Sub Panel Question

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  #1  
Old 05-09-05, 02:46 PM
Don W's Avatar
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Sub Panel Question

I'm feeding from the main box in a new home to a sub panel in the basement .

A description of interior of the sub box is 2 supplies in the middle lugs, flanked by a neutral to either side of the supplies (lug on right neutral) with a strap going across and joining the neutrals together, and a separate ground bus. I disconnected the strap, and am using the "right-side" bus as neutral and the "left-side" as ground. I did this because I was picking up continuity from the additional ground bus and the sub-panel, and thought that when I energized the circuit, that there would be a potential on the box itself.

I'm using 6/4 wire with a 60 amp breaker at the main from 200 amp service. At the main, the interior is 2 supplies flanked by a ground/neutral bus on either side. The electrician used each bus for ground and neutral.

I energized the breaker feeding the sub and I am picking up 120v going across either of the supplies and the box itself! Is there a potential on the box? HELP! What did I do wrong? The circuits being fed from the sub are working o.k. (2 circuits for basement, total 4 outlets). I am afraid that the box is hot.
 
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Old 05-09-05, 02:54 PM
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You should be getting 120 volts between each of the supplies and the box. This is normal (and essential).

Whether or not it was correct to use the "right side" for neutrals and the "left side" for grounds is not possible to tell from here. The grounds should be on the bar that is electrically bonded to the panel, and the neutrals should be on the bar that is electrically isolated from the panel. Note that you can't use an ohmmeter to verify this now that the panel is all connected up, because even though the correct neutral bar is not bonded to the subpanel (as it must not be), it is electrically connected to the subpanel via the bonding in the main panel.
 
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Old 05-09-05, 03:13 PM
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Old 05-09-05, 03:14 PM
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John,
Thank you for responding. My thought was not to connect the grounds to the bus that was electrically bonded to the box, because if a neutral opened on a circuit it would take the ground and then the box would become "hot". I will change the "left-side" bus back to an unbonded neutral, and take the grounds to the bonded ground bus. Thanks again.
 
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Old 05-09-05, 04:18 PM
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Since you won't have the ground and neutral bonded together in the subpanel(as is required for any properly wired subpanel) that scenario will never happen. An open neutral in your subpanel would result in a high-voltage situation for your 110 circuits (sharing a neutral from opposite legs of the incoming service) in your subpanel. Possibly just dead circuits for the supanel in certain instances.

You want the ground bar to be bonded to the cabinet of the subpanel.

The only place you want the neutral and ground bonded together is in the main service panel. You want to make certain an open neutral condition will likely never happen for your subpanel. (This is why you never want to fuse a neutral.)
 
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Old 05-09-05, 04:49 PM
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Oh, and an open neutral on one of your 110 circuits in the subpanel will result in an inoperative circuit (essentially open). There is nothing I can think of that would cause an outlet to start using the ground wire as a neutral in the event of an open true neutral.
 
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Old 05-09-05, 05:23 PM
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Originally Posted by Don W
John,
Thank you for responding. My thought was not to connect the grounds to the bus that was electrically bonded to the box, because if a neutral opened on a circuit it would take the ground and then the box would become "hot". I will change the "left-side" bus back to an unbonded neutral, and take the grounds to the bonded ground bus. Thanks again.
Your best bet is to try not to re-think something that was engineered and works properly.
Put the bar across the neutrals back in, leave out any bond strap/screw, keep the two hot busses for the hots and ground the ground bar.
A sub-panel is a very simple thing with little to go wrong. Basically you either leave the bond screw in or take it out. The rest you leave alone.
 
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