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Electrical Subpanel Grounding Safety Issues (Read 4374 times)
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Electrical Subpanel Grounding Safety Issues
May 7th, 2015 at 4:49am
 
You may have a Subpanel already or may be adding one for your grid ties out in a shed.

THIS INFO IS FOR INFORMATIONAL AND EDUCATIONAL PURPOSES ONLY - HAVE A PROFESSIONAL ELECTRICIAN DO ALL WIRING.

At a normal main electrical panel on a house, the neutral and ground connection busbars are electrically connected together and to the box metal case, and also to a long 6 or 8 ft. buried ground rod. This is your main ground location for the house.

On a subpanel some distance away, say 100 ft. the neutral and ground will be separated, not electrically connected together. A subpanel is wired using a 4 conductor wire, one wire is for ground from the main panel.

The reason you want your neutral isolated on a subpanel is this:

Voltage drop across the long wire or possible connection fail at a wire end.

For an example, picture a 1800W 120V heater connected between one side of the 240V wire and neutral wire at the shed end (normal 120V circuit) WITH NO BOX for this THOUGHT PROCESS - NOT REAL - example only . Say the wire run is 100 ft.

First, both connected wires are going to have a voltage drop both the same amount, let's say 2V per side. Why? Because there are amps flowing down both wires, about 15A.

There is also a separate ground wire coming to the shed, it's connected also to the neutral at the main box side.

If you got your meter out at the shed end and measured from that ground wire to neutral, you would see the 2V drop and also NOTE THERE IS NO VOLT DROP FOR THE GROUND WIRE, because it has no amp flow. Same as if you had super long meter leads running over to the main panel for testing.

The ground should never have any amps flowing down it, it is there for safety only.

So if you ever connected the ground wire at the shed to the neutral at the shed, you have caused the ground to take some of that amp flow from the 2V difference.

If the neutral connection to the main panel started to fail the ground wire would have to take all the current flow! THAT IS THE DANGER because now everything metal out at the shed is now part of a amp flowing circuit.

If you didn't get that then you should draw it out on paper.

So, what you should have on a subpanel is a insulated and thus isolated busbar for neutral connections (white wires) and another metal busbar for ALL ground wires and that busbar is screwed to the metal box to ground it too.

Now your box metal case and any other grounds are completely separate from a neutral that might be loose or something. Meaning no matter what they are ground only. GFCIs will work correctly too.

If you wanted to you could also add another ground rod near the subpanel and connect it to the subpanel ground busbar. Your local electrical code may require one so check that.

THIS INFO IS FOR INFORMATIONAL AND EDUCATIONAL PURPOSES ONLY - HAVE A PROFESSIONAL ELECTRICIAN DO ALL WIRING.
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