Really short MWBCs

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zbang

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Roughly 5346 miles from Earls Court
Need another opinion on this-

A couple of months ago, a group I work with had some panels installed for outdoor event use. This is a permanent install, not temporary. All of these have an even number of 20a GFCI receptacles and few 50a 2p RV-style receptacles . Pairs of 20a outlets were installed in quad boxes with in-use bubbles, mounted to the side of the panels. OK. No problem.

When I looked inside, every single 20a receptacle on the entire site (10 panels, about 60 outlets) were installed as 2 pole MWBCs! :mad: (I think this must have saved 20' of 12g solid per panel.)

IMHO this is, well, just stupid. At these events, you're fairly likely to trip a 20a circuit when some food vendor stacks all their coffee makers on one extension cord, and using 2-pole breakers will make it much more difficult to quickly troubleshoot things. Especially as each circuit may run a different vendor. It also leads to problems with the other vendors...

Am I off base here or were the installers either cheap, lazy, or just trying to make things difficult? (IMHO, the overall quality of install wasn't so hot, anyway.) If it were me, I'd just add the missing neutrals and remove the handle ties from the breakers (can't remember if they used them or 2p breakers), but I won't see this site for another 8 or 9 months.

Yes, I could have specified no MWBCs. On the other hand, I don't think I've ever seen anybody use them when the breaker-to-receptacle distance is less than 10'. Having the installation changed wasn't an option at the time. Most of the panels are 200a & 320a 120/240v SqD meter combos, with a couple of 200a 120/208v 3p ones.

Other than that I have to live with it... what do people think?
 
I may have done the same thing these guys did if you didn't specify otherwise.

BTW, your O may also be considered stupid by some.:D

But,
(IMHO, the overall quality of install wasn't so hot, anyway.)
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this may be another issue.

Roger
 
30 runs of #12 at about 10 feet is 300 feet of wire. That's a little more than a roll. I'd say they did a good job. 50 bucks saved.
 
I think it is a “non-problem.” You are not at risk of nuisance tripping. If a single circuit, a single 20 amp, two pole, MWBC serves two vendors, and if each vendor puts 20 amps worth of load on his or her receptacle, the circuit will not trip. That is because it is a 120/240 volt, 20 amp circuit, capable of supplying a total of 4800 VA. If you draw 20 amps on one receptacle, that’s 2400 VA. If you draw 20 amps on the other receptacle, that’s another 2400 VA. The total is not in excess of the rating of the circuit.

What actually happens here is that, during one half of the AC cycle, the 20 amps serving one receptacle wants to flow out from the source via its “hot wire,” go through its load, and return to the source via the neutral wire. At the same half cycle, the 20 amps serving the other receptacle wants to flow from the source via the neutral, go through the load, and return to the source via its “hot wire.” As a result, what you actually see is current leaving the source via one hot wire and returning via the other hot wire, and you see no current flowing through the neutral. Neither hot wire sees a current in excess of 20 amps, despite the fact that you have two receptacles each supplying a 20 amp load.
 
charlie b said:
What actually happens here is that, during one half of the AC cycle...........,
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I am going to assume the poster was talking about when one vendor does over load their circuit and it takes out both which is a guaranteed with a common trip breaker and quite possible with just handle ties.
 
peter d said:
If I had a piece of utilization equipment within the panel itself that could be fed with a MWBC, I would do that.
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I probably would too but I would have used single pole breakers. Now that I have to work under the 2008 I might have used all two wire circuits.

I certainly have nothing against MWBCs but given the use of the receptacles I can see the OPs point to some extent.
 
Interesting perspectives all around, which is why I asked :grin:.

iwire said:
I am going to assume the poster was talking about when one vendor does over load their circuit and it takes out both which is a guaranteed with a common trip breaker and quite possible with just handle ties.
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Exactly. The usual drill is that multiple vendors each request a single 20a circuit and one of them actually needs two (or three, or four). The offender gets chewed out by me*, told "don't connect anything else until I get back" (so I can run more cable over to them), I leave and get waylaid by someone, the vendor connects all their heating devices to the single circuit anyway, it trips. This takes out the lights of the vendor next to them, or possibly a couple of booths over. Usually this happens when I'm at least a quarter mile away and in the middle of something.

*nicely, after all, we provide a service to them, and sometimes there are free donuts :D

mdshunk said:
30 runs of #12 at about 10 feet is 300 feet of wire. That's a little more than a roll. I'd say they did a good job. 50 bucks saved.
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In reality here- less than half that, but if they used 2p breakers, they more than made up for the cost "savings". Last I looked two 1p QO breakers plus 5' of wire (plus labor) still costs less than one 2p breaker, albeit not much less.

Let me rephrase the question- in circumstances such as this, is there any reason TO wire them as MWBC instead of running a few extra wires? The cost savings really are minimal.
 
zbang said:
Let me rephrase the question- in circumstances such as this, is there any reason TO wire them as MWBC instead of running a few extra wires? The cost savings really are minimal.
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I am a huge fan of MWBCs but given your description about the only reason I would have to use them would be that I had a bunch of 2-pole 20s hanging around and I wanted to get rid of them.

In the end though it comes down to lack of specifications, if you wanted dedicated neutrals and breakers you should have said so. (Hind sight is always so clear. :smile: )
 
I think if you wanted 2 wire circuits, you should have specified them. Although I can certainly see your point, there is no reason for the installers to have wanted them.

After all, if the vendors are blowin the breakers, it is their "fault", not the electricians' . Hey, it's a festival... lighten up. :smile:
 
Is the OP clear in the details if this was a single phase or three phase installation? I didn't see that.
It would effect my response if they ran MWBC without a third ungrounded conductor and the associated 3P CB if it was in the panelboard.
 
I am assuming that the reason for the handle tie is to identify ( and de energize )all the phase conductors associated with the MWBC.

Couldn't there be some reasonable effective alternative ? Maybe something colored that would snap on the two breakers? Maybe some sort of tie that did not operate all the poles, but would identify them?

I can see good points on all sides of this. I think MWBC are good on more than one level, but I do think they should be well identified.
 
realolman said:
I am assuming that the reason for the handle tie is to identify ( and de energize )all the phase conductors associated with the MWBC.

Couldn't there be some reasonable effective alternative ? Maybe something colored that would snap on the two breakers? Maybe some sort of tie that did not operate all the poles, but would identify them?

I can see good points on all sides of this. I think MWBC are good on more than one level, but I do think they should be well identified.
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I use the legend or labels to identify breakers;)
 
It would be nice if one of the breakers tripped the other would stay on. Of course you would want the breakers that indicate they are tripped to help with trouble shooting.
 
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