEC not counting the neutral as a current carrying conductor.
The neutral can be carrying current all the time and still not need to be counted as a current carrying conductor. As Ken pointed out, check out 310.15(B)(4)(a) - (c).
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09-09-2009, 10:18 AM
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#13
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Senior Member
Join Date: Sep 2009
Location: USA
Posts: 165
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Quote:
Originally Posted by 480sparky
I'm glad you live in a perfect world where you never get a defective lamp or luminaire. |
Jealous?
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09-09-2009, 01:31 PM
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#14
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Licensed Pro
Join Date: Jan 2007
Location: Rahway, NJ
Posts: 2,737
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I predict as soon as the electrician understands derating factors and what is and what is not a current carry conductor, that electrician moves up to another level. You need to fully understand derating and CCC's before moving on to motors and motor calculations. Buy some Mike Holt books, you'll get it.
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09-09-2009, 10:11 PM
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#15
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Senior Member
Join Date: Feb 2008
Location: Atlanta, Ga/Hamilton, Al
Posts: 2,035
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The loads on an MWBC don't need to be balanced to discount the neutral, only non-inductive. The reason is, if only one circuit is loaded, then only that wire and the neutral are carrying current. The other two wires are non-CC. If two circuits are loaded, then one wire is non-CC, and the neutral only carries the largest current of the two loaded circuits. If all three circuits are loaded, then the neutral is non-CC.
If all three are loaded differently, then the neutral will carry minimal current, but then so will the least loaded circuit(s). In other words, whether the loads are balanced or not, the heating effect is similar, so the neutral can be discounted as non current carrying.
Inductive loads are a different story according to the Code, so the neutrals are counted.
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09-10-2009, 01:26 AM
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#16
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Senior Member
Join Date: Nov 2007
Location: Yakima, Wash.
Posts: 101
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Quote:
Originally Posted by InPhase277
The loads on an MWBC don't need to be balanced to discount the neutral, only non-inductive. The reason is, if only one circuit is loaded, then only that wire and the neutral are carrying current. The other two wires are non-CC. If two circuits are loaded, then one wire is non-CC, and the neutral only carries the largest current of the two loaded circuits. If all three circuits are loaded, then the neutral is non-CC.
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Nice explanation, I hadn't formed a mental picture of that before, Thank you
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09-10-2009, 04:13 AM
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#17
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Not Peter D
Join Date: Apr 2009
Location: South Eastern MA
Posts: 2,882
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Quote:
Originally Posted by InPhase277
Inductive loads are a different story according to the Code, so the neutrals are counted.
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Not 'inductive loads' it's linear vs non-linear loads.
A motor would be a linear load.
A computers power supply would be a non-linear load.
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09-10-2009, 07:49 AM
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#18
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Senior Member
Join Date: Feb 2009
Location: us
Posts: 696
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Quote:
Originally Posted by InPhase277
The loads on an MWBC don't need to be balanced to discount the neutral, only non-inductive. The reason is, if only one circuit is loaded, then only that wire and the neutral are carrying current. The other two wires are non-CC. If two circuits are loaded, then one wire is non-CC, and the neutral only carries the largest current of the two loaded circuits. If all three circuits are loaded, then the neutral is non-CC.
If all three are loaded differently, then the neutral will carry minimal current, but then so will the least loaded circuit(s). In other words, whether the loads are balanced or not, the heating effect is similar, so the neutral can be discounted as non current carrying.
Inductive loads are a different story according to the Code, so the neutrals are counted.
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No, it carries the difference of the two circuits.
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09-10-2009, 04:33 PM
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#19
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Senior Member
Join Date: Feb 2008
Location: Atlanta, Ga/Hamilton, Al
Posts: 2,035
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Quote:
Originally Posted by mcclary's electrical
No, it carries the difference of the two circuits.
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That would be true for a single phase MWBC. We are talking about 3-phase.
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09-10-2009, 10:13 PM
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#20
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Senior Member
Join Date: Feb 2008
Location: Atlanta, Ga/Hamilton, Al
Posts: 2,035
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Quote:
Originally Posted by Bob Badger
Not 'inductive loads' it's linear vs non-linear loads.
A motor would be a linear load.
A computers power supply would be a non-linear load.
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You're right. I could have been a little more descriptive. Not all inductive loads are non-linear.
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