Neutral Grounding

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busman

busman

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Northern Virginia
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Master Electrician / Electrical Engineer
I don't think the NEC covers this question because it involves an old wiring method. The question is, if you have an old run of BX for clothes dryer without a bonding strip under the armor, and 3 conductors in the cable, should this be a neutral grounded circuit? Or, was the armor without a bonding strip considered to be a ground at the time these were installed and therefore it should have been a 4-wire circuit? I think we all agree today, that the armor without a bonding strip doesn't make an effective ground because of the inductance of the spirals.

Thoughts?

Thanks,

Mark
 
the dryer being feed from the service panel or a sub-panel. If it is the service panel, then the white conductor is being used for the neutral current and the fault clearing path no equipment ground is required in this existing dryer circuit.

If the dryer is being supplied from some other panel than the service panel then an equipment grounding conductor is required for this dryer circuit,
 
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david said:
is the dryer being feed from the service panel or a sub-panel. If it is the service panel, then the white conductor is being used for the neutral current and the fault clearing path no equipment ground is required in this existing dryer circuit.

If the dryer is being supplied from some other panel than the service panel then an equipment grounding conductor is required for this dryer circuit,
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It is fed from the service panel. About a 50 foot run across a finished basement, so I don't want to run a new cable if not necessary. At the moment it is connected with a 50A 3-pole, 3-wire plug and receptacle. I obviously need to change the cord and receptacle to 30A. Unless someone gives me a good reason otherwise, I'd plan to install another 3-pole, 3-wire and bond the neutral to the outlet box and the dryer frame.

Thanks,

Mark
 
Mark
my post was hasty, the reference I was thinking of was for branch circuits being supplied from the service using service entrance cable. I am not sure the code ever allowed BX cable to be used this way. Others nay no older codes that allowed this. Any how look at 250.140
 
As you look at 250.140 there is more than one qualifier I was only remembering the fact that it needed to be run from the service panel. ?if all the following conditions are met.? (2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum.
 
busman said:
...and bond the neutral to the outlet box and the dryer frame.
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Dryer frame: yes. They come with a bonding strap now in case you have a 3 wire system but if you bond to a metal box you are going to be carrying some (objectionable) current on the MC jacket back to the panel as a parrallel path with your grounded conductor; unnecessary and unsafe.
 
busman said:
Unless someone gives me a good reason otherwise,
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You would have to ask yourself if you believe there is good reason for the Code to move away from bonding metallic surfaces with conductors being used to carry current as part of the normal circuit.

In you circumstance the metal sheath providing physical protection of the circuit conductors will also be caring normal circuit current and any metal surfaces like a furnace duck system the sheath come into contact with also becomes a potential conductor for current
 
Sparky3141 said:
Dryer frame: yes. They come with a bonding strap now in case you have a 3 wire system but if you bond to a metal box you are going to be carrying some (objectionable) current on the MC jacket back to the panel as a parrallel path with your grounded conductor; unnecessary and unsafe.
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Thanks for all the responses. You've hit my point exactly. 250.140 allows me to connect the grounded conductor (all the exception conditions are met in this installation) to the frame of the dryer and the JB holding the receptacle. That will bond the armor of the BX to the grounded conductor. The code clearly permits this, but doesn't seem to require it. My question is, which is the greater hazard:

1) Bonding it provides a fault current path to operate the OCPD, but also places current on the armor and anything else the armor touches.

2) Not bonding it provides no fault current path and therefore the armor could become energized and stay that way if there is a fault from ungrounded conductor to the dryer, the JB or the armor.

It seems that #1 is less of a hazard, but I started this thread to try to come to consensus.

Thanks,

Mark
 
I never seen BX cable that was not bonded to the panel that supplied the circuit so there should be a reference to the neutral in the panel if a ungrounded conductor makes contact to the sheath. i think what was in question was the idea of bonding the neutral at the device (receptacle) box.

The idea presented was to keep the neutral isolated from the metal sheath at the receptacle end.

I think the issue is more to the metal sheath conducting normal circuit current, and the hazard as already been addressed. I think the choice you have is to continue to use the wiring method or change it
 
Prior to the early 1950's AC cable did not have the bonding strip but the jacket was suitable for grounding. Not sure if they ever de-listed the jacket as an EGC.
 
david said:
I never seen BX cable that was not bonded to the panel that supplied the circuit so there should be a reference to the neutral in the panel if a ungrounded conductor makes contact to the sheath. i think what was in question was the idea of bonding the neutral at the device (receptacle) box.

The idea presented was to keep the neutral isolated from the metal sheath at the receptacle end.

I think the issue is more to the metal sheath conducting normal circuit current, and the hazard as already been addressed. I think the choice you have is to continue to use the wiring method or change it
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Ok, with all your help I have this narrowed down. 250.140 says you can do neutral grounding if there is NOT an EGC in the box. So the question is simply,

Is BX without a bonding strip an EGC or not?

Thx

Mark
 
busman said:
I don't think the NEC covers this question because it involves an old wiring method. The question is, if you have an old run of BX for clothes dryer without a bonding strip under the armor, and 3 conductors in the cable, should this be a neutral grounded circuit? Or, was the armor without a bonding strip considered to be a ground at the time these were installed and therefore it should have been a 4-wire circuit? I think we all agree today, that the armor without a bonding strip doesn't make an effective ground because of the inductance of the spirals.

Thoughts?

Thanks,

Mark
Click to expand...


250.118 (5) (6) I think is where you'll find the answer.
 
Fladude38 said:
250.118 (5) (6) I think is where you'll find the answer.
Click to expand...

Although Type AC cable without the bonding strip is not FMC, I think your answer is as close as we're going to get to definitive using the current NEC. Thanks for the help.

Mark
 
busman said:
Ok, with all your help I have this narrowed down. 250.140 says you can do neutral grounding if there is NOT an EGC in the box. So the question is simply,

Is BX without a bonding strip an EGC or not?

Thx



Mark
Click to expand...
It once was, is not anymore. NEC doesn't address existing installations that were compliant at one time though. Is that maybe somewhat a hint the CMP's don't support using it, IDK? After hearing some stories of what can happen, I would want to eliminate these installs wherever possible.
 
The old AC cable without the bonding strip was code compliant as an EGC when installed and the NEC is silent on any prohibition for using it as such. Same thing would be true for old NM cable with a reduced size EGC. Although the NEC tells us what to do when we have NM cable with no EGC is silent on the cable with a reduced size EGC.
 
infinity said:
The old AC cable without the bonding strip was code compliant as an EGC when installed and the NEC is silent on any prohibition for using it as such. Same thing would be true for old NM cable with a reduced size EGC. Although the NEC tells us what to do when we have NM cable with no EGC is silent on the cable with a reduced size EGC.
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I agree with that. I also have to say I would trust the reduced EGC in the old NM cable to have less impedance then the sheath of an old AC cable in general.
 
I would tend to concur. But if it was important enough to not run new wire And I was going to use it as an egc then I would feel a little better about it after slappin' a megger on it.

I just did 3 of these in a domitory. They had 2 blacks and a green in the conduit and when they got new machines they just slapped the green on the neutral terminal and left the grounding strap on. Well, they had the old surface mount 3 prong receptacle and the neutral got smushed when replugging it in and didn't connect as it should. So the residence were coming down to the concrete laundry room floor barefooted and getting hammered when they turned on that machine and touched it.

fortunately I did have 1" conduit, plenty of wire, and plenty of time and only about 25 feet to go. I ended up just changing all 3 on that floor with new recep's, plug cords, and pulling the jumpers off of the equipment. I realize that not everyone has this luxury though and just hope the OP makes it a safe installation.
 
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