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Old 02-16-2010, 09:02 AM   #16
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It looks like, according to 310.13, you should be using #8 wire for a 40 Amp breaker.
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Old 02-16-2010, 09:04 AM   #17
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Use the 75c degree column for THHN
If buying cable use the 60c column





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Old 02-16-2010, 09:08 AM   #18
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If I am unable to read the outer sheathing/insulation of the exisitng feed is there another way of determining the size/type of the existing wire?
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Old 02-18-2010, 08:15 AM   #19
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Getting back to the grounding issue. I noticed the existing sub panel has a gound wire going from the neutral bus to a copper water line. Should I wire the replacement sub panel the same or do as previously stated and install a seperate grounding bus?
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Old 02-18-2010, 08:24 AM   #20
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A subpanel MUST have its neutral and ground bars isolated from each other.

Are your water lines ONLY bonded to the subpanel (ie. not to the main panel)? I am thinking it should be bonded to your main panel, not a subpanel. The ground wire feeding your subpanel may not be large enough to handle the amount of current that could be in that bonding wire if an item on a breaker larger than your 40Amp feeder shorts to a water pipe.

If you had, say, a 60 amp breaker in your main panel feeding a second subpanel. On this other subpanel is a water heater. This water heater shorts to the water pipe. Now you have 60 amps of current traveling through the water pipe, to your 40 amp subpanel, then back to the main panel through the ground wire feeding that 40amp subpanel. Since that wire is feeding a 40 amp panel, it's probably not large enough to handle 60 amps, you could have a fire.

I hope this makes sense, it does to me
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Old 02-18-2010, 09:17 AM   #21
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So what you are saying is to get rid of the existing groud wire to the copper pipe and install a seperate ground bus in the replacement panel?
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Old 02-18-2010, 09:25 AM   #22
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Make sure your water pipe is bonded to your main panel's ground bar, both before and after the meter.

And, yes, install a separate ground bar that is isolated from the neutral bar in the sub.
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Old 02-18-2010, 10:08 AM   #23
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If I get rid of the wire from the sub to the pipe, why would I have to make sure the ground is bonded to the main panel?
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Old 02-18-2010, 10:25 AM   #24
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I'm not exactly sure what you're asking, but here's some info about why water pipes are bonded to your ground bar in the panel.

Your water pipe needs to be bonded to the main panel in case your water pipes get electrified due to a short in a washing machine, water heater, etc. If, say, the frame of your washing machine inadvertently got electrified (black wire shorted to the case), the water pipes could now be electrified. This electricity would travel down the water pipe, to your bonding wire, back to the main panel, and back to the PoCo so fast that it will trip your breaker almost instantly, before you accidentally get the clothes out of the washer
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Old 02-18-2010, 10:33 AM   #25
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Here is a quick diagram of how your main panel should be bonded with your water pipes. It has to make contact both before and after the meter, in case the meter is removed, your entire system is still grounded.


Here is what a clamp looks like that connects your ground wire to the water pipe. Wire goes through the hole on top, pipe through the big hole.
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Old 02-18-2010, 10:53 AM   #26
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gottcha. i believe there is a ground from the meter to the main panel, but I will double check. If this is the case, is it ok to remove the wire now at the sub and install new grounding bar as explained above?
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Old 02-18-2010, 10:58 AM   #27
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I think yes, since your water pipes will be bonded to your main panel's grounding bar, but wait for one of the more experienced guys to verify this.

When it's done, it should look like this:

You will have 4 wires feeding the subpanel from the main panel. 2 hots (from double pole breaker), neutral (from neutral buss bar), ground (from ground bar or shared neutral/ground bar).

The two hots will connect to the two lugs (or breaker) in your subpanel. The neutral wire will connect to the the neutral bar in your sub. The ground wire will connect to your ground bar, which will be isolated from the neutral bar.
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Old 02-18-2010, 11:57 AM   #28
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SOunds good , thanks!
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