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-   -   Is This MWBC Wiring Setup Legal? (http://www.diychatroom.com/f18/mwbc-wiring-setup-legal-84922/)

DenverBob 10-26-2010 10:20 AM

Is This MWBC Wiring Setup Legal?
 
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Hi All,

I just bought a 1925 house in Denver (city proper) with a basement that was finished relatively recently, I don't know exactly when.

The basement circuits are fed through conduit from the main panel on the rear of the house and into a junction box in the basement ceiling. The existing setup has multiple issues, including using a 4 11/16 square junction box for splicing together 22 12 AWG conductors plus ground.

My current problem is that the conduit is 3/4 in. It will be very difficult to increase the conduit size, and I can't derate all the circuits to 15A due to the boiler and sump pump loads, so I either need to replace all the wires with 10 AWG or run an MWBC for one of the circuits to get me down to 9 current carrying conductors in the conduit.

If I were to run an MWBC, is it ok to splice together multiple neutrals? (see the schematic) I can't think of an electrical reason why not, and I don't see it prohibited in the code.

Thanks!

frenchelectrican 10-26-2010 11:55 AM

Is your basement ceiling is finshed ?? if so which one ? drywall or supened ceiling ?

If latter it very easy to add new conduit and move the circuit over to this conduit.

The way the situation you have now it really crowed in the conduit and I did come up with figures to drop down and it still way too many in the conduit with your exsting size there.

The derating with your exsting set up now you only can get 40% out of what you can get on 20 amp circuit which it mean you only safely run 8 amps.,, I know you will say WHAT ?? but I am not joking on this one so I did change the numbers a little with revised with your plans and came up 17 conductors and still too many you only can have 10 amps per MWBC due the numbers of conductors in there.

As I mention above the best move is run other conduit along next to the exsting conduit that way you can actally reduce the numbers of conductor down and get it to more mangable level so you can use pretty good level without any issue.

Merci,
Marc

DenverBob 10-26-2010 01:00 PM

I apologize, I wasn't very clear in my description of the existing wiring.

There are 5 circuits feeding the basement, so if I didn't use an MWBC, I would have 10 current carrying conductors plus grounds in the 3/4 inch rigid conduit. The conduit comes down the exterior brick wall from the main panel, goes through 3 layers of brick into the basement ceiling, and connects to a junction box where the THHN in the conduit is spliced to 6 NM-B cables (4 circuits have one cable coming into the box, and one circuit has two)

The circuit with 2 nm cables coming into the junction box is one of the circuits I wish to replace with an MWBC.

So I'm ok on conduit fill and junction box fill (I'm replacing the old box with a 6x6x4) and if I use the MWBC, I derate 30A to 21A (actually 20A) and I'm ok on overcurrent protection with 3 SP 20A breakers and 1 DP 20A breaker. As long as I can use the MWBC I proposed.

Thanks!

frenchelectrican 10-26-2010 06:22 PM

Ok now I can undestand clear now what your indent with this set up once you go to MWBC you will have to use the two pole breakers or get a handle tie peice to tie the breaker together it is required after 2008 NEC code { I think Canada is simauir to this situation }

10 current carry conductors you will be ok but the grounding conductor in the conduit you only need one and it will be sized according the largest breaker you have in that conduit so it will be little more eaiser to deal with it.

Just becarefull with netural when you run in MWBC set up.

Merci.
Marc

AllanJ 10-26-2010 09:23 PM

Quote:

Originally Posted by DenverBob (Post 523202)
If I were to run an MWBC, is it ok to splice together multiple neutrals? (see the schematic) I can't think of an electrical reason why not, and I don't see it prohibited in the code.

The neutral and the hot wire(s) associated with it must run in the same conduit.

This includes from the junction box to each light fixture.

So a multiwire branch circuit can come to a junction box and various 2 wire (hot and neutral) subcircuits run from that junction box to the respective lights and receptacles. At the same time the multiwire branch circuit can daisy chain from that junction box to other junction boxes. All the neutrals from the various downstream branches (or subcircuits) whose hot wires are taken from the incoming MWBC are tied to the neutral of the incoming MWBC.

oleguy74 10-26-2010 09:40 PM

3/4 " conduit.pull all that wire out,run 3 #6 and 1 #8 for ground and set up a sub panel,job done.you can also use a #10 for ground.50 amp breaker in main panel.will free up some breaker space...in main panel.

DenverBob 10-26-2010 09:43 PM

Thanks AllanJ, that's exactly my situation! The MWBC will save me a lot of time and money pulling new wire!

DenverBob 10-26-2010 09:59 PM

Quote:

Originally Posted by oleguy74 (Post 523523)
3/4 " conduit.pull all that wire out,run 3 #6 and 1 #8 for ground and set up a sub panel,job done.you can also use a #10 for ground.50 amp breaker in main panel.will free up some breaker space...in main panel.

I thought of that too, but it would be quite difficult to mount and run two conduits to the sub without tearing out large chucks of drywall. It'd be cheaper and easier to run the #10!

oleguy74 10-26-2010 10:35 PM

the #6's with thwn will carry 65 amps.that should be plenty ...

AllanJ 10-27-2010 08:04 AM

Hmmm. Does a 120/240 volt MWBC count as three wires or two when determining how much you need to de-rate closely juxtaposed wires? The total current carried is no more than a regular (2 wire) 240 volt only circuit given that the neutral carries just the difference in current between the two hot wires.

If the MWBC counted as two wires, you could run two #10 MWBC's in the existing conduit for a total of four current carrying conductors, but one #6 MWBC seems to have more flexibility.

If you run a 10 gauge circuit, you still need a subpanel if you want to use heavier than 20 amp breakers.

For multiple branch circuits including multiple MWBC's coming into the same junction box, the neutrals for the loads on each circuit are connected only to the neutral for that respective circuit.

oleguy74 10-27-2010 11:36 AM

mwbc ckts the neutral counts beacause it is a current carring wire.


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