Multi-wire circuits

Wasn’t too hard to see if these multi-wire circuits shared the same bus bar:shock:

Ward 210.jpg

Not to mention the 408.21 violation in the BACK GROUND !

Charles, that looks like it was intended to be a multiwire circuit but with both sides on the same breaker it is now only one circuit. In this setup there would be no danger of overheating the neutral since it is limited by the one breaker. Of course the double tap is still a violation.

Paul, do you think if the two were nutted together with one pigtail to the breaker it would be compliant? Or would it constitute paralleled conductors?

but Brian…are you saying IF it was a multi-wire circuit…and sharing a neutral and done in that fashion that it is OK?

is it Paralleled to a single device…Article 310.4 would have to be met…

IE: brian my reference was to 408.21 in the back ground…I did not call it a multiwire circuit…for clarrification meaning…he did not mention the issue RIGHT behind his concern…

lol brian…forgive me…it is early…are you asking me what i would think if they were connected together?

Considering if the two go to different devices…and they share a neutral…I am not overly concerned if tied together…in a splice as it would just be one large circuit…lol…chances are it would throw the OCPD if needed anyway as long as the OCPD is sized accordingly to the conductors.

This application is no different…just other violations involved…

Now are they parralleled…well considering they are under 1/0 and probably do not meet the other requirements of 310.4…which I do not think they meet.

The issue of double tapping is compounded if this is truly a multi-wire circuit where a neutral is being shared…so it happens to be on the same OCPD…and should limit it lets say to 15A…since we can’t see the breaker rating…however is it a problem if one conductor feeds a 8 A load and the other feeds a 8A load…on the same sin.wave…do we not have the possible load on the neutral as 16A…could it be a concern???

Are we relying on the OCPD to protect the entire function…their is much understanding in the design of installations…and think that is where the problem lies…

Great points guys!

Additional info about multiwire circuits I posted many months ago, if in FACT we are speaking of a Multiwire circuit…lol…that has not really been established yet…lol…enjoy Charles…:slight_smile:

Consider a three-conductor cable (red, black, and white) that originates at two circuit breakers which are connected to opposite poles, and the neutral buss bar. One can connect a 120-volt load between the red and white wires, placing that load on one of the circuit breakers, and one can connect another 120-volt load to the black and white wires to place it on the other circuit breaker.

The breakers, of course, are sized appropriately to protect the wiring. But what about the current in the white wire? The white wire is the return for both loads. Won’t it be carrying more than its rated current?

The answer is that the neutral does not carry the sum of the two currents; it carries the difference. If both loads draw the same amount of current, the neutral will carry no current. The voltage on one “hot” wire is always of opposite polarity to the voltage on the other “hot” wire. Therefore, the current returning by way of the white wire for each circuit flows in the opposite direction to the current for the other circuit, so that the two currents subtract, and the white wire can never carry more current than one of the two “hot” wires. This technique is called a multiwire circuit.

Beware, however, of connecting to the wrong breaker. If the two breakers supplying the two “hot” wires are connected to the same pole, the voltage on both hot wires will be of the same polarity, and the current for each of the two circuits will return in the same direction through the white wire, and thus add together. The white wire may carry as much as twice the rating of the circuit so look for those browned white wires in those panels fella’s. This is a dangerous condition.

To check that a multiwire circuit is supplied correctly, measure the voltage between the two “hot” wires. If the two “hot” wires are connected to opposite poles, as they should be, there will be a difference of 240 volts between them. If they are incorrectly connected to the same pole, there will be zero (or nearly) volts between them.

There are some restrictions on the use of multiwire branch circuits, including these (2002 NEC 210.4):

  • “All conductors must originate from the same panelboard.”
  • If a multiwire circuit in a “dwelling unit” supplies “split-wired” receptacles, a means must be provided to disconnect both ungrounded conductors simultaneously. This can be done with a two-pole switch, but usually it is done by tying the handles of the two circuit breakers together.
    *]The same simultaneous disconnection requirement also exists if the circuit supplies a mixture of 120v and 240v loads (whether or not in a “dwelling unit”), but the disconnection means must be the "branch circuit overcurrent.

Paul, it may have originally been intended as a multiwire circuit but it ended up a double tapped breaker (inappropriate at that):slight_smile:

lol…so it was a RED conductor from a different set of 3 wire NM cable…if that is the case…not a problem…other the double tap…:slight_smile:

But we learned about Multiwire circuits so it is all GOOD…:slight_smile:

Is that smoke coming out of that breaker?
Also the grounded conductors are double tapped at the bus bar.


we have called that out which is a violation of 408.21 in the NEC.

I dont see any smoke…however

How do you see this as a parallel conductor unless it was going to the same terminal of the device at the far end?

greg…???..I don’t see it as a Parallel conductor…OH wait…sorry…you were talking to brian…sorry.

I was thinking they would be paralled up to the point at which they split, but now that I think about it you are right they wouldn’t be supplying the same load(s), it would be no different than in a true multiwire as far as that goes. Thanks, these forums are great learning for everyone.:slight_smile: