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Old 02-07-2008, 08:53 AM   #1
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Subpanel installation in outbuilding


I've read many questions here relating to my situation, but still have questions. Since this installation will be permitted and inspected by the county, I'd like not to have any major errors in my plan. Here's the situation: I need to provide a 240V 40 amp circuit to a small greenhouse about 50 feet from my house. The main panel in the house is a 200A Square D. I purchased an outdoor Square D 100A panel to use as a subpanel in the greenhouse. The circuit will be fed from a 40A breaker in the main panel and will be powering a 240 volt portable heater (rated at 23.3 amps, with a 30A socket), a 15A 120V convenience outlet (GFI in the subpanel), and a 15A 120V dedicated circuit (no outlets) powering a couple of small air circulation blowers (2.4A total load). The total distance from the main panel to the subpanel is 85 feet, of which 45 feet will be underground and 40 feet will be through the house crawlspace to the main panel.

The underground section will be 1" schedule 40 PVC buried 18 inches. I plan to run individual #8 THWN wires (black, red, white) and a #10 green insulated ground wire through the conduit. I spoke to the chief inspector for the county (he's NOT an electrician and doesn't do the actual inspections) and he didn't think that I needed a ground rod and to separate the neutral and ground connections at the subpanel. Answers I've read here lead me to believe that might not be correct.

There are other 240 volt circuits in my house right now, including A/C, and
those plus many of the 120V circuits are NOT in conduit nor are they run through holes in the floor joists. They're just stapled to the joists, so thought I'd do the same with the new circuit. But, this seems to be at odds with other answers I've read.

So - what are the problems with what I plan to do? Thanks for any help.

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Old 02-07-2008, 09:10 AM   #2
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Subpanel installation in outbuilding


"There are other 240 volt circuits in my house right now, including A/C, and those plus many of the 120V circuits are NOT in conduit nor are they run through holes in the floor joists. They're just stapled to the joists, so thought I'd do the same with the new circuit. But, this seems to be at odds with other answers I've read."

So what part are you questioning? You can run the conduit as stated but the individual conductors cannot be run exposed to the main panel without protection of conduit. You can run conduit all the way back to the main (much work!) or alternately, you could terminate the individual wires at a junction box where you convert from the conduit to the appropriate cable to go to the main panel.

Regarding the ground rod, you would have a ground conductor running from the subpanel back to the main and the main should have the only connections to ground rods so this should be fine. You must keep the neutral and ground seperated in the subpanel.

You might want to locate the electrical inspector to see if the plan is acceptable there.

Electricians on this board may have some other ideas.

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Last edited by handyman78; 02-07-2008 at 09:18 AM.
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Old 02-07-2008, 09:18 AM   #3
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Subpanel installation in outbuilding


I had planned on a junction box in the crawl space to terminate the individual conductors and then run a jacket cable from there to the main panel. Regarding the ground in the subpanel, I guess I'll have to find a ground bar for the subpanel as it didn't come with one. There's a bar for the neutrals, but not for the ground wires. Are the ground bars unique to the particular make/model of subpanel, or can I use any of the ground bars sold at Home Depot for this? I believe that they must be attached to the housing using a threaded screw hole.
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Old 02-07-2008, 09:22 AM   #4
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Subpanel installation in outbuilding


I was in Lowes the other day and saw that ground bars are offered by different manufacturers for their own panels so this shouldn't be a problem. Make sure that there is a bond from the subpanel to the ground bar only, not to the neutral bar.
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Old 02-07-2008, 10:15 AM   #5
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Subpanel installation in outbuilding


The greenhouse is a detached structure. If you are installing a subpanel, you are required to also install a grounding electrode (rod). This will attach to the grounding bus.
Use a grounding bus specific for your panel. It should fit in holes already in the panel.
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Old 02-07-2008, 10:21 AM   #6
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Subpanel installation in outbuilding


The 240 volt circuits in your house (dwelling) are branch circuits. The wires running to the sub panel are feeders. That is why it is a different circumstance.
As handyman said, seperate the grounds and neutrals in the subpanel. Drive one ground rod at the sub panel and bond the rod to the ground terminal strip in the sub. Connect the ground from the source to this terminal strip also. If you have metal water lines at the green house bond to them and land them on the ground terminal. No action needed if the water pipes are plastic.
You must have four wires running to the sub. 2 Hots 1 Neutral and 1 Ground.
Green houses are wet environments. You may need to protect the feeder with a GFCI breaker. For sure you will need GFCI for any recepts in the green house. Also you may need to drive 2 ground rods. Might need a weather resistant panel enclosure? Talk to the inspector about wet locations.
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Old 02-07-2008, 10:24 AM   #7
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Subpanel installation in outbuilding


Thanks Househelper and J.V. for the correction and clarification re: ground rods at the subpanel.
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Old 02-07-2008, 02:04 PM   #8
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Subpanel installation in outbuilding


I don't think they will flag you but the minimum feeder in your situation must be 60 amps.

Copied section Art. 225 Outside feeders and branch circuits

Note the below requirements are talking about the number of branch circuits that supply utilization equipment within the structure unless otherwise noted.

The feeder or branch-circuit disconnecting means for a structure must have an ampere rating not less than the calculated load determined per Art. 220 [225.39]. But observe the following:
  • One-circuit installation The disconnecting means must have a rating at least 15A.
  • Two-circuit installation The feeder disconnecting means must be rated at least 30A.
  • One-family dwelling The feeder disconnecting means must be rated at least 100A, 3-wire.
For all other installations, the feeder or branch-circuit disconnecting means must be rated at least 60A.
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Old 02-07-2008, 02:23 PM   #9
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Subpanel installation in outbuilding


Since code states that 60A is minimum for sub feeder... why not just run a #6 for 60A. You will pay a little more but will be within code and never have to worry about not having enough power.
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Old 02-07-2008, 02:24 PM   #10
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Subpanel installation in outbuilding


Quote:
Originally Posted by Stubbie View Post
I don't think they will flag you but the minimum feeder in your situation must be 60 amps.

Copied section Art. 225 Outside feeders and branch circuits

Note the below requirements are talking about the number of branch circuits that supply utilization equipment within the structure unless otherwise noted.

The feeder or branch-circuit disconnecting means for a structure must have an ampere rating not less than the calculated load determined per Art. 220 [225.39]. But observe the following:
  • One-circuit installation The disconnecting means must have a rating at least 15A.
  • Two-circuit installation The feeder disconnecting means must be rated at least 30A.
  • One-family dwelling The feeder disconnecting means must be rated at least 100A, 3-wire.
For all other installations, the feeder or branch-circuit disconnecting means must be rated at least 60A.

Stubbie, those are disconnect ratings not feeder sizes... No where does it state what size feeder to pull only what the minimum disconnect must be...

I can easily pull # 10's out to a shed install a 60 amp disconnect and meet code... the OCP would be installed at the house...

Last edited by chris75; 02-07-2008 at 02:26 PM.
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Old 02-07-2008, 04:53 PM   #11
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Subpanel installation in outbuilding


I thought that is what I was trying to say and I quickly read over this (my mistake for the day) and read it to say 40 amp breaker at the main panel feeding an mlo 100 amp sub panel. Feeder in the first sentence was a real bad typo should have been disconnect. I was trying very badly to say a 40 amp breaker for a disconnect also serving as the ocpd for the feeder was not going to be compliant for a 3 branch circuit installation. The six handle rule under my understanding doesn't apply to a detached building like this..... 225.36 requires the disconnecting means to be marked as suitable as service equipment. Most panelboards have instructions that they are suitable as service equipment when less than six handles are installed and the panelboard is not used as a Lighting & Appliance Branch Circuit Panelboard .... often overlooked on feeders to detached structures. A panel containing three branch circuit handles would make the panel not suitable for service equipment, and a main disconnect would be required.
In most shed type installations this will limit the disconnect to one handle and in this case it would need to be 60 amps not 40.

That being said.. good catch and I apologize for really goofing that one up.
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Old 02-07-2008, 05:06 PM   #12
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Subpanel installation in outbuilding


Quote:
Originally Posted by HouseHelper View Post
The greenhouse is a detached structure. If you are installing a subpanel, you are required to also install a grounding electrode (rod). This will attach to the grounding bus.
Use a grounding bus specific for your panel. It should fit in holes already in the panel.
OK - thanks to all for comments on the grounding. I understand and will handle that properly.
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Old 02-07-2008, 05:48 PM   #13
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Subpanel installation in outbuilding


Stubbie - I'm afraid I don't follow your point. I just read through the section of the NEC you referenced and that seems to apply to situations where the subject circuit will be outdoors or on the outside of a building. I don't understand how it applies to this situation. According to the inspector I talked to, I don't need any other disconnect than the 40 amp breaker in the main panel that will feed my greenhouse circuit. No one else has brought up the point about the breaker or feeder size for this circuit having to be 60 amps. Seems to me that if that were true in every case, there wouldn't ever be a need for UF cable of #8 size. Maybe I'm just not interpreting the terminology used correctly.
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Old 02-07-2008, 08:22 PM   #14
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Subpanel installation in outbuilding


[quote=J. V.;95734]
You must have four wires running to the sub. 2 Hots 1 Neutral and 1 Ground.
quote]


Not necessarally, you can run 3 wires (no egc) under certain circumstances. If your state hasn't adopted the 2008 NEC, the 05 states this

250.32(B)(2) Grounded Conductor Where (1) an equipment grounding conductor is not run with the supply to the building or structure, (2) there are no continuous metallic paths bonded to the grounding system in each building or structure involved, and (3) ground-fault protection of equipment has not been installed on the supply side of the feeder(s), the grounded conductor run with the supply to the building or structure shall be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded.

The 08 states this

250.3(B)(2)Exception: For existing premises wiring systems only, the grounded conductor run with the supply to the building or structure shall be permitted to be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded where all the requirements of (1), (2), and (3) are met:
(1) An equipment grounding conductor is not run with the supply to the building or structure.
(2) There are no continuous metallic paths bonded to the grounding system in each building or structure involved.
(3) Ground-fault protection of equipment has not been installed on the supply side of the feeder(s).
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Old 02-07-2008, 09:42 PM   #15
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Subpanel installation in outbuilding


All true, but I'll bet being a greenhouse, there is a water pipe in there somewhere. Even if the water is piped in PVC, he's running conduit, why NOT throw a EGC in? Still though, you are thorough.

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