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3 Wire or 4 Wire for sub-panel?

TP_Bosworth's picture

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Kevin, you're gonna get hosed if you follow the electrician's suggestion. My bet is that the existing range wire - which was just fine a few years ago - has only 3 conductors. That's simply not sufficient to wire a subpanel. Today, the code requires 4-wire to ranges (two hots, neutral, ground). You might have been able to pull that one off IF the wire was sized sufficiently to accommodate the range plus whatever else needs circuits.

(post #176446, reply #17 of 19)

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Who is paying him?
And who covers the warranty?
For a 240v sub-panel you need 4 wires. red, black, white and the "ground". Ground and neutral are not the same.
If only 120v sub panel then you might sneek by with 3 wire but I would feel safe with a new wire run.
If you are paying him you tell him what you want!!

(post #176446, reply #18 of 19)

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I just had a client tell me that the electrician I sent to look over the job, a kitchen remodel,stated he could cut the range plug off and use the same wire (a 3 conductor) for the line to the sub panel located under the kitchen,where I asked that it be put. Other sub-panels that I have had installed were fed w/ 4 conductor cables. Several jobs ago this fellow cost me quite a bit of time when he damaged floors in the process of rushing through a heating installation. Is he just trying to rush through this job without any regard for doing the job right.

(post #176446, reply #1 of 19)

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Hope it's OK, because I did exactly what he proposes. The cable was "3-wire plus ground," maybe that's the confusion? ground and neutral should NOT be bonded at the subpanel.

(post #176446, reply #2 of 19)

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Kevin - It almost seems like you are asking about this guy, rather than what he is doing. Whenever I have doubts about someone like that, I start looking around for someone else. Your reputation will live and die with the subs you use. Sorry for the presumption, if I misunderstood your question. - yb

(post #176446, reply #3 of 19)

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Article 384-20 of the NEC says all panel boards are to be grounded in accordance with article 250. The only place I see where Article 250 will let you subfeed a panel with 3 conductor wire is a seperate building with it's own grounding electrode system.

If the subpanel in question is going to be 110 volt only, then I can see how 3 total conductors would be fine. 1 hot, 1 neutral, 1 ground.

Was your electrician going to simply pull a seperate ground wire for this panel so that it could be 220v?

-Rob

(post #176446, reply #4 of 19)

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Is he proposing that the sub panel not be grounded? Wrong. Is he proposing that the ground and "neutral" be combined in the sub panel? Wrong. Is he proposing that this cable be installed in metalic conduit that will serve as the ground? OK.

(post #176446, reply #5 of 19)

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It depends on what loads you want to serve. NEC section 215-7 allows you to tap the two hot conductors without bringing over the neutral. Of couse, you could then only feed 240 loads from the sub-panel. If you want 120V loads as well, you need 4 wires.

(post #176446, reply #6 of 19)

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Young Bob,
I already know about the guy, that's why I questioned the use of the old feed wire. I know that he's just trying to save time, figuring that I'm no wiser to 3 or 4 con.
I feel the same way, I'm in the hotseat when problems come up in, say 6 months. Problem is that he's sorta friends w/client ( both in by-pass unit at same time) and if I contravene his authority as knowing what he needs to do for the job I'll p*** both of them off.
Or I need to explain why and decline the job.
My reputation is worth more than somebody saving a hundred or so on doing the job right.

Why don't exchanges like this take place at the local lumberyard?
thanks kevin

(post #176446, reply #7 of 19)

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Are there no inspections?

(post #176446, reply #8 of 19)

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I learned at one of those personal growth seminars years ago that the best way to shut down a relationship is to give unasked for advice.

Nonetheless, if I had a choice between accepting (and taking responsibility for) unprofessional - and in this case possibly dangerous - work, or losing the job, I'd walk away from the job 10 days out of 10.

One of the big problems with our trade is that we are constantly exposing ourselves to liability that far exceeds our potential for profit. I try to limit my exposure and there is no way I would assume any responsibility for less than safe work. If your customer is a reasonable person, he will respect this. - yb

(post #176446, reply #9 of 19)

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He should still have a ground conductor(insulated) even if he runs metallic. Conduit should "be grounded," not "be the ground".

Scott

(post #176446, reply #10 of 19)

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The "grounded conductor" is the neutral.

The "grounding conductor" is the ground and the use of metalic conduit as a "grounding conductor" is approved under the NEC and is standard practice.

I was wrong however to suggest that he could use this cable with conduit as a ground. If the cable is indeed 2 wire with a ground than that third wire is not an acceptable neutral. Use as a neutral requires inuslation the same as the other two conductors and most cable ground wires are not insulated, only sheathed.

(post #176446, reply #11 of 19)

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This separation of the gnd and neutral is required in the sub panel in instances where the the two are bonded in the main panel, right.(?) I do remember seeing this.But does this mean that the main panel gets un bonded or you just install a lug for the ground conductor ? I'm sure about what has to be done just concerning myself unnecessarily with the details.
thanks
kevin zale

(post #176446, reply #12 of 19)

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Kevin, If you are going to have a 120/240 volt subpanel, you must have the following: Three INSULATED feed wires ( 2 hots and a "neutral") and some type of grounding conductor (wire or metal conduit) Most new buildings require an insulated grounding wire inside the conduit so you don't have to rely on the conduit as your grounding conductor. At the subpanel, you must not bond the neutral busbar to the panel enclosure ( it must float) You will have to install a separate bar for the grounding wires and this will be bonded to the enclosure. If you have a subpanel with a strap connecting a neutral and grounding bar ( usually located on opposite sides of the panel) , simply remove the connecting strap and make sure the neutral bar is isolated from the enclosure and the grounding bar is bonded to it.

(post #176446, reply #13 of 19)

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The Square D subpanel I bought spelled all this out really clearly. I think the default condition was for the box not to be bonded. The idea is to have only one path to ground, all the way back to the master panel.

(post #176446, reply #14 of 19)

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Kevin, you're gonna get hosed if you follow the electrician's suggestion. My bet is that the existing range wire - which was just fine a few years ago - has only 3 conductors. That's simply not sufficient to wire a subpanel. Today, the code requires 4-wire to ranges (two hots, neutral, ground). You might have been able to pull that one off IF the wire was sized sufficiently to accommodate the range plus whatever else needs circuits.

(post #176446, reply #15 of 19)

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In reference to TP's post- same goes for the dryer, actually I think this applies to any 240VAC device that is a host to a 120VAC powered accessory.

S.

(post #176446, reply #16 of 19)

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... of course he could do a 120V subpanel, right? kind of rinky-dink I guess

(post #176446, reply #19 of 19)

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The cable might be bx or greenfield which might be sufficient to carry the ground . This stuff, three-wire, usually carries an insulated neutral. Good idea? I don't know, I'm not there. Lee