3 wire range on subpanel

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apprenticebill

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St. Augustine, FL
Hello, I believe I have interpreted 250.140 correctly and provided the correct estimate for a job. However, I can't answer the question about why the code calls for this work or effectively describe the safety issue behind it.

I will helping on a job where we will install a Service Entrance ATS in a house built in the late 1980s. The load center will become a subpanel. The have 3 wire SEU cabling to the range and 3 wire NM cabling to the dryer.

The dryer is fine based on 250.140 exception (3) because the grounded conductor is insulated. The range has to be re-wired because that circuit will now originate at a subpanel and the grounded conductor is not insulated. (Personally, I would change the dryer out too because it's 2 feet away from the breaker panel but that's above my pay grade. It won't cost much.)

250.140 (3): “The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service entrance cable and the branch circuit originates at the service equipment.”

Is the issue that clearing a ground fault will now have to go through the subpanel to the ATS to reach an ECG? Does that put the devices downstream of the subpanel at risk? I can't wrap my head around the additional risk this creates terminating in a subpanel or why an insulated conductor makes a difference.

We're going to do all the work correctly either way but I would feel better just understanding the theory behind it.

Thanks

Bill
 
Actually, the fault current must reach the service neutral to clear a fault.

I had the exact same issue years ago. The panel was in a garage connected to the house (both of which were brick construction) with a breezeway. It would have been almost impossible to run 4-wire circuits to the range and dryer, unless run underground outside.

The inspector agreed and allowed me to cover the entire exposed length of the range's and dryer's SE neutral conductors in the panel with white tape and connect them each to the now-isolated neutral bar in the now-sub-panel.
 
Thanks. So, if I understand correctly, we'd rather clear that fault differently but it's not a major safety issue. They don't have dedicated grounding conductors in the first place.

You're taping up the neutral so someone doesn't come along in the future and think this is a grounding conductor? This makes a little more sense why an insulated neutral is allowed in the subpanel. It has white insulation and is obvious.

I'll need to check with someone else about what our AHJ might say if we didn't replace that range wiring. It does appear the other bids didn't include doing this and the homeowner really didn't want to hear about us doing work in their kitchen for a generator install. I sort of opened a can of worms that no one is happy about.
 
The whole intention of running separate grounding and grounded conductors is to prevent having current on normally non current and resulting voltage on normally non current carrying parts. Problem is the ranges and dryers were once permitted to bond the frame to the grounded conductor and they felt they needed to leave some rule there to allow them to remain that way if they were in compliance at the time of installation.

I suppose the idea with wanting an insulated grounded conductor if not in the service equipment is to minimize the possibility of interconnection between grounded and grounding conductors even if just because of casual contact of conductors laying next to one another in the panel. Just a guess, if you can assure there is no contact to other items then electrically they are the same thing for the most part, but at same time range or dryer could still contact other items and allow neutral current to pass through the frame.
 
That 3 wire SEU can not originate from a subpanel. You need to run 4 wire over to the range. Reason being is that the bare neutral from the SEU could short to the sub-panel can inadvertently bonding neutral to ground. Notice I use the word neutral, in that it must be terminated to the neutral bar and not the ground bar. Wording of the code is that you can use the neutral for grounding, but not grounding as the neutral in a 3 wire range or dryer circuit.
 
Always wondered about the reasoning behind that rule. No 3 wire for ranges and dryers in subpanels unless the neutral is insulated. Makes perfect sense now. Keep accidental contact between neutral and ground from happening downstream of service equipment. Learn something new all the time.

:thumbsup:
 
DBoone said:
Always wondered about the reasoning behind that rule. No 3 wire for ranges and dryers in subpanels unless the neutral is insulated. Makes perfect sense now. Keep accidental contact between neutral and ground from happening downstream of service equipment. Learn something new all the time.

:thumbsup:
Click to expand...

Yep, I get it now. Explains why the AHJ allowed LarryFine to keep the uninsulated 3 wire so long as he covered it with tape. Glad I bid this correctly even if I couldn't explain the theory. This particular range can't be more than 35 feet away, accessible attic, and drywall. Not too bad except I think we're charging $500 for this part of the job.

Thanks everyone.
 
mbrooke said:
That 3 wire SEU can not originate from a subpanel. You need to run 4 wire over to the range. Reason being is that the bare neutral from the SEU could short to the sub-panel can inadvertently bonding neutral to ground. Notice I use the word neutral, in that it must be terminated to the neutral bar and not the ground bar. Wording of the code is that you can use the neutral for grounding, but not grounding as the neutral in a 3 wire range or dryer circuit.
Click to expand...

which would in essence be another MBJ.....~RJ~
 
DBoone said:
Always wondered about the reasoning behind that rule. No 3 wire for ranges and dryers in subpanels unless the neutral is insulated. Makes perfect sense now. Keep accidental contact between neutral and ground from happening downstream of service equipment. Learn something new all the time.

:thumbsup:
Click to expand...
But at same time if your range or dryer happens to contact something else connected to the EGC, maybe some other appliance, ductwork that has continuity back to the furnace, metal water piping, etc. you still have a connection between the two conductors they are trying to separate:huh:
 
apprenticebill said:
Yep, I get it now. Explains why the AHJ allowed LarryFine to keep the uninsulated 3 wire so long as he covered it with tape. Glad I bid this correctly even if I couldn't explain the theory. This particular range can't be more than 35 feet away, accessible attic, and drywall. Not too bad except I think we're charging $500 for this part of the job.

Thanks everyone.
Click to expand...


Weclome :)


FWIW here are the connection diagrams to these plugs, notice the term "none grounding" in that there is no EGE and the pin is labelled "W", then compare to the grounding types.


how-to-wire-240-volt-outlets-and-plugs-of-nema-l6-20-wiring-diagram-3.jpg





4 wire grounding:


how-to-wire-240-volt-outlets-and-plugs-of-nema-l6-20-wiring-diagram-1.jpg



240 volt, no neutral:

how-to-wire-240-volt-outlets-and-plugs-of-nema-l6-20-wiring-diagram.jpg




Also if you ever need the configurations and diagrams of every NEMA plug there is, I present you:


https://mafiadoc.com/wiring-devices...utomationdirect_59c5dc8e1723dd42ad224776.html
 
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kwired said:
But at same time if your range or dryer happens to contact something else connected to the EGC, maybe some other appliance, ductwork that has continuity back to the furnace, metal water piping, etc. you still have a connection between the two conductors they are trying to separate:huh:
Click to expand...

Like a grounded washer next to an ungrounded dryer

mama pajama puts a hand on both

aaaaannnnnnnddd.......


~RJ~
 
romex jockey said:
Like a grounded washer next to an ungrounded dryer

mama pajama puts a hand on both

aaaaannnnnnnddd.......


~RJ~
Click to expand...
If they are connected they should be same potential, if there is voltage drop on the dryer neutral then that potential will exist between the two appliances if not connected together.
 
kwired said:
If they are connected they should be same potential, if there is voltage drop on the dryer neutral then that potential will exist between the two appliances if not connected together.
Click to expand...


I am in agreement. Adding to what you wrote, it is that lack of connection which makes it so deadly.
 
DBoone said:
Connected together where?
Click to expand...
Anywhere, whether intentional or inadvertent.

Problem is with using the current carrying grounded conductor on the dryer to also use to bond the frame, you place any voltage drop on that grounded conductor on the frame as well. With separate EGC on both appliances as has been required for around 20 years now, no current on the EGC does leave them at same potential, a fault condition could leave them at different potential though
 
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