NEC problems with sub-panel for detached building
#1
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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!
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!
#2
Are there and "grandfathering" options,
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
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.
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".
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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.
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?
#4
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.
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....
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....
#7
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.
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.
#8
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.
Perhaps a break could be installed in the water line with a PVC coupling to isolate the metal piping.
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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?
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?
#10
(and assuming the inspector OK's keeping the granfathering).
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?
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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.
#12
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.
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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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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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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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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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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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 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!