NEC problems with sub-panel for detached building

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Old 04-10-11, 03:00 PM
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NEC problems with sub-panel for detached building

I've found myself between a rock and a hard place with the NEC, and need some advice. I have a block building behind my house that I have gutted and am renovating. I've stripped all the wiring, pulled the old subpanel, and am trying to install a new one (100A). There is a hot-hot-neutral 3-wire run to the building from the main panel, and the neutral and ground were bonded in the old sub-panel. As far as I can tell there is no ground electrode, so I intend to drive a ground rod or two.

The part I am having trouble with is that there is also a metal water pipe at the corner of the building, with spigots on either side of the block wall. I have to assume that the pipe is continuously metal all the way to my house. My understanding of the code is that having the 3-wire is OK as long as there is no continuous metal path bonded to the grounding system in each building, but then the code also says I MUST connect the pipe to my grounding electrode. Due to landscaping, pavement, etc installed since the house and building were built, it is not possible (practically speaking) for me to run a 4th wire from the main breaker. So basically, if I bond the ground to the pipe I lose my exemption in 250.32 B.1.2, but if I do not I am then in violation of 250.50.

So, do I have any reasonable solutions here? Are there and "grandfathering" options, or should I consider something like digging up a section of the pipe and replacing it with PVC (not ideal, but more realistic than running 4-wire)?

I should also add that there is a box that contains something like knife switches in the circuit between the main panel and the subpanel, which I assume is the "disconnecting means". Not sure if that matters, but the wording of 250.32 D confuses me a little.

Thanks for any help!
 
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Old 04-10-11, 03:13 PM
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Are there and "grandfathering" options,
When you gutted the building and removed the wiring you invalidated the grandfathering. Ground rods are for lightning not a primary route for fault clearance back to the main panel.

My understanding of the code is that having the 3-wire is OK as long as there is no continuous metal path bonded to the grounding system
Not under current code but it would depend on which version of the NEC your AHJ uses and any amendments.

Due to landscaping, pavement, etc installed since the house and building were built, it is not possible (practically speaking) for me to run a 4th wire from the main breaker.
At 100 amps I would think this was conduit not cable.

there is a box that contains something like knife switches in the circuit between the main panel and the subpanel, which I assume is the "disconnecting means".
If there is no fuse or breaker this is not code compliant.
 
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Old 04-10-11, 03:27 PM
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Originally Posted by ray2047 View Post
When you gutted the building and removed the wiring you invalidated the grandfathering.
Was afraid of that.

Originally Posted by ray2047 View Post
Not under current code but it would depend on which version of the NEC your AHJ uses and any amendments.
I was looking at the 2011 code, using the exception in 250.32 B(1) for installations in compliance with previous versions of the code. I know gutting the building killed the grandfathering for the building, but does that apply to the existing supply cable that I am not replacing?

Originally Posted by ray2047 View Post
At 100 amps I would think this was conduit not cable.
Could be. It goes into the ground from the main panel inside conduit, but I could not find conduit at the other end. Maybe I'll try digging deeper when I get home.

Originally Posted by ray2047 View Post
If there is no fuse or breaker this is not code compliant.
It is fed from a double pole breaker in the main panel.

Thanks for the response. So, under the assumption that I am not able to replace the 3-wire supply, is there anything I can do?
 
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Old 04-10-11, 05:22 PM
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It probably be like K&T or ungrounded cable. You can't add to it or modify it. Final call on this would be your inspector but for safety it should be 4 wire. The unfused disconnect isn't needed if it is at the house/main panel.
NEC 2008
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....
I think number two will be the hang up depending on the use of the building. Even a phone line or data cable is a metallic pathway. Again this is something to ask your AHJ.
 
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Old 04-10-11, 08:09 PM
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If the outbuilding is served by more than one circuit it will need a means of disconnect.
 
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Old 04-10-11, 08:31 PM
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I suggest talking with your local inspector because IMO, since you did not remove the feeder, it would still be grandfathered in.
 
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Old 04-10-11, 09:34 PM
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Originally Posted by ray2047 View Post
It probably be like K&T or ungrounded cable. You can't add to it or modify it. Final call on this would be your inspector but for safety it should be 4 wire. The unfused disconnect isn't needed if it is at the house/main panel.
NEC 2008
I think number two will be the hang up depending on the use of the building. Even a phone line or data cable is a metallic pathway. Again this is something to ask your AHJ.
I thought a water pipe is classified as a conductive path even if it is PVC, given that a solid column of water is conductive.
 
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Old 04-11-11, 06:25 AM
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Water is not as conductive as people think. A PVC water line between the building would not be an issue. It is the metallic paths like a phone, cable or metal water line the code is concerned with.

Perhaps a break could be installed in the water line with a PVC coupling to isolate the metal piping.
 
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Old 04-11-11, 08:07 AM
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Thanks for all the help, I will give my local inspector a call and hope for the best.

Regarding metallic pathways and installing a PVC "break" in the water line, I may need some clarification or correction. The way I read it, the "continuous metallic path" described in 250.32 (B)(1)(2) only applies if it is bonded to the grounding system in both buildings. That said, I did not see where phone lines or other wires qualify as electrodes (per 250.52) that are required to be bonded to the grounding system per 250.50.

So, the way I read it, ONLY the water pipe has to be bonded to the grounding, allowing me to keep the exemption in 250.32 (B)(1) IF I install a PVC break in the water pipe (and assuming the inspector OK's keeping the granfathering).

I don't know, maybe I'm reading too much into it... One thing is for sure, trying to do this right is sure getting time consuming and expensive!

One more question, just to satisfy my curiousity... aside from no longer being in code, from a safety standpoint what in theory is wrong with the previous configuration of having the neutral bonded with the ground bar in the sub-panel?
 
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Old 04-11-11, 10:25 AM
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Originally Posted by benhar View Post
The way I read it, the "continuous metallic path" described in 250.32 (B)(1)(2) only applies if it is bonded to the grounding system in both buildings.
Yes, but any properly installed plumbing system must be bonded to the electric service ground.

(and assuming the inspector OK's keeping the granfathering).
My interpretation of your situation is that you need to install the new four wire feeder. Of course you can take a shot at the inspector and see if he will give you pass on this one.

Do you know what kind of cable / conduit feeder you have right now? Based on the described age of everything else you may be reaching the end of the service life of the feeder anyway. If you have a conduit then it should be a much smaller job to repull new conductors. Sometimes old conduits can be tricky to pull, but you can clean them out pretty good with an air compressor, shop vac, swab line and even some specialty cleaning products.

One more question, just to satisfy my curiousity... aside from no longer being in code, from a safety standpoint what in theory is wrong with the previous configuration of having the neutral bonded with the ground bar in the sub-panel?
If there is a break, corrosion, loose connection, etc, in the neutral wire (which is fairly common), it can cause enough voltage rise and current flow on the bonded ground wire to become a shock hazard to people touching exposed metal appliances which also connect to the ground wire. When the neutral/ground bond is only in the main panel it limits the potential shock hazard in the event of a neutral failure to only the service entrance conduit, meter pan, aerial mast -- things that are less likely to be touched by the homeowner instead of a professional.
 
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Old 04-11-11, 10:59 AM
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Originally Posted by ibpooks View Post
Yes, but any properly installed plumbing system must be bonded to the electric service ground.
Yeah, that's why I was considering installing a short PVC run.

Originally Posted by ibpooks View Post
Do you know what kind of cable / conduit feeder you have right now?
Meant to take a picture yesterday, but it is three thick stranded copper cables in black insulation, where the neutral has a yellow stripe. It enters the ground from the main panel in conduit, but as far as I can tell comes out of the ground at the other end without conduit. I'm considering digging the cable as far as I can (to the sidewalk) to see if I can find conduit, but I'm not getting my hopes up.
 
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Old 04-11-11, 11:12 AM
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Good that it's copper and good that all three conductors are insulated. That does suggest the cable may have some useful life left if your inspector agrees to allow the three-wire feeder. That type of cable would typically not be installed in conduit, so you probably do not have pipe.

If you do have to bury a new line, there are sometimes tricks to get around obstacles so the situation may not be hopeless. For example you can often bore under a driveway slab using a garden hose and a conduit as a homemade hydraulic drill, or route in the line from another wall of the house to avoid the driveway. You can rent a concrete saw to cut out a channel, then carefully put it back and caulk it like expansion joints.
 
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Old 04-11-11, 11:27 AM
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Originally Posted by ibpooks View Post
For example you can often bore under a driveway slab using a garden hose and a conduit as a homemade hydraulic drill
That's a good idea. I was considering something like that to help with the ground rod installation, but didn't think about it for this.

About how much does the appropriate 4-wire feeder cable cost (probably need 75'-100')? I hate making money an issue but am already way over budget
 
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Old 04-11-11, 12:08 PM
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You could use #1 quadplex aluminum mobile home cable, URD, to save money.
 
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Old 04-11-11, 12:55 PM
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What electrical loads need to be served? If it is just going to be a storage building with a couple of lights and maybe a convenience receptacle then you might be able to re-purpose the existing feeder to a 120 volt circuit.
 
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Old 04-11-11, 01:10 PM
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Originally Posted by Furd View Post
What electrical loads need to be served? If it is just going to be a storage building with a couple of lights and maybe a convenience receptacle then you might be able to re-purpose the existing feeder to a 120 volt circuit.
I was planning on putting in about 4 flourescent lights and 6-8 receptacles. I'm assuming I should stick with the 240?
 
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Old 04-11-11, 01:21 PM
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Not necessarily, it depends on the size and type of the lights and what you plan on plugging in to the receptacles. If you use two-tube four foot lamps they take about 70 watts each so four of them would be about 280 watts or less than 2.5 amperes. If you want a beer refrigerator it will take maybe 8 amperes when starting and about 4 while running. If you mostly want to use the receptacles for small tool battery chargers or a radio or the like then the current flow is very low. A 20 ampere, 120 volt circuit may be quite sufficient and with the size of the feeder you have you could go to a 40 ampere (or even larger) circuit breaker at the house and install a sub-panel in the building to break it down to individual 15 and 20 ampere circuits; all at 120 volts. The only problem would be if you have some large loads, like electric heaters, you might have a voltage drop problem.
 
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Old 04-11-11, 01:43 PM
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Good idea Furd, this could be a great solution.
 
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Old 04-11-11, 02:29 PM
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I heard back from the inspector, he OK'd the 3-wire, since that is not what I am replacing.

I also like the idea of 120V, which sounds sufficient for my needs. Though I'd hate to lose the future capactiy, I may do that anyway to be safe.

Thanks all for the help!
 
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Old 04-11-11, 03:03 PM
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What size is the 3 wire. What size breaker at the main panel.
 
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Old 04-11-11, 05:31 PM
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Originally Posted by benhar View Post
I heard back from the inspector, he OK'd the 3-wire, since that is not what I am replacing.

I also like the idea of 120V, which sounds sufficient for my needs. Though I'd hate to lose the future capactiy, I may do that anyway to be safe.

Thanks all for the help!
I would go with it then. Thousands of 3 wire feeds have been installed over the years with no issues.
 
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