Timeline for answer to Testing and proper breakers for a multi wire branch circuit? by TDHofstetter
Current License: CC BY-SA 3.0
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| when toggle format | what | by | license | comment | |
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| May 19, 2016 at 23:25 | comment | added | Harper - Reinstate Monica | The requirement is for a disconnecting device which disconnects both sides of an MWBC together, to protect service personnel. There is no requirement that the sides trip together. So for instance if you used fuses, you would need to provide a gang switch that disconnects all poles in one motion. It would be weird if you had untied breakers and also a disconnecting switch. So I don't recommend that. | |
| May 19, 2016 at 23:21 | comment | added | Harper - Reinstate Monica | @TDHofstetter nothing requires matched breaker sizes on MWBCs. Matched wire sizes, perhaps, but you are always allowed to downsize a breaker. For instance if you convert an old dryer circuit to MWBC serving an RV via a NEMA 5-30, and a string of outlets in a garage (5-15 and 5-20). You must put a 20A breaker on the latter. It must be handle-tied with the 30A breaker unless you provide another disconnecting means. | |
| Aug 17, 2014 at 23:29 | comment | added | user24125 | The amp meter should show you the imbalance between the circuits only. | |
| Aug 17, 2014 at 23:08 | comment | added | TDHofstetter | @user24125 - I'm having a bit of trouble finding the specification for matched breaker sizes. I know I've run across it, though... and it makes sense. Since all (including common neutral) conductors serving an MWBC are required to be the same size, then the breakers serving those wires also need to be the same size to match the wire size. | |
| Aug 17, 2014 at 23:05 | comment | added | TDHofstetter | @Mazura - you can nearly always shuffle breakers around in the panel until you can get both MWBC breakers adjacent to each other, then tie them together or swap out to a double-pole breaker. If you DON'T, what happens when some innocent electrician shuts off ONE breaker and unwires the neutral? That neutral will now be HOT through the load on the second branch. If she's got a dicky ticker, she might now be dead. Yep, an amp clamp will show current through a neutral IF the load is unbalanced. If balanced, no. | |
| Aug 17, 2014 at 21:34 | comment | added | Mazura | @mazura "not in tandem", I meant not in double pole breakers. Also not necessarily adjacent either. | |
| Aug 17, 2014 at 20:26 | comment | added | Mazura | What if they are improperly installed? I am dealing with many unidentified not in tandem, nor pigtailed, MWBCs. How to test if I'll regret disconnecting the neutral from an outlet? Will a clamp on amp meter show a reading over the neutral? | |
| Aug 17, 2014 at 9:03 | comment | added | user24125 | @user24125 can you show me the code reference that disallows a 20A and 30A linked if#10 AWG is used for all three conductors even though I have never done that. | |
| Aug 17, 2014 at 8:51 | comment | added | user24125 | The nec does say that the link needs to be listed for the purpose but yes I always felt that a 2 (or 3) pole is better. | |
| Aug 17, 2014 at 6:24 | comment | added | wallyk | It is also the case that a three phase MWBC must have a triple pole breaker ganged together. | |
| Aug 17, 2014 at 5:04 | history | answered | TDHofstetter | CC BY-SA 3.0 |