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240v sub panel wiring


John Dirks Jr
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Take a look at the picture at the link. It is a 240v sub panel that runs a clothes dryer and an AC unit.

http://home.comcast.net/~mailmanusa/240sub.JPG

The white sheathed cable runs to the AC unit and the cloth sheathed cable to the dryer. If you look closely you will see these things;

The common for the AC is grounded to the case. The common for the dryer is connected to the bus bar. The bus bar is grounded to the case.

Now there is more than one way to skin a cat for sure but is everything ok in this box? Everything is working fine.

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Bob, the way I understand it 22 volt appliances don't need a neutral, common or grounded conductor (depending on your choice of words) but should have a grounding conductor (the uninsulated wire).

The dryer is a special case because it has a 110 volt motor, so needs the neutral and the ground.

Jim K had a nice instructive diagram in a recent post showing how the 220 and 110 come from different areas of the transformer at the street, why 110 needs a neutral and 220 doesn't. Not sure if that's what you were asking. Brian or Jim can probably answer better.

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Originally posted by AHI

. . . The common for the AC is grounded to the case.

The AC doesn't have a common. That's its grounding wire. No, it shouldn't be terminated to a screw in the back of the enclosure.

Why does this grounding wire come out of the cable sheath, make a hairpin turn, go up and out of the enclosure, then turn around and come back in again?

The common for the dryer is connected to the bus bar. The bus bar is grounded to the case.

Improperly. You can't use a screw in the back of the enclosure as a grounding connection.

Now there is more than one way to skin a cat for sure but is everything ok in this box? Everything is working fine.

No, this panel is screwed up.

  • There's no ground/neutral separation.

The grounding connections are screwed (literally).

The dryer circuit lacks a grounding wire. (Maybe ok for an older installation, but since someone's going to have to fix this mess, that should probably be upgraded.)

You've got a breaker that's "off" in the up position.

Here's what happened. Originally, this panel just served the dryer. Back in the day, it didn't need a separate grounding wire and the breaker could be installed upside down. When they added the AC circuit, someone should have realized that they now needed a proper grounding wire and he should have upgraded the panel at that time. I'll bet that this happened after 1996, in which case the feeder should have been changed and the upside down breaker issue resolved.

- Jim Katen, Oregon

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I suppose the reason the AC ground wire loops out and back in again was to give it a second point for grounding to the box. Its clamped between the knockout connector.

So, I should;

1. Replace the dryer feed circuit to a 4 wire with ground.

2. Replace the breaker for the dryer for proper orientation.

3. Remove the wire that grounds the neutral bus to the case to keep grounds and neutrals separate.

4. Add a ground bus to the panel to provide proper connection point for grounds.

Can a ground bus be added to this panel or does the entire panel have to be replaced?

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Originally posted by AHI

I suppose the reason the AC ground wire loops out and back in again was to give it a second point for grounding to the box. Its clamped between the knockout connector.

So, I should;

1. Replace the dryer feed circuit to a 4 wire with ground.

3-wire with ground ought to do it.

2. Replace the breaker for the dryer for proper orientation.

I don't know of any way to do this other than turning the panel 90 degrees.

3. Remove the wire that grounds the neutral bus to the case to keep grounds and neutrals separate.

Yes.

4. Add a ground bus to the panel to provide proper connection point for grounds.

Can a ground bus be added to this panel or does the entire panel have to be replaced?

I believe that you can get a grounding terminal kit for the panel. Your electrician might have to drill & tap holes.

You'll need to separate the grounding and neutral conductors in the feeder. If the conduit is continuous, it can be the grounding conductor. However, if you re-orient the panel, you'll need to add a bend or come in through a different knockout.

- Jim Katen, Oregon

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OK great. Now I am very inquisitive so I will continue. I am comfortable being told it has to be, but I usually want to know why.

In the main panel the neutral bus should be bonded to the case, correct?

If so then in the main panel there is not separation of grounds and neutrals, correct?

The reason I ask is this;

The main and sub panels are bonded together via metal conduit. The main neutral bus is grounded to the main case.

Given these facts, no matter how you hook it up, how can you have separation of neutrals and grounds in the sub?

Dont the sub connections wind up bonding in the main via the metal conduit anyway?

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