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Electrical Code - USA Commercial or Residential 1999 / 2002 / 2005 versions - for UNITED STATES

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  #1   IP: 204.210.131.186
Old July 29th, 2008, 07:05 PM
Ashokan1 Ashokan1 is offline
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Default small subpanel grounding

A job I'm on has a small 240v 20A - 2 space/4 circuit - subpanel located outside for some minor use on a farm. One 20A circuit will feed a small mobile trailer on the property. There was no existing fourth wire run from the main panel in the house so the subpanel needs to be grounded. Can I drive in grounds at the panel if there is no other grounded lines (tel, CATV...) from the house nearby? Do I need two 8' ground rods for such a small panel? Thanks.
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  #2   IP: 75.11.36.143
Old July 29th, 2008, 07:50 PM
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Roger Roger is offline
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Is this sub-panel on the same structure as the main house panel and is the main disconnect located in the house panel ...ie...main breaker panel.... or is the main disconnect out on a farm pole and a 4 wire feeder going from the disconnect to the house panel?

I assume you mean a 20 amp 120/240 volt circuit to the sub panel.

Last edited by Roger : July 29th, 2008 at 07:52 PM.
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  #3   IP: 204.210.131.186
Old July 31st, 2008, 06:51 PM
Ashokan1 Ashokan1 is offline
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This subpanel is located on a post on the side of a field where a small RV is parked. It is about 100' from the house where the main service panel is located.
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  #4   IP: 76.104.172.149
Old August 1st, 2008, 08:04 AM
suemarkp suemarkp is offline
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Yes, all detached panels require a ground electrode. And code would require two for this panel, as much of a waste as it seems. The only out, which isn't a reliable one, is to say that the one ground rod you drive at the outside panel is supplemented by another electrode on the property. In order to use that rule, you'd need to have a conductor sized per table 250.66 going from the subpanel ground electrode to the main panel ground electrode. Your neutral is this conductor, bus it is #8 copper or larger (which is the smallest value in table 250.66)?
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  #5   IP: 75.11.36.143
Old August 1st, 2008, 12:10 PM
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Take the panel off and convert to a weather proof switch box. Install a double pole snap switch. You didn't clarify but convert to 120/240 multiwire at the house panel if this isn't already what you have. A 20 amp multiwire branch circuit to that pole would not require any ground rods at all. The switch would be your required disconnect for denergizing both ungrounded conductors. You would then have the option of splitting the multiwire into two 20 amp 120 volt branch circuits from the switch box. The switch must be rated 15 amps if one branch circuit is supplied from the pole or 30 amps if two are supplied.

Last edited by Roger : August 1st, 2008 at 12:14 PM.
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  #6   IP: 64.83.225.185
Old August 1st, 2008, 01:54 PM
junkcollector junkcollector is offline
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Quote:
Originally Posted by Roger View Post
Take the panel off and convert to a weather proof switch box. Install a double pole snap switch. You didn't clarify but convert to 120/240 multiwire at the house panel if this isn't already what you have. A 20 amp multiwire branch circuit to that pole would not require any ground rods at all. The switch would be your required disconnect for denergizing both ungrounded conductors. You would then have the option of splitting the multiwire into two 20 amp 120 volt branch circuits from the switch box. The switch must be rated 15 amps if one branch circuit is supplied from the pole or 30 amps if two are supplied.
It's a good idea, the problem is that he says that there are only 3 wires going out to the pole (hot, hot, and nuetral) You would still need a 4th ground wire to the pole, ground rods or not. (feeder or mulitwire)
Quote:
There was no existing fourth wire run from the main panel in the house
Ashokan, which version of the electrical code are you under?
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Old August 1st, 2008, 03:13 PM
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Thanks Junk, I didn't read the 4th wire not being there. He says only one branch circuit to a trailer so he could convert to 120 volts by re-identifying a hot for egc. I am assuming here he is smaller than #6 with what he has in the ground now. Technically he would likely need his inspectors approval to re-identify smaller than #6 but it is an option if only one 20 amp branch circuit is needed. My personal opinion is that a ground rod or two likely isn't going to add much protection if you don't know the resistance. Under the NEC if the 1st ground rod had 100 ohms resistance then all I have to do is add a second and I'm good to go and I may still have 100 ohms of resistance. So I wonder what the point is...


Maybe we could wish for metal conduit....

Last edited by Roger : August 1st, 2008 at 04:02 PM.
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  #8   IP: 64.83.225.185
Old August 1st, 2008, 05:01 PM
junkcollector junkcollector is offline
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Quote:
Originally Posted by Roger View Post
He says only one branch circuit to a trailer so he could convert to 120 volts by re-identifying a hot for egc. I am assuming here he is smaller than #6 with what he has in the ground now. Technically he would likely need his inspectors approval to re-identify smaller than #6 but it is an option if only one 20 amp branch circuit is needed. My personal opinion is that a ground rod or two likely isn't going to add much protection if you don't know the resistance. Under the NEC if the 1st ground rod had 100 ohms resistance then all I have to do is add a second and I'm good to go and I may still have 100 ohms of resistance. So I wonder what the point is...
Well said, Roger, I couldn't agree more.
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  #9   IP: 204.210.131.186
Old August 1st, 2008, 07:32 PM
Ashokan1 Ashokan1 is offline
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In NY we are under the 2005 NEC and the 2007 NYS building code for residential.

The homeowners idea was to have circuit breakers at the outside location for convenience. The subpanel is fed with three old #10 direct buriel single strand wires. I have 2x 20A circuit breakers in the subpanel and it is protected in the house by a 20A 2-pole.

It's an old farm I'm just trying to straighten it up a bit to make it more safe. We spent today putting open-air splices into j-boxes w/covers in the basement. It's a real old mess. Unfortunately, that's the condition of a lot of the old farmhouses in NY state. Old wiring with bad workmanship over the years.
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  #10   IP: 64.83.225.185
Old August 1st, 2008, 08:05 PM
junkcollector junkcollector is offline
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Quote:
Originally Posted by Ashokan1 View Post
In NY we are under the 2005 NEC...
2005 NEC does allow 3 conductor feeder, but you need a grounding electode system. So you have two options either do that or downgrade it to a single 120 volt circuit.. In my opinion you could keep the 20 amp breaker in the small subpanel. It can be argued that having a 2 redundant breakers of the same size like that is still a branch circuit. (Others, including the inspector, may disagree)

Is there another circuit in this panel too? You mention the one, and it is a two space loadcenter. I was just wondering if 240 is even needed out at the pole.
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