MWBC

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domnic

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Electrical Contractor
Can I run a mwbc from fuse box for two 20 amp outlets ? ( by code )
 
domnic said:
Can I run a mwbc from fuse box for two 20 amp outlets ? ( by code )
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Sure. There are some caveats: MWBC neutral cant splice on a device, handle tie/double pole breaker required, Use to be a deal breaker in dwellings where AFCI's are required, however everyone seems to have two poles now that cost the same as 2 single poles...
 
ActionDave said:
By code you need a way to simultaneously disconnect both ungrounded conductors where they originate. A case could be made for using a double pole switch next to the fuse box to satisfy 210.4(B).
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I don't even think it is a 'case' I think it is a for sure you can do that.

It blows my mind that some insist it has to be a breaker. :)
 
iwire said:
I don't even think it is a 'case' I think it is a for sure you can do that.

It blows my mind that some insist it has to be a breaker. :)
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How does a two-pole switch satisfy the condition of disconnecting at the point where the branch circuit originates? Under Code, a MWBC circuit originates at the terminals of the branch-circuit OCPD(s) and the grounded conductor terminal bus.
 
Smart $ said:
How does a two-pole switch satisfy the condition of disconnecting at the point where the branch circuit originates? Under Code, a MWBC circuit originates at the terminals of the branch-circuit OCPD(s) and the grounded conductor terminal bus.
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Yeah OK. :lol:

You are free to live in the world linguists I will live in the world of electricians and commonsense. :cool:

You really believe the CMPs intention was to prevent the use of MWBCs with fuses? I find that notion absurd.
 
iwire said:
Yeah OK. :lol:

You are free to live in the world linguists I will live in the world of electricians and commonsense. :cool:

You really believe the CMPs intention was to prevent the use of MWBCs with fuses? I find that notion absurd.
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I live in the same world as everyone else whether you REAL-ize it or not. ;)

I discovered I am not a 100%-accurate mind reader way back when... so I am not going to claim I know what the CMP's intent is. I do however attempt to see the logic or lack thereof in these matters. Your suggested use when the ocpd's are panelboard fuses has merit...

...but that's all it has because it's just not written into the requirement or an exception thereto. :eek:hmy:
 
I see nothing in the code that would prohibit fuses from serving a MWBC. But I do not know, from a practical installation point of view, how to satisfy the rule about simultaneous disconnection. Let me echo Smart $'s question from post 5. Can anyone speak to that?
 
Who knows what the CMP thinks on this topic.:huh:

The statement in this 2011 proposal could be read in more than one way IMO.

2-32 Log #598 NEC-P02
Final Action: Reject
(210.4(B))
_______________________________________________________________
Submitter:
Herbert S. Pharo, Cape May, NJ
Recommendation:
Revise text as follows:
Each multiwire branch circuit shall be provided with a means that will
simultaneuosly disconnect all ungrounded conductors (Insert= nearest) ( Delete=at) the point
where the branch circuit originates.

Substantiation:
The problem is that the point where the branch circuit
originates is at the overcurrent protective device. Nearest as used in 230.70(A)
(1). The new word will make it clear that the disconnect need not be overcurrent
protective device.

Panel Meeting Action: Reject
Panel Statement:
The term “nearest” is vague and unenforceable. The present
language is clear and enforceable by the authority having jurisdiction.
Number Eligible to Vote: 12
Ballot Results:
Affirmative: 12
 
Smart $ said:
He can run a MWBC from a fuse box...


...if the fuse box is a 2- or 3-pole fused disconnect.
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Is the fused disconnect as a whole considered to be OCPD or just to contain the fuses which are the actual OCPD?
With a breaker the OCPD contacts and the disconnect contacts are one and the same. :angel:
 
GoldDigger said:
Is the fused disconnect as a whole considered to be OCPD or just to contain the fuses which are the actual OCPD?
With a breaker the OCPD contacts and the disconnect contacts are one and the same. :angel:
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I believe Code considers a fused disconnect as an integral unit consisting of a switch and fuse-type OCPDs. Under relevant sections, each is considered separately. A breaker is considered without differentiation for the very reason you stated.
 
Smart $ said:
I believe Code considers a fused disconnect as an integral unit consisting of a switch and fuse-type OCPDs. Under relevant sections, each is considered separately. A breaker is considered without differentiation for the very reason you stated.
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I just realized the other major difference between a fused disconnect and a disconnect after a fuse box:

In the fused disconnect the switch is normally on the line side of the fuse, so that the load wires connect directly to the OCPD terminals with nothing other than some bus and terminals in between.
 
Smart $ said:
He can run a MWBC from a fuse box...


...if the fuse box is a 2- or 3-pole fused disconnect.
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Or (and I know this will be tough for you to wrap your head around :p ) the AHJ does not interpret this section in the same way you do.

I know ... how could that be? Don't they know that they should be calling you first to tell them how it is. :D
 
charlie b said:
I see nothing in the code that would prohibit fuses from serving a MWBC. But I do not know, from a practical installation point of view, how to satisfy the rule about simultaneous disconnection. Let me echo Smart $'s question from post 5. Can anyone speak to that?
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I will give my opinion on it.

Smart is interpreting the rule in one way but not the only way.

It is up to the AHJ to interpret the code sections.
 
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