210.4(b) & (d)

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tortuga

tortuga

Code Historian
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Oregon
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Electrical Design
Is it really necessary for people to group and handele tie multiwire branch ckts? Is anyone proposing to delete this in there jurisdictions?
Is there any study that demonstrates qualified trained persons get zapped of these?
 
I personaly like 210.4(D). It can be a major PITA to identify what neutral goes with what ungrounded wires when you have 2 or 3 sets in a conduit.

210.4(B)... not a big fan of that one, but if it is required, I will do it.
 
"hey what happened to the lights"

"hey what happened to the lights"

iwire said:
I think the end result of the required handle ties will be more live work instead of less.
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more "darkness" also, now rather than having 1 breaker trip on a fault, you will have three.
There may have been method in their maddness. I can see more designs prohibiting MWBC in the future.
 
more "darkness" also, now rather than having 1 breaker trip on a fault, you will have three.
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Only if you use common trip breakers. In most cases the use of handle ties does not cause the other breakers to open when on breaker trips.
Don
 
The '08 says "simutaniously disconnect" - I would assume 'common tip' is implied????? (As it doesn't mention under what circumstances.)

IMO this is a code meant to idiot proof the trade - but will only produce more idiots through eventual dumbing down.
 
e57 said:
The '08 says "simutaniously disconnect" - I would assume 'common tip' is implied?????
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Nope, at least that is not the intent.

Its all about being forced to shut them all of when working on one.
 
I agree with the handle tie rule on MWBC. Anyone with any experience working on these circuits know the neutral can pack quite a bite when you are not expecting it. At least when you trace this cicuit now you will notice the handle tie and realize you are dealing with a MWBC. As far as 210.4(D) this is a good practice but I don't see why it should be a code requirement.

Joe
 
Earlier in the year, I encountered a client who insisted on implementing the 2008 requirement for handle ties immediately (it was even before the 2008 was first published). They considered it a safety issue. I started thinking of ways to convince them that it really was not a safety issue, but I wound up convincing myself that it is. It was certainly counter-intuitive, but I saw the truth when I drew a couple of sketches.

I am not good at drawing and posting sketches. So let me just say that the hazard comes into play if, and only if, you join the grounded conductors internal to an outlet box with a wire cap. That is, you have one incoming white wire, and you attach two other white wires under a single cap. One of them leads out of the box, and the other leads to a device installed in the box.

In order to work on (or replace) that device, you may need to unscrew the wire cap, and you might just let all three wires dangle for a while. You turned off the breaker feeding your device, so you think the circuit is safe. However, unknown to yourself the white wire leading out of the box is part of a MWBC, and the “other breaker” is still closed. You have disconnected the second half of the MWBC, but that dangling wire is now at 120 volts to ground. Touch it, and you complete the circuit.

I might not have described the issue completely right. But there is a danger, and it is related to the description I tried to give. Perhaps someone with greater drawing skills can post a sketch that explains the issue better.
 
Charlie, how about this?

In the first diagram, the MWBC can only shock you in the ceiling box with both ungrounded conductors in it.

In the second diagram, the MWBC can shock you in the device box, but it would require ignoring the fact that there is a hot conductor in that box sharing the neutral.

Is there a third scenario I didn't think of (not in violation of 300.3)? I think in both scenarios, a qualified individual would notice the MWBC and behave accordingly, IMO.
 
charlie b said:
Earlier in the year, I encountered a client who insisted on implementing the 2008 requirement for handle ties immediately (it was even before the 2008 was first published). They considered it a safety issue. I started thinking of ways to convince them that it really was not a safety issue, but I wound up convincing myself that it is. It was certainly counter-intuitive, but I saw the truth when I drew a couple of sketches.

I am not good at drawing and posting sketches. So let me just say that the hazard comes into play if, and only if, you join the grounded conductors internal to an outlet box with a wire cap. That is, you have one incoming white wire, and you attach two other white wires under a single cap. One of them leads out of the box, and the other leads to a device installed in the box.

In order to work on (or replace) that device, you may need to unscrew the wire cap, and you might just let all three wires dangle for a while. You turned off the breaker feeding your device, so you think the circuit is safe. However, unknown to yourself the white wire leading out of the box is part of a MWBC, and the ?other breaker? is still closed. You have disconnected the second half of the MWBC, but that dangling wire is now at 120 volts to ground. Touch it, and you complete the circuit.

I might not have described the issue completely right. But there is a danger, and it is related to the description I tried to give. Perhaps someone with greater drawing skills can post a sketch that explains the issue better.
Click to expand...


So we need to dumb our trade down because someone is not smart enough to look at what is actually going on with a splice?
 
georgestolz said:
In the second diagram, the MWBC can shock you in the device box, but it would require ignoring the fact that there is a hot conductor in that box sharing the neutral.
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It is not the presence of the second hot conductor in the first box that creates the hazard. It is the fact that the white wires are connected with a wire cap. When you open the black circuit?s breaker, and take off the white wire?s wire cap, you create an open circuit in the load served by the red wires. So the if you touch the white wire that is the return from the red circuit, you get a shock.

Ronald?s link shows the situation perfectly.
 
stickboy1375 said:
So we need to dumb our trade down because someone is not smart enough to look at what is actually going on with a splice?
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As an engineer, not an electrician, I know that I am speaking out of turn here, so please forgive that. But I believe that that notion of, "open breaker, undo wire cap, and you still have a shock hazard in the neutral wire" is advanced knowledge, not early apprentice level knowledge. It is not dumbing down the trade to attempt to prevent an unexpected situation from becoming a shock hazard.
 
I agree with Charle

Try telling that to someone who has gotten in a hurry on a trouble shooting job and got knocked off a ladder and injured their self.

I get Engineers Like Charlie and Rattus mad at me some time,when I tell
them you can not explain everytihng with numbers.

I am not trying to dumb them down.

They have a gift a lot of us don't and our hands would be tied if we didn't
have those gifted in that expertise.
 
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