Multiwire branch circuit question

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GoldDigger said:
In addition to the obvious reduction in number of CCCs counted for derating, there is a quite significant reduction in voltage drop on a balanced circuit.
With two circuits and two neutrals, the VD on. 120V circuit is twice the drop in the hot conductor.
With an MWBC the voltage drop on each circuit is just that of its own hot wire.
An earlier post stated that the MWBC is more efficient, and this difference in voltage drop is where that comes from.
Cost is also lower for less copper.

mobile
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I wonder if the Energy Code crusaders are going to catch on to this and start promoting MWBCs.
 
GoldDigger said:
In addition to the obvious reduction in number of CCCs counted for derating, there is a quite significant reduction in voltage drop on a balanced circuit.
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The long term operating cost is lower, too. Lower voltage drop amounts to a lower I²R loss.
 
ActionDave said:
I hate that too. I hate the code change that brought it about. I hate 210.4(B) so much it is one of the only code references I know by heart. Over a century of safe wiring practice hobbled up by a useless change.



That sounds like a load problem not a circuit problem.
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Partly that too. We ran some new ckts to lighten loads but in some cases 1 ckt was ok but tripped when one her was overloaded. Machines & coolers had been moved around over time and loads changed. Fortunately no lost neutrals and burned out equipment. /


I know mwbc's do not al aye cause problems but I have seen enough problems not to like them. Then when code required double pole and triple pole breakers I liked them even less. I avoid them as much as possible.
 
jmellc said:
Partly that too. We ran some new ckts to lighten loads but in some cases 1 ckt was ok but tripped when one her was overloaded. Machines & coolers had been moved around over time and loads changed. Fortunately no lost neutrals and burned out equipment. /


I know mwbc's do not al aye cause problems but I have seen enough problems not to like them. Then when code required double pole and triple pole breakers I liked them even less. I avoid them as much as possible.
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Services and feeders are almost always multiwire circuits.
 
jmellc said:
Partly that too. We ran some new ckts to lighten loads but in some cases 1 ckt was ok but tripped when one her was overloaded. Machines & coolers had been moved around over time and loads changed. Fortunately no lost neutrals and burned out equipment. /


I know mwbc's do not al aye cause problems but I have seen enough problems not to like them. Then when code required double pole and triple pole breakers I liked them even less. I avoid them as much as possible.
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jmellc said:
There is no advantage to doing this, except maybe saving the cost of a 2nd neutral conductor, or not exceeding conduit fill.
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You need to look into the performance of the circuit.

Not worth doing if you don't have to.
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In the large scheme of things it is an energy saver.

3wire-modified.GIF


Roger
 
jmellc said:
I know mwbc's do not al aye cause problems but I have seen enough problems not to like them. Then when code required double pole and triple pole breakers I liked them even less. I avoid them as much as possible.
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Your soul is light and will probably go to heaven. My soul is heavy from practicing the dark art of MWBC wiring and may be iredeemable for I have only respect for those that taught me to use them and no remorse for continuing to install them.

The number of MWBC circuits in service operating trouble free today has to be in the hundreds of millions, any problems with them are either overstated paranoia or under qualified people working on them.
 
ActionDave said:
Your soul is light and will probably go to heaven. My soul is heavy from practicing the dark art of MWBC wiring and may be iredeemable for I have only respect for those that taught me to use them and no remorse for continuing to install them.

The number of MWBC circuits in service operating trouble free today has to be in the hundreds of millions, any problems with them are either overstated paranoia or under qualified people working on them.
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:thumbsup:

It could be billions, when you factor in that every service in this country is technically a MWBC.
 
ActionDave said:
Your soul is light and will probably go to heaven. My soul is heavy from practicing the dark art of MWBC wiring and may be iredeemable for I have only respect for those that taught me to use them and no remorse for continuing to install them.
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:D:D


ActionDave said:
The number of MWBC circuits in service operating trouble free today has to be in the hundreds of millions, any problems with them are either overstated paranoia or under qualified people working on them.
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or miswires. I do not know what code cycle required MWBC to have all ungrounded disconnected by the same breaker, but having been smashed once by a neutral on a MWBC that did not have all ungrounded disconnected by the breaker, I'd say there is a bit more hazard with an incorrect install.

When I write smashed, I mean lit up badly. Leaning on a metal building in July 16' off the ground, I dont want to know how low my body's resistance was. I was just very glad it was 120V and I wasnt leaning off the ladder, or I'd probably be typing this with software made for quadraplegics.
 
JFletcher said:
or miswires. I do not know what code cycle required MWBC to have all ungrounded disconnected by the same breaker, but having been smashed once by a neutral on a MWBC that did not have all ungrounded disconnected by the breaker, I'd say there is a bit more hazard with an incorrect install.

When I write smashed, I mean lit up badly. Leaning on a metal building in July 16' off the ground, I dont want to know how low my body's resistance was. I was just very glad it was 120V and I wasnt leaning off the ladder, or I'd probably be typing this with software made for quadraplegics.
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210.4(4)(B) 2008 NEC. I know it well because I hate it with a fervor that is usually reserved for ex wives and tax collectors. We have had MWBC since the 1800s and they are perfectly safe and reliable without a common disconnect.

You have my sympathy and full appreciation that you got away from a bad situation in good shape. We have all been there. Sometimes it's the wires that zap us, sometimes it's just a ladder that slips and brings one crashing to the ground. The thing is unless you were dealing with a bootleg neutral then what happened to you was not an incorrect install and entirely preventable.
 
iwire said:
Surely you seen problems with two wire circuits?

Did those problems cause you to stop installing those as well?
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Of course I have seen problems with 2 wire ckts. But they seldom caused problems on other ckts. A mwbc with critical loads puts a lot at risk. If 1 machine has problems and trips a breaker, it kills the other ckt too, now that we are required to use double pole breakers, or 3 phase, in which it kills 2 other circuits. A disaster in a science lab with coolers and incubators containing long term experiments.
 
jmellc said:
Of course I have seen problems with 2 wire ckts. But they seldom caused problems on other ckts. A mwbc with critical loads puts a lot at risk. If 1 machine has problems and trips a breaker, it kills the other ckt too, now that we are required to use double pole breakers, or 3 phase, in which it kills 2 other circuits. A disaster in a science lab with coolers and incubators containing long term experiments.
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Yes but the association of breakers with circuits is a design decision. It in no way demotes MWBC. We do not ban all rail traffic because some conflicts can arise.
 
I see problems nowadays with mwbc's and don't use them if I have a choice. If you like them go for it.
 
jmellc said:
I see problems nowadays with mwbc's and don't use them if I have a choice. If you like them go for it.
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The problems nowadays is handle tie rules and AFCI requirements and some lack of AFCI breakers that can operate on multipoles.

Loose neutral issues always was going to be a problem when it happens. Services and feeders typically are multiwire circuits and have the same problem if the neutral is compromised.
 
this code change (2008) before 3-pole handle ties were available increased installed costs in commercial wiring in a big way. It still does. none of the suppliers stock handle ties, especially 3-pole. I just ordered 30 3-pole Siemens handle ties and they came with lock/tag out attachment and were just under $4. other jobs I pulled separate neutrals. others I cheated.

In a existing installation remodel job we're doing my co-worker blew up 6 led drivers when he disconnected that shared neutral in a 3-phase MWBC and he said we were lucky.
 
farmantenna said:
In a existing installation remodel job we're doing my co-worker blew up 6 led drivers when he disconnected that shared neutral in a 3-phase MWBC and he said we were lucky.
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burning something up when disconnecting a neutral is really the best way I have seen to make people learn about that situation. You can tell them all you want about what might happen, but until they see it first hand, it just doesn't soak in.

I still cringe once in a while when disconnecting a neutral conductor when I thought it was unloaded, and then see it arc when I disconnect it. First thought usually is what did I just burn up that is sharing this conductor?

Was changing out a panelboard one time, incoming feeder was shut off and still I had an arc when I disconnected a neutral - corridor lights did go out, so they were obviously somehow tied on to a neutral from a different panel then where the branch circuit originated.
 
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