Multiwire Branch Circuits

[letgomywago]

Because you often have to work the circuit hot because you can't turn off a full boat of circuits. The labs I work in it's a nightmare to turn off one circuit let alone 3.

Why if this is the issue you can't just pull the 1 circuit off the breaker and put it back. I'd rather work 1 hot once than 10 devices or fixtures.

 

[kwired]

Don't I know it. But don't expect the guys who put both hots of an MWBC on the same tandem to remember that.

I don't even expect them to understand why they shouldn't put both both hots on the same tandem, some still won't understand even after you explain why.

 

[kwired]

I am absolutely opposed to making requirements that dumb down the trade to accommodate unqualified people doing work. Where does it end if you go with that philosophy?

so am I. I will say is probably a good design practice to group neutrals with their corresponding "hots", but probably has no business being a code requirement either. Handle tie? Maybe should at least revert back to being required if both "hots" land on the same device.

 

[electrofelon]

Why if this is the issue you can't just pull the 1 circuit off the breaker and put it back. I'd rather work 1 hot once than 10 devices or fixtures.

Yup that certainly works and I have done that at times.

 

[RAKocher]

I saw in a 30 year old office bldg where an area of about 4,000 sq ft was recently reconfigured, and the EC only reused about half of the existing receptacle circuit, black, red, blue & white thhn, homeruns in a conduit back to the panel. I then noticed that just about all of the unused circuits are phase 3's; about 6 of them. They didn't run anything new back to the panel, so how did they do that without overloading some neutrals?

 

[kwired]

I saw in a 30 year old office bldg where an area of about 4,000 sq ft was recently reconfigured, and the EC only reused about half of the existing receptacle circuit, black, red, blue & white thhn, homeruns in a conduit back to the panel. I then noticed that just about all of the unused circuits are phase 3's; about 6 of them. They didn't run anything new back to the panel, so how did they do that without overloading some neutrals?

Original was overkill?

Not sure I fully understand what you are asking though.

 

[ptonsparky]

I saw in a 30 year old office bldg where an area of about 4,000 sq ft was recently reconfigured, and the EC only reused about half of the existing receptacle circuit, black, red, blue & white thhn, homeruns in a conduit back to the panel. I then noticed that just about all of the unused circuits are phase 3's; about 6 of them. They didn't run anything new back to the panel, so how did they do that without overloading some neutrals?

Loading two of three phases on MWBCs does not overload a neutral. Either by accident or purposely.

 

[Dennis Alwon]

I saw in a 30 year old office bldg where an area of about 4,000 sq ft was recently reconfigured, and the EC only reused about half of the existing receptacle circuit, black, red, blue & white thhn, homeruns in a conduit back to the panel. I then noticed that just about all of the unused circuits are phase 3's; about 6 of them. They didn't run anything new back to the panel, so how did they do that without overloading some neutrals?

The one way to overload the neutral is if you share a neutral with 2 or 3 circuits and they are on the same phase. For instance a shared neutral and one circuit is on Phase A and the other circuit is on Phase A. If each circuit is on a different phase then the neutral is fine.

 

[kwired]

Loading two of three phases on MWBCs does not overload a neutral. Either by accident or purposely.

It will make the neutral count as a current carrying conductor though for ampacity adjustment reasons, but at same time if you lost one ungrounded out of every set in a raceway you reduced the number by same quantity as you increased it so still have same overall adjustment factor.

 

[wwhitney]

It will make the neutral count as a current carrying conductor though for ampacity adjustment reasons

Loading only 2 of 3 ungrounded conductors doesn't do that. But disconnecting one ungrounded conductor from the supply does do that. And then if you leave that conductor in the conduit as a spare, you have to count it as a CCC as well. So that increases the number of CCCs for ampacity adjustment.

Cheers, Wayne

 

[kwired]

Loading only 2 of 3 ungrounded conductors doesn't do that. But disconnecting one ungrounded conductor from the supply does do that. And then if you leave that conductor in the conduit as a spare, you have to count it as a CCC as well. So that increases the number of CCCs for ampacity adjustment.

Cheers, Wayne

with a 3 phase 4 wire wye supply, a "full boat" MWBC, and all loads being linear loads as well as line to neutral, disconnecting one ungrounded makes the neutral go from non current carrying to current carrying for the sake of ampacity adjustment reasons. But either way there is three current carrying conductors when it comes to ampacity adjustments.

If you have line to line only loads and take away one ungrounded conductor then you go from three to two current carrying conductors, but at same time you had no neutral to apply so whether it would be current carrying or not doesn't matter, it's not there or is not used if it is there. How it would be counted if it is there? Good question, will depend on how it is used when and if it is used I would think. Could be used with one or two of the ungrounded and wouldn't count as current carrying, or could be used with all three ungrounded and would not count as current carrying. Again either way with an individual "full boat" you end up with a maximum of 3 CCC's.

 

[wwhitney]

with a 3 phase 4 wire wye supply, a "full boat" MWBC, and all loads being linear loads as well as line to neutral, disconnecting one ungrounded makes the neutral go from non current carrying to current carrying for the sake of ampacity adjustment reasons. But either way there is three current carrying conductors when it comes to ampacity adjustments.

That's not correct as far as the NEC. Table 310.15(B)(3)(a) footnote states that "spare conductors" need to be counted.

So if you a 4 wire wye circuit hooked up to a 3 pole breaker, that's 3 CCCs. If you now disconnect one wire everywhere in the circuit, but leave it in your conduit, you have 1 spare plus a circuit with 3 CCCs, so the CCC count is 4.

Counterintuitive, but that's what it says.

Cheers, Wayne

 

[kwired]

That's not correct as far as the NEC. Table 310.15(B)(3)(a) footnote states that "spare conductors" need to be counted.

So if you a 4 wire wye circuit hooked up to a 3 pole breaker, that's 3 CCCs. If you now disconnect one wire everywhere in the circuit, but leave it in your conduit, you have 1 spare plus a circuit with 3 CCCs, so the CCC count is 4.

Counterintuitive, but that's what it says.

Cheers, Wayne

If you have 4 conductors total one of them being the grounded conductor, how can you ever get a combination of 4 CCC's? other than situations with high levels of non linear loads?

Disconnect one the ungrounded and the grounded becomes a CCC. Reconnect the grounded and the grounded becomes non CCC. Connect two of the ungrounded to same phase- now you've maybe created an additional problem and potentially can overload the grounded conductor

 

[ESolar]

The work was 1994, before the 2008 change. As this conversation reveals, even experienced ECs are doing things that are questionable or wrong (e.g., mixing neutrals). And sure, if you open a box and find a red wire, and you flipped one breaker, you know that you might have a problem. But then you're into the hot box, and you can't always tell what you got without pulling out the wires. Plus, what about down the line, e.g., receptacles on one conductor, where it's just black wires? Relying on the presence of a red wire is not an answer (a detector works). And if you want to work with half the circuit hot for some sort of trouble shooting, pull the tie, and replace it afterwards. I like the tie because even if you know what you are doing, mistakes happen - like overlooking that it is a MWBC and wiring on the same phase. And it's not a lot of work to locate breakers adjacent. Plus, most homeowners are not calling in an electrician to replace a receptacle or a switch. That's a simple task that most people can DIY. I guess I'm saying - I like the 2008 code change - it makes sense - which is why it's there. And actually, I think that when you get into an older panel, and it's not a lot of trouble, it should be rearranged to comply with that.

 

[RAKocher]

Original was overkill?

Not sure I fully understand what you are asking though.

The original probably was an overkill and true they didn't necessarily overload any neutrals if just a bunch of phase 1's, 2's & neutrals were used, but I was hoping to get 3 MWBC's from the panel for 3 groupings of 8 cubicles each that are factory wired with a common neutral, so except for the 6 or so spare phase 3's the panel's full.

 

[wwhitney]

If you have 4 conductors total one of them being the grounded conductor, how can you ever get a combination of 4 CCC's? other than situations with high levels of non linear loads?

You are missing that spare conductors (not connected to anything) count as a CCC, per the citation I gave.

If you have 4 conductors in a conduit on a 3 phase 4 wire wye supply system, the only way they count as just 3 CCCs is if they are ABCN on a single circuit. Any other combination will count as 4 CCCs. Use S for spare, ABNS, AABN, etc. are all 4 CCCs.

Also, if you have two circuits, one A-N, and one B-C, that counts at 4 CCCs. That means you can't tell the number of CCCs just by looking at the panelboard end of the circuit (that could be supplied by one 3 pole breaker), you need to know the arrangement of the loads on the circuit to determine the number of CCCs.

Cheers, Wayne

 

[kwired]

You are missing that spare conductors (not connected to anything) count as a CCC, as per the citation I gave.

If you have 4 conductors in a conduit on a 3 phase 4 wire wye supply system, the only way they count as just 3 CCCs is if they are ABCN on a single circuit. Any other combination will count as 4 CCCs. Use S for spare, ABNS, AABN, etc.

Also, if you have two circuits, one A-N, and one B-C, that counts at 4 CCCs. That means you can't tell the number of CCCs just by looking at the panelboard end of the circuit (that could be supplied by one 3 pole breaker), you need to know the arrangement of the loads on the circuit to determine the number of CCCs.

Cheers, Wayne

The line to line load can make for a 4th CCC as you mentioned, I also did bring that up. But for line to neutral only situations which I maybe didn't make clear enough but is what I was referring to, as well as all being linear loads - you can't get 4 CCC's without also committing other violations. Connecting A,A, B and N would give you 4 CCC's but also gives you a potentially overloaded N, you would have to connect that spare to C if and when you use it.

 

[jaggedben]

...

Also, if you have two circuits, one A-N, and one B-C, that counts at 4 CCCs. That means you can't tell the number of CCCs just by looking at the panelboard end of the circuit (that could be supplied by one 3 pole breaker), you need to know the arrangement of the loads on the circuit to determine the number of CCCs.

That arrangement would be a violation on a 3-pole breaker, I'm pretty sure. It should be on a 1-pole and a 2-pole, and the wires need to be grouped separately at each j-box they pass through.

To the larger point, in practice we rely very much on what we see in the panelboard to know which wires belong to which circuit. Which is why I'm generally supportive of the code rules requiring grouping.

 

[wwhitney]

That arrangement would be a violation on a 3-pole breaker, I'm pretty sure.

Citation? I don't think there is one.

Cheers, Wayne

 

[kwired]

That arrangement would be a violation on a 3-pole breaker, I'm pretty sure. It should be on a 1-pole and a 2-pole, and the wires need to be grouped separately at each j-box they pass through.

To the larger point, in practice we rely very much on what we see in the panelboard to know which wires belong to which circuit. Which is why I'm generally supportive of the code rules requiring grouping.

Citation? I don't think there is one.

Cheers, Wayne

I don't either. Would/could potentially be 4 CCC's like you said though.

If supplying a single utilization equipment and that is how load is arranged within that equipment most would call this an individual multiwire circuit and probably figure the neutral is not a CCC ,though it kind of actually is.
Same thing would happen to a service or feeder if only one line to load and one line to neutral load were supplied by it, though we would normally consider the service/feeder neutral to not be a CCC in most cases.