210.4(b) & (d)

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ronaldrc said:
Charlie,Don,Roger or anybody if I have this wrong,Please let me know.
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I think the way the neutral wire is disconnected under bulb #1 is not the way it would actually be done in the field. You removed the path for neutral current back to the source, but you left the neutral from bulb #1 connected to the rest of the downstream loads. I should think that that point would be disconnected at the same time, and that the three wires would have been under the same wire nut.
 
I giving up this trade and becoming a dentist.

hermey.jpg


Obviously MWBCs will kill me. :roll:

Give an experienced worker an amp clamp and a volt meter and MWBCs present no more danger then any other ciruit.

Again I point out the result of this requirment will be more live work, not less.
 
Charlie

I wasn't really thinking so much alone the lines of how we would actually
troubleshoot the circuit.

I wanted to show different configures of how it could be a shock hazard and destroy equipment.

But you're right I should have shown all three neutrals under the one wire nut. Thanks :)
 
Charlie

Lets say you're right and the electrician does tries to keep the neutrals
intact, he better be really good and not mess up and let them loose connection.

Because where that conduit goes through the wall ,there could very well be my scenario of a 5000 watt heater on the blue circuit and a computer on the red circuit and it wouldn't take a half a second to smoke something small with the resistance of that heater coil and a voltage of 208 voltage on a 120 volt piece of equipment.
 
Da gone it ,I thought someone would have donged me on than 5000 watt heater .I'll change that to a 2000 watt heater.

We don't normally run 50 amp 120 volt circuits :D
 
iwire said:
Give an experienced worker an amp clamp and a volt meter and MWBCs present no more danger then any other ciruit.
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I again will agree, and there is no better way to gain experiance than experiance itself. From now on all apprentices will think that it's OK crack that noodle - until he finds himself a building built from before the cieling lowered, and it won't be long. That person won't know what to do in those cases, and yes - will be dumber for it. This attemp to control practice by design will only be effective once every building standing now is gone IMO.

It's also a great way to sell breakers and handle ties - panels and copper too. (How are you gonna handle tie at the transition of twins and singles?)
 
Give an experienced worker an amp clamp and a volt meter and MWBCs present no more danger then any other ciruit.

Again I point out the result of this requirment will be more live work, not less.
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Well said and I couldn't agree more.

Chris
 
iwire said:
Give an experienced worker an amp clamp and a volt meter and MWBCs present no more danger then any other ciruit.
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Once again I will speak out of turn. Let me pose a situation to those of you who do electrical repairs, as opposed to those like me who merely talk about them.

SITUATION:
HO reports receptacle accidentally damaged (e.g., something bumped into it, and cracked the plastic). You check the contacts with your meter (i.e., insert test probes into receptacle slots), and see that the receptacle is live. You figure out what breaker serves the receptacle, open that breaker, and test the contacts again. The meter tells you the receptacle is de-energized. You undo the screws and pull the receptacle a few inches out of the box. You test it again, and it still shows de-energized.

QUESTION:
When you start to disconnect the wires, are you going to treat this job as ?live work?? Are you going to use the PPE that is appropriate for 120 volt live work? Are you going to assume that the neutral wire is de-energized, or will you treat it as though it were live?

MWBC:
If this receptacle is served by a MWBC, then you are doing live work, whether you know it or not. If you treat it otherwise, you are at risk, and you don?t know you are at risk. The real point is that you might not know, going into the project, whether this is a MWBC or not. So you don?t know whether to treat the job as live work.
 
Charlie, an experienced gun handler knows all guns are loaded and treats them as such until he has opened the chamber, a qualified electrical worker should treat all his potential dangers the same as the gun handler treats his.

Getting lax and being injured, injuring someone else, or doing property damage would be human error attributed to carelessness.

Roger
 
charlie b said:
If this receptacle is served by a MWBC, then you are doing live work,
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That is false.

If after I verify the receptacle is dead and pull it out of the box and all that is in the box is one circuit I know I do not have a MWBC.

On the other hand if I see the other conductors feeding through then I have to do more checking.

That is where this tool, which I always have in my bag comes in....

fluke-t5-600.jpg


Without breaking the circuits I can easily check for current flow on the neutral or any conductor.

I mean no disrespect but it seems you may not understand the typical wiring of MWBCs. Not how they work but how they are installed.


Edit..fixed img tags
 
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I made a couple of examples, yeah they are ugly but I think they may help.

In Example number 1, if I turn off circuit 3 (the red circuit) I can safely...without any risk to property or personal, replace the devices connected to that circuit.

MWBC1_001.jpg


In example 2 things change a bit, if I was to break the single connection already required by 300.13(B) without turning off the other circuits I could be hurt and damage equipment connected to other parts of the circuit.

However the point of 300.13(B) is to allow the removal of the device without breaking the neutral.

Personally I would try to avoid working on Example 2 without killing it all, but I would have no problem working on one circuit of example 1 without killing the other two circuits.

Example 1 is the most common layout of MWBCs that I find.



MWBC2_001.jpg
 
charlie b said:
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.
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I actually ran into this exact situation last week. I was changing out some receptacles and the original installer did not pigtail any of the neutral wires, and they were part of an MWBC. I took out a receptacle and lo and behold, the white was was "live," which I discovered with my wiggy's. So I went and shut off the breaker right next to the one I had already shut off for the receptacle circuit and it killed the other half of the circuit. Granted, I should have checked that neutral wire with an amp clamp but I didn't. I'm still opposed to the handle tie rule even though I had a close call with a live neutral.
 
peter d said:
I actually ran into this exact situation last week. I was changing out some receptacles and the original installer did not pigtail any of the neutral wires, and they were part of an MWBC. I took out a receptacle and lo and behold, the white was was "live," which I discovered with my wiggy's. So I went and shut off the breaker right next to the one I had already shut off for the receptacle circuit and it killed the other half of the circuit. Granted, I should have checked that neutral wire with an amp clamp but I didn't. I'm still opposed to the handle tie rule even though I had a close call with a live neutral.
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Usually my first clue to the presence of a MWBC is that there are more than one ungrounded circuit conductors in the box to begin with. If this is the case, and the neutral isn't pigtailed, I will shut off all branches of the MWBC as a precaution. I learned that lesson many years ago wiring checkstands in a grocery store- neutral splice came lose that was feeding the "dirty" power to the line-up. The resulting voltage on the circuit burned out three of the photoeye modules that controlled the belt motors.

IMO, the new handle-tie rule is completely unnecessary and the "qualified" electrician can easily mitigate any hazards associated with the neutral wire current. The ones that WILL be protected are the countless handymen, and homeowners that try to do electrical work themselves. (What does the announcement always say at Big Orange... "you can do it, we can help!")


- Greg -
 
Wow thanks for all the discussion! I think this code change is in violation of 90.1 and should not be adopted by local jurisdictions.
Basicly I do allot of troubleshooting and I am qualified to. I got zapped on a MWBC back when I was an trainee 277 to G.
I carry a wiggy and It always goes off on hot neutrals. I also carry a tool similar to the one shown above.
In the jurisdiction where I currently work everyone has to have a license to be on a job. They think that having that license means they are qualified for it all.
Where I grew up they stressed "qualification" they have got too much going on to try to license anyone more than contractors. I got a mantra drilled in to my head: Are you qualified to work on the system infront of you? If you are staring at a malfunctioning outlet, nurse call system, fire alarm panel, 4180 Xformer, MV cable, or a fiber optic patch panel the first question you ask is am I qualified to troubleshoot this?
Sure you have been thru 5 years of OJT and maybe some school but are you safe to work on anything?
IMO Part of being qualified to troubleshoot a outlet is knowing the status of available power inputs.
roger said:
an experienced gun handler knows all guns are loaded and treats them as such until he has opened the chamber, a qualified electrical worker should treat all his potential dangers the same as the gun handler treats his
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The first thing I do on a service call is take off the panel cover and look inside. I verify the voltage comming in to the ML or MB is correct. I test grounding and impedance at outlets with an Ideal sure test. I also look at the conduits leaving the panel for MWBS's.
I have seen to many loose connections, burning connections, mains that only 1 leg is blown or tripped and voltage 1 leg of 120 on all 2 or 3 legs.
Open neutrals where the bonding bushing is acting as the neutral. etc.
I have arrived at a service call for a "outlet not wokring right" an thought slam dunk easy fix and spent hours finding a toasted main breaker on some other floor etc. I have even see computer equipment running fine on a mistakenly high legged 208V to G circuit. Sure enough I looked at the pwr supply's and they were self adjusting 100V-240V. The problem had persisted for months until somebody plugged in a lamp.
So my point here is that 90.1 uses the word "practical" and it makes no practical sense to protect a unqualified person from themselves. If people start trying to turn the NEC into a training manual and a throw in protection of unqualified persons in every article then you will see the competing electrical codes again.
Things like this are a very large crack in the foundation of the code.
 
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Wow

Wow

Nice follow up tortuga.

I agree with your thought of adjusting to unqualified masses.
I also see this as MORE live work,just a little more carefull.

On the resi side, shut them off. Oh "Boo-Hoo".. we must reset clocks and the like.:mad:

On the commercial side.. Now (Ask any manager) their looseing $10,000,000
a minute of down time (even for Mable on her PC).:rolleyes:

So schedule shutdowns and charge a premium.;)

If done as intended, we will be safer pros'.
 
charlie b said:
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.
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One of the worst shocks I've had is when replacing a ballast while MWBC was energized, and coming in contact with an open neutral; completed a 277 volt lighting ckt. Hurt like hell and was sent to the hosp for observation. Result: I favor the new code requirement FWIW:smile:
 
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masterelect1,
In theory, you shouldn?t even need to worry about turning off those wall switches or circuit breakers to change fluorescent ballasts anymore, since the CMP has also graciously implemented 410.130[G][2] and [3] which requires disconnecting both the ungrounded and neutral conductors feeding ballasts. :wink:
 
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