Need help with multiwire branch circuit serving both 120V and 240V loads

[BarryO]

I guess a picture is worth a thousand words:

If an overcurrent condition trips the breaker on the black conductor, that would be OK if there were only line-to-neutral loads. But with the line-to-line Load C, we end up with Load C being in series with all the line-to-neutral loads on the black conductor. However, Exception 2 will not permit this to happen, as it requires an OCPD to trip both the black and red legs simultaneously.

 

[George Stolz]

I guess a picture is worth a thousand words:

You're darned right it is.

Thanks for the clarification.

 

[LarryFine]

Barry's diagram shows why the BWBC must be protected by a multi-pole breaker, and that such an installation is safe.

As with any MWBC, the integrity of the neutral must be maintained, and is why it cannot depend on device terminals for continuity.

To me, it is clear that line-to-line and line-to-neutral laods may be supplied from such MWBC's.

 

[iwire]

If an overcurrent condition trips the breaker on the black conductor, that would be OK if there were only line-to-neutral loads. But with the line-to-line Load C, we end up with Load C being in series with all the line-to-neutral loads on the black conductor. However, Exception 2 will not permit this to happen, as it requires an OCPD to trip both the black and red legs simultaneously.

Barry's diagram shows why the BWBC must be protected by a multi-pole breaker, and that such an installation is safe.

I agree with what can happen I disagree with your interpretation of the NEC requirements.

First of all if the multiwire branch circuit is supplied with fuses there is no way to provide common trip.

Handle ties are all that is required, not common trip.

Have a look at some handbook examples

Exhibit 240.6 Examples of circuits in which approved handle ties are permitted according to 240.20(B)(2) or 240.20(B)(3).

 

[BarryO]

240.20(B)(2) and 240.20(B)(3) don't apply to the circuit we've been discussing in this thread, IMHO, since they only apply to line-to-line loads. Line-to-neutral loads are outside their scope. I don't see how they apply to the first two of those diagrams, either.

210.4(C) Exception 1 allows a MWBC to supply a single piece of utilization utilization equipment, even if it includes both line-to-line and line-to-neutral loads, without the need for common trip; e.g. a dedicated circuit supplying a clothes dryer does not need common trip. This would permit the configuration shown in the first two diagrams.

 

[iwire]

Barry Let me ask a question as this thread is all messed up.

What type of circuit do you have in your mind?

 

[milwaukeesteve]

Man I thought this discussion was over. There were so many that were quiet for a long time on this thread, and now they are not quiet.

Iwire, the drawings you have are great representations of the handle tie usage. However, in relation to this thread, and what has been argued over the last several pages, is that we are talking about field wired 240V and 120V loads. Your graphics are showing MWBCs feeding fixed, manufactured appliances or loads. Close, yes, but not what we are arguing. Rather we are argueing whether a 120V load can be combined with a 240/120V load, and what are the limitations or requirements or justifications for that?

I continue...

Jim Dungar, I got your point about the conductors, not the loads in your post. Take a look back through this thread again. I believe I had an installation example of a 240V baseboard Heater with an integral 120V duplex recept. The installation was in 12/3 cable, protected by (2) 20Amp breakers, Handletied. Now what happens when we plug in that 11Amp vacuum cleaner? OK, we plug in the vacuum somewhere else. What about that window A/C unit? Do we increase one of the breakers, to allow this new load. We might be able to legally put a 25Amp breaker in, along with the existing 20, and handle tie them. But that is still your convenience duplex receptacle. What happens if we plug things in now? How about 30? 35? That's not going to work.

Maybe we should only allow this for fixed loads, not general or convenience loads? But neither the code rule nor the exception address anything like that.

Furthermore, if you want to address this like a 15 or 20Amp Branch Circuit, then you have something else to consider. 210.23 (A) (2) Utilization equipment fastened in place shall not exceed 50% of the branch circuit ampere rating. In my example 424.22(A) Fixed Electric Space Heating Equipment OCP refers you back to Art 210. This is getting scarrier by the minute. Now that baseboard is limited to 10A, but that only allows for 10Amps for the convenience outlet. Yet because it is the only outlet on a 20A circuit, you must install a 20A device.210.21(B)(1)

Now I am confusing myself while trying to confuse myself... I think.




If we look at the original rule and its exceptions (210.4), nothing is mentioned about any of this. Are we to assume that it meant only fixed loads? Are we to assume nobody will plug a vacuum cleaner in? Are we to size the conductors to the largest load or combination of loads? That might be difficult, but could be done if loads are fixed loads.
Yet the blanket arguement that says that this style wiring is allowed, is hard for me to swallow on face value. There is a lot more to this type of circuit, if we try and wire this way and make it right, and my arguement is I could probably throw enough safety and workmanlike manner codes at this to make it wrong.

If all the planets were to line up, I could see this installation actually working. If the last one to leave could please turn out the light...

 

[iwire]

However, in relation to this thread, and what has been argued over the last several pages, is that we are talking about field wired 240V and 120V loads. Your graphics are showing MWBCs feeding fixed, manufactured appliances or loads. Close, yes, but not what we are arguing. Rather we are argueing whether a 120V load can be combined with a 240/120V load, and what are the limitations or requirements or justifications for that?

Well Steve that is easy.

Yes you can 'field wire' 120 and 240 loads together and without looking it up at the least they require handle ties maybe even a common trip.

But I have purposely stayed out of this exchange because no matter what was presented to you by any other member you refused to listen and kept on talking about how we know nothing about safety or electrical theory when we talked about this before.

By the way, electrons don't know if they are travailing on filed wiring or factory wiring so the safety is the same.

 

[iwire]

If all the planets were to line up, I could see this installation actually working. If the last one to leave could please turn out the light...

Boy I must be a gluten for punishment but here I go.

What is unsafe about it or what will not work about it?

 

[jim dungar]

Jim Dungar, I got your point about the conductors, not the loads in your post. Take a look back through this thread again. I believe I had an installation example of a 240V baseboard Heater with an integral 120V duplex recept. The installation was in 12/3 cable, protected by (2) 20Amp breakers, Handletied. Now what happens when we plug in that 11Amp vacuum cleaner? OK, we plug in the vacuum somewhere else. What about that window A/C unit? Do we increase one of the breakers, to allow this new load. We might be able to legally put a 25Amp breaker in, along with the existing 20, and handle tie them. But that is still your convenience duplex receptacle. What happens if we plug things in now? How about 30? 35? That's not going to work.

Why would we increase the size of an existing breaker? If the circuit was designed and installed to be 20A then that is what it stays regardless of what loads get plugged in. Are you talking about trying to create a MWBC where one leg has a different ampacity than the other?

 

[milwaukeesteve]

I thought I made pretty good sense on that last post. I guess not.

Jim, you had brought up the 11 Amp vacuum. How is that supposed to be protected? If you have a 220V load, fixed as I mentioned before, it can't be larger than 50% of the circuit. So let's say it is 50%. That would be 10Amps on a 20Amp circuit, because the 120V side of this circuit IS a Branch Circuit, as I described. Therefore, if you had the 10A load ON, you wouldn't be able to run the vacuum. That is why I said this needed to be for fixed loads ONLY. And again, because this would be a branch circuit that is the only recept on the circuit, the device must be rated at the ampacity of the circuit. But the code rule or the exception 2 doesn't state that.

Bob, I know they are the same thing in how the OCPD sees it, yet they are different. The wiring inside the dryer/range is fixed, the load is fixed, the 120V loads are most likely fused. But that doesn't matter because UL, the manufacturer and the NEC tell me it is fine (exception 1 for one utilization equipment). The question is when we field wire loads, what kind of loads, how are they protected, and what other codes are involved (as I just pointed out to Jim in the last paragraph), will allow us to use Exception 2.

Exception 2 is incredibly all encompassing, compared to the Exception 1. In Exception 1 we are told exactly what, why and how many.
Exception 2 just tells us how, if we were to do IT. Now I feel like an Ebay commercial. What is IT? We are told only how to protect IT.

Nothing tells me I have to watch out for other code rules, like I showed with the vacuum cleaner example.
Nothing tells me to look at motors, appliances, HVACR equipment, Branch Circuits or any other specific area. I need to do that on my own.

I have a problem with someone telling me I CAN do this. That is not what the code reads. It only ALLOWS. And that is a HUGE difference to me. 'CAN' sounds so simple. 'ALLOWED" sounds like there is an IF, BUT or a HOWEVER that is attached to it. Especially because if someone younger in the trade and experience, that looks at the handbook for reference, and says 'Hey I can do this, it says so right here', and completely disregards all else. Just because the HandBook and Mike Holt say you can, doesn't mean you should.

It is too hard to look back, but I believe George just mentioned that he wouldn't wire this way, or to the effect. I wouldn't do it, but someone may, and that person better know what they are doing. And that person would rightfully be ALLOWED to do it if done right.


My arguement for safety is what I just mentioned, and probably should have stated before, but it never quite formulated in my head.

That is why I said if all of the planets... This would have to be done right to be correct. Yet when you read the exception, it sounds so simple to just say 'I can'.

 

[milwaukeesteve]

And I just reread you question Jim. Sorry.


That is what I was talking about. Would it be alright, if the circumstances called for it, to put two different Ampacity breakers to protect the circuit? Given that they would be handletied.

Exception 2 does allow this. As I just stated in my lengthy post, that the exception only tells us how: if all are simultaneously disconnected. Nothing tells me I can't, in the exception. Now I have to use other codes to figure out if I can.

This again is my whole arguement.

 

[George Stolz]

I'm glad somebody finally said something about this aspect, as it's one of the first things that caught my eye about the Mike Holt picture.

First of all if the multiwire branch circuit is supplied with fuses there is no way to provide common trip.

Handle ties are all that is required, not common trip.

I think the wording of the exception precludes the use of fuses to protect this circuit.

Exception No. 2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device.

They didn't say "disconnecting means", they said "opened by the BC OCPD". I'd say that is very enforceable as "common trip", and the picture Barry O posted cleared that up for me. I could not see a reason for common trip to be required, until I saw that circuit path.

 

[George Stolz]

Now I am confusing myself while trying to confuse myself... I think.
...

Yet the blanket arguement that says that this style wiring is allowed, is hard for me to swallow on face value. There is a lot more to this type of circuit, if we try and wire this way and make it right...

Steve, have you considered that you're trying to eat an elephant all at once? You have to go one bite at a time.

IMO, you need to ascertain whether 210.4 exception 2 means what the majority thinks it means, and resolve this piece. This piece is yes or no.

After that, we can sail the seas of green cheese with the weird loads and OCPD ratings and general goofiness. But if you jump into the green cheese before you've determined that there is indeed a boat there to sail it with, you're going to drown.

So, for my sake in my little stake in this discussion, can you believe that the 18 words in exception two allow all permutations of loads? That is not to say that there are not words elsewhere that would prohibit weird green cheese goofiness, but that these words do not?

 

[DesertRat]

Jim, you had brought up the 11 Amp vacuum. How is that supposed to be protected? If you have a 220V load, fixed as I mentioned before, it can't be larger than 50% of the circuit. So let's say it is 50%. That would be 10Amps on a 20Amp circuit, because the 120V side of this circuit IS a Branch Circuit, as I described. Therefore, if you had the 10A load ON, you wouldn't be able to run the vacuum. That is why I said this needed to be for fixed loads ONLY. And again, because this would be a branch circuit that is the only recept on the circuit, the device must be rated at the ampacity of the circuit. But the code rule or the exception 2 doesn't state that.

so you have a 10 amp load and an 11 amp vacuum, and it doesn't add up on a 20 amp breaker? Lets say you have a duplex recept. dedicated to a 20 amp circuit. Now lets say the homeowner decides to plug two space heaters drawing 14 amps apiece into that recept. How is that scenario any different in context than yours? Both are uncontrollable. Neither is patently unsafe, at least not if the OCPD is doing it's job correctly.

 

[milwaukeesteve]

We deal with the idea of plugging in too many things into a general, bathroom, laundry or SA circuit all the time.

In relation to 210.23 Branch circuits Permissable Loads, 15 and 20 Amp Branch circuits have limits when combined with utilization equipment. Again, we deal with that all the time that someone could plug in too many things. But is this the same thing? Because it is part of the MWBC, is it still part of the circuit (as seen by 210.23)?

My point is not in this specific example, but rather that this example shows that more than just 210.4 (C) Exception 2 is involved here. But the Handbook doesn't elude to that, neither does anybody's arguement here.

Did you not read through my example about the duplex recept in the baseboard heater? There is a lot to consider in that installation, and several other code rules that are affected.

The blanket acceptance of this rule as a 'CAN' is the reason I am standing on this darn island, by myself.

 

[iwire]

Did you not read through my example about the duplex recept in the baseboard heater? There is a lot to consider in that installation, and several other code rules that are affected. .

No I did not, I will try to find it.

But in a nut shell

What other code sections are you thinking about?

I really have no idea what you see as unsafe or a code violation.

 

[milwaukeesteve]

Try and imagine a circumstance where you would use this code rule.

Any situation I can think of would require you to look elsewhere and combine several rules. If you have a motor or a combination of motors, 430. HVACR, 440. Appliances, Heating equipment, Snow Melt equipment...

There are requirements to these circuits. And a MWBC does have circuit in the title. So we need to consider all other applicable 'circuit' rules when trying to use Exception 2.

Yet, the Exception 2 does not say anything about that. However, throughout the code book there are constant references to other applicable codes.
For example:
'identified in accordance with 200.6'
'values permitted by 210.6(A) through 210.6(E)'
'connected in accordane to 400.7'

All of these examples are found on the same pages with the 210.4. Yet, this exception doesn't mention anything like that or even reference anything else. It simply tells you what needs to be to do IT. Here is my IT reference again. What is IT?
I have argued that IT should not involve 120V loads. I have argued that exception 2 shouldn't even see 120V loads due to the wording in the rule itself. DeserRat had a problem with my train of thought there due to linguistics, verbage and semantic arguements.

by DesertRat
Try this. If I tell you:
"You are required to stand on your head and sing"
"Exception: if you are an electrician"

Will you be standing on your head? Will you be singing?

That is not accurate as a representation of this rule. Try this one:

Rule: If you have an orange ball, you are the only one that can play ball.
Exception: If you have blue balls, then you can play with them.

Does it say anything about playing with the orange ball while playing with blue balls? NO. The exception is there for those that don't have an orange ball, so that they can play with something, too, as long as they meet the exceptions criteria. Also, the wording 'play with them' has no quantifier on it. What does 'them' actually mean? Do I have to play with my orange ball, too. But wait, if I had an orange ball, I wouldn't even be looking at the exception.

This is what I mean when I describe 210.4 (C) and the exception. There is no qualifier, quantifier or limiter on the exception (only a what to do), and if I had a 120V load, I shouldn't even be reading the exception, I should have stayed with the rule itself, because I would have satisfied the original rule.

 

[LarryFine]

Try and imagine a circumstance where you would use this code rule.

Okay, how about a home woodshop where there are a couple of 20a, 120v receptacles for, say, a miter saw and a drill press, as well as a 20a, 240v receptacle for a table saw.

It's clear to me that, as long as a 2-pole breaker is used (saving whether a handle tie alone is adequate for another discussion), the two 12v receptacles and the 240v receptacle may share a 3-wire 20a circuit.


I'd also like to keep the color of my balls out of the discussion.

 

[iwire]

Try and imagine a circumstance where you would use this code rule.

Any situation I can think of would require you to look elsewhere and combine several rules. If you have a motor or a combination of motors, 430. HVACR, 440. Appliances, Heating equipment, Snow Melt equipment...

So look and show me any of the rules in those sections that prohibit what we are talking about.

Here is one example that is OK code wise.

I use a 12/3 NM to feed a 240 volt 20 amp air handler and 120 volt outlet.

I supply the 240 to the air handler with black and red, than I use black and white to supply the receptacle at the unit for service of course I do this on the line side of the disconnecting means.

Try to find code or safety reason why I can not or should not do this.