I have a subpanel at my garage. It has 2 hots, one neutral and one ground from the service panel. The neutral and ground in the subpanel are not bonded. All the outlets including GFCIs all work correctly. When I test outlets with my DMM in the garage I have 120v between neutral and hot but do not have 120v between ground and hot like I do in the main house. Is this normal?
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Turn off the feeder breaker for the sub in the main panel, then go to the subpanel and use your DMM to check for continuity between the neutral and the ground there, then report the results, please.– ThreePhaseEelDec 31, 2018 at 4:17
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Also, is this an attached or a detached garage, and is the feeder wired in conduit or using a cable?– ThreePhaseEelDec 31, 2018 at 4:19
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Detached garage, feeder in underground conduit. I'll check that tomorrow. I assume since the ground and neutral are not bonded in the subpanel that not getting 120v between ground and hot on an outlet is normal?– Don_JuanDec 31, 2018 at 5:20
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Is the feeder conduit metal or plastic?– ThreePhaseEelDec 31, 2018 at 12:19
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Thanks! I did find the ground wire coming out of the garage going into the conduit next to the garage not connected yesterday. I connected them and also connected them to the galvanized underground water pipe next to the conduit. But still no 120v between hot and ground at subpanel or outlets. I will have to check for continuity between neutral/ground at subpanel today. Yes the neutral and ground are bonded at the main panel and the ground goes into the conduit toward the garage.– Don_JuanDec 31, 2018 at 15:20
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1 Answer
You were right to be suspicious and it does sound like you have a problem. You should see 120V from hot to ground in your garage, just like in your house. The fact that you don't is an indicator that there is a problem with the ground to your garage.
The ground and neutral should not be bonded at the sub-panel, so the wiring there sounds correct.
Does the ground you see at the sub-panel actually run along with the feeder back to the main panel?
If not: Your installation is not compliant with the NEC1. A sub-panel needs to have a ground wire (separate from the neutral) which runs back to the main panel, as well as a local ground rod. See this Q&A: Is it ok to use a ground rod at a subpanel instead of a fourth wire?
If so: Then it sounds like you have a problem with the ground wire somewhere between the main panel and the sub-panel.
Turn off the feeder breaker for the sub in the main panel, then go to the subpanel and use your DMM to check for continuity between the neutral and the ground there, then report the results, please.
You should see ~0 ohms between neutral and ground, indicating that both conductors are good back to the main panel, and that they are properly bonded together (hopefully at only one point).
Based on your voltage readings, you will probably see ~∞ ohms. From there I would look in the main panel where the sub-panel feeders enter. You should see the neutral and ground enter and connect to their respective bus bars (or the same bus bar if that is the main bonding point).
I'm guessing that your sub-panel ground either doesn't go back to the main panel, or it isn't connected.
1 - The NEC used to allow the use of a single wire for neutral and ground, both for sub-panels and for 120/240V appliances, but this is not the case any more. All 120/240V circuits must now be four-wire.
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Depending on the year built 3 wire was legal and even though codes change that install is still legal(if installed prior to the code change) the ground wire can now be separate from the cable (now allowed to ground old 2 wire systems). As far as the resistance it should be close to zero but may be much higher based on distance and the quality of the meter.– Ed BealDec 31, 2018 at 9:40