Checking ground on a subpanel

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Old 03-13-13, 09:17 AM
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Checking ground on a subpanel

We have a subpanel in our condo (no main shutoff). We had an issue with neutrals and grounds intermixed. I made sure the neutral buss was not bonded to the panel and then put all ground wires on grounding buss and all neutrals on neutral buss.

Panel appears to be connected with metal conduit in order to ground it and 3 wires (red, black, white).

If I get 110 v between hot and ground at any outlet, wouldn't that mean that subpanel is correctly grounded back to service entrance (or at least before it enters our condo), which is all I can really check?
Thanks
Dave
 
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Old 03-13-13, 09:30 AM
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Actually the reading should be closer to 120 volts 10% since nominal voltage is 120 volts. The metal conduit is serving as your ground. As long as it doesn't become disconnected at a joint it should be okay. The down side is exposed conduit often gets abused* if exposed in public areas and can loosen at the joints.

*Dogs tied to it, kids swinging on it, movers bumping into it, 280 pound drunks needing a little extra support, etc.
 
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Old 03-13-13, 10:08 AM
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But with a subpanel, there really is no way to ground it besides using the existing conduit. Unless a new ground wire were run down 7 stories.
 
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Old 03-13-13, 11:13 AM
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Yes, that would be the way.
 
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Old 03-13-13, 11:45 AM
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We have a subpanel in our condo (no main shutoff). We had an issue with neutrals and grounds intermixed. I made sure the neutral buss was not bonded to the panel and then put all ground wires on grounding buss and all neutrals on neutral buss.
That was a needed improvement. Glad you got that corrected.

But with a subpanel, there really is no way to ground it besides using the existing conduit. Unless a new ground wire were run down 7 stories.
If that's where the conduit feeding your unit originates, then yes. The odds are, though, that your supply conduit originates in an electrical closet on your floor, and that the panel that it starts from, where your main breaker is located, is bonded to ground. Pulling an insulated ground wire from that main panel to your subpanel sould help insure the low-resistance path to ground.

Reading 120V hot-to-ground does not measure the resistance of the path to ground, unfortunately.
 
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Old 03-13-13, 05:46 PM
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If I get 110 v between hot and ground at any outlet, wouldn't that mean that subpanel is correctly grounded back to service entrance (or at least before it enters our condo), which is all I can really check?
If the metallic conduit was properly installed with bonding bushings at concentric knockouts and the conduit bushings bonded to the panel boxes (at both ends) there should be no problem.
 
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Old 03-13-13, 07:36 PM
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If you have access to an accurate voltmeter....... check between ground and neutral.
You should see no voltage.
 
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