Your opinion of existing 3 wire Dryers and Ranges on Sub panel

[GoldDigger]

I think the reasoning behind not using an SEU in a sub in that the neutral is bare. Landing it on the neutral bar could cause it to also short against the can bypassing the separation in a subpanel.

+1
I think you've found the explanation behind the wording that justifies the interpretation that an insulated neutral is OK in a subpanel.
You can have the same problem with accidental contact between neutral and ground wherever the SEU comes in through a bushing, etc.

 

[jwelectric]

Why can't I see page two?

 

[GoldDigger]

Why can't I see page two?

It happens sometimes when the forum database gets screwed up temporarily.
You can reduce the problem by setting a higher number of posts per page.

 

[curt swartz]

I think the reasoning behind not using an SEU in a sub in that the neutral is bare. Landing it on the neutral bar could cause it to also short against the can bypassing the separation in a subpanel.

In my opinion that is definitely the reason. If not I don't understand why if all 3-wire circuits had to originate for the service equipment it would not be be the number 1 requirement in the list. They tacked the requirement on to the bare neutral permission in number 3 for a reason. You can use a bare neutral if it comes from the service panel. If it comes from a sub it must be insulated.

 

[hurk27]

I agree that the two sentences in (3) are separate and independent allowances of the exception, and the bare wire reason makes sense up until you realize that the dryer ductwork or frame of the dryer being bonded to the grounded conductor can also inadvertently cause this grounded conductor to be grounded via water pipes or other paths such as the exhaust work touching other grounded items which could also cause the grounded and grounding to have a connection in a subpanel.

How many of these dryers have we seen people connect a ground wire to the frame of the dryer and run it to the water pipes serving the washer because the wife gets a small shock when starting the dryer standing barefoot on the concrete basement floor?

but I agree the wording in the above exception is what it is, and the referance to the service panel is only in the second sentence that has a comma and the word "or" that seperates it from the first sentence.

Those two and three letter words gets the best readers all the time.

, or the grounded
conductor is uninsulated and part of a Type SE service entrance
cable and the branch circuit originates at the
service equipment.

 

[mbrooke]

I agree that the two sentences in (3) are separate and independent allowances of the exception, and the bare wire reason makes sense up until you realize that the dryer ductwork or frame of the dryer being bonded to the grounded conductor can also inadvertently cause this grounded conductor to be grounded via water pipes or other paths such as the exhaust work touching other grounded items which could also cause the grounded and grounding to have a connection in a subpanel.

How many of these dryers have we seen people connect a ground wire to the frame of the dryer and run it to the water pipes serving the washer because the wife gets a small shock when starting the dryer standing barefoot on the concrete basement floor?

but I agree the wording in the above exception is what it is, and the referance to the service panel is only in the second sentence that has a comma and the word "or" that seperates it from the first sentence.

Those two and three letter words gets the best readers all the time.

Even if the dryer is touching something grounded like a cold water pipe I think it doesn't make much of a difference whether its from a service or sub panel. In both cases there is impedance between the utility transformer and main neutral, which means a potential exists between any neutral and something connected to the earth. Some current will always flow through the water bond, ground rod and so fourth back to the transformer's ground as will if the dryer touches anything earthed be the unit 4 wire or 3 wire. Ive gotten many times current readings the ground wire headed to an AC unit siting on top of moist grass.

Even that aside the dryer's neutral itself has impedance so a running dryer motor at 6 amps might pass 3 through the neutral and another 3 through say a water pipe regardless if the 3 wire dryer circuit comes from a sub-panel or service. Of course 4 wire would eliminate this over the 3 wire.

The reason behind the change from 3 to 4 wires isnt so much normal operation but rather should the neutral become disconnected at any point.