3 current carrying wires for sub panel?

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Old 10-24-10, 07:05 PM
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3 current carrying wires for sub panel?

I have moved a sub panel from a shed to the house. This sub panel was wired by the previous owner.

The sub panel used 8-2 UM (plus ground) to feed the sub panel. This sub panel was protected by a 40A 240V breaker in the main panel. Was this code?

I have been told that the feeder must have 2 hots, a neutral, and the bare ground wire, since the neutral and ground busses in the sub panel must remain segregated when routed to the main panel.

True or not?
 
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Old 10-24-10, 07:13 PM
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Yes thats true. 4 Wire feeders must be used for all sub panels and all neutrals and grounds must be isolated. If the sub panel is a detached structure then a ground rod must also be installed.
 
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Old 10-24-10, 09:44 PM
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There is an exception IF the sub-panel ONLY serves 240 volt line-to-line loads. There would have to be a nameplate on the panel that stated 240 VOLTS ONLY.
 
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Old 10-25-10, 07:34 AM
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Sorry, I left something out.

For the sub panel, I may not need the 3 wire (+ ground) cable after all. The breakers in the box will be serving 120V circuits exclusively. The feed can be 120V. I plan to route the white and bare copper leads as in a main panel (to the neutral and ground busses, respectively) and install a jumper between the two hot busses and splitscrew the single hot to it.
 
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Old 10-25-10, 11:12 AM
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Originally Posted by Jon Fisher View Post
Sorry, I left something out.

For the sub panel, I may not need the 3 wire (+ ground) cable after all. The breakers in the box will be serving 120V circuits exclusively. The feed can be 120V. I plan to route the white and bare copper leads as in a main panel (to the neutral and ground busses, respectively) and install a jumper between the two hot busses and splitscrew the single hot to it.
i dont think you can do that. Ive never seen it done that way before, i dont think thats legal, and i wouldnt do it myself. That being said. Your installing a subpanel. Get a 4 wire cable, or run 4 wires in a conduit and do it the right way. Your on a forum asking for advice, and people here are telling you the proper, code wise way to do it.
 
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Old 10-25-10, 01:00 PM
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Electric Joe from previous posts in this forum I have seen pros suggest converting 240 breaker boxes to 120v breaker boxes is ok. The box of course must be labeled 120 volts only.

If this is a main lug panel he will need to jumper the two sides with a 2 pole breaker sized to the feeder as per Marc's instructions in a previous post.
 

Last edited by ray2047; 10-25-10 at 01:20 PM.
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Old 10-25-10, 02:07 PM
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Ok, then ill still ask why bother. Hes moving the panel anyway, which means moving or shortening the wire anyway. Whats another couple bucks to do it the right way. Im still not sure about that for 120v panels, i know you can do that for 240v only panels, and you dont need a neutral. But im just not understanding how it could be legal to split the wire and feed both busses from one hot conductor, and then also split it to attach to a breaker. even if you did do it, you would have to cut the amps in half on that conductor for the breaker sizing. You now have the ability to have both busses on the same conductor, so a 100 amp sub instead of being 2 conductors rated for 100 amps each, its now 2 busses both capable of 100 amps connected to one 100 amp rated conductor. Im just not understanding that, or how it can be legal.
 
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Old 10-25-10, 03:47 PM
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While a 120 only subpanel would be the exception rather than the norm, as long as the load is less than the feeder capacity I don't see a problem with this. At least it allows the ground to be isolated from the neutral.
 
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Old 10-25-10, 05:57 PM
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Decision time

Thanks, all of you, for your input.

The local consensus (of those who know about the local inspectors) is that the jumper wire caper would likely get flagged. The rule about using equipment for its "original intended purpose" holds some water, but the idea would work technically. In fact the box cover label even shows two wiring diagrams (one for 120V and one for 240V), but I have no idea how old the box is. The box did not have a grounding bus, it only had an insulated bus which was originally intended for neutrals AND grounds I suppose).

But, since I don't want to get into a big thing over code, I bit the bullet and got the 8-3 (+G) cable and delivered the 2 hots, the ungrounded neutrals and the grounds to the main panel for grounding there. The wire was not cheap ($2.65/foot), but it is bulletproof code.
 
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