Code for underground conduit
mjp24coho
Solar Expert Posts: 104 ✭✭✭
What does the NEC (2011) and IRC dictate about appropriate conduit for buried THHN wire? I'm thinking of doing an expanded array 400' from a new cabin I want to build, and want to run 6 awg THHN wire from the array to the cabin. I'd like to run the wire in black 1-1/2" poly pipe. Would NEC and IRC allow for this as appropriate conduit? How deep would it have to be buried? Note this would be an off-grid installation. Could I fit 3 runs in each pipe (3 DC +, 3 DC -), or would only 2 be best?
Comments
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1. Since any PVC pipe except electrical conduit (gray) does not qualify as a wiring method under the NEC, you would have to use wire or cable rated for direct burial.
2. Black poly? Not black ABS? Are you referring to the flexible pipe used for irrigation?
3. Even when you use NEC compliant conduit, the inside of outdoor or buried conduit is considered a wet area. THHN is for dry areas only. But most often the THHN you see in the big box stores is dual rated as THWN or other wet rated insulation type also. But the ampacity based on highest allowed conductor temperature is lower when the THWN rating is used. Look for THWN-2 instead.
4. The NEC gives depth of burial rules.
5. The NEC also gives tables of the number of identical sized conductors that can be contained in each trade size conduit. If I have done the math correctly 1-1/2" pipe could hold 15 #6 conductors.SMA SB 3000, old BP panels. -
Code aside, I usually use polyethylene pipe as conduit (always trying to reduce the chlorinated compounds/dioxin) but its a medium density grade, used for pressure water mains. If you were to take a sharp spade to either with a good old whack it wouldnt be that pretty. One would slice, the other shatter.
Code likes things to be standardised. So when poly is allowed for things like telecom, water, it has to be the right color. Green for telecom, blue for water, etc. That way all the trades understand whats what. Dosent stop them taking diggers to 11kV power mains from time to time though.1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar -
I also lay several, larger diameter, pipes (I am cheap, I use ABS sewer pipe and use underground rated cable pulled in pipe--not legal, I know--But to be honest, I never checked the pricing of legal plastic conduit--It is just black sewer pipe is available in large diameters at the local home center). Dig once, and you can/update wiring later (add networking cable, some AC wiring back out, etc.). Leave a cord in the pipe so you can pull other wiring later.
-Bil
PS: I also use two 45 degree bends instead of a short 90 degree bend. Easier to get wiring through (also, for sewer/outside storm/water drains, easier to snake).Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Requirements for under ground you will need.
-1 bare 10AWG copper conductor for ground fault (500' spool), this requires a ground rod at point of array and a ground rod at the point of inverter with 6AWG.
-400' from cabin means you will need 6AWG( 500' spools)
-1" PVC (electrical grey), IF you want 1.5" you may do so but it costs more. NEC requirement for pack and fill doesn't exceed 60% with 1" conduit, if it is (2) 6AWG, and 1 bare copper 10AWG, ground does not count in pack and fill.
-1" rigid long sweeps to come out of the ground, and require ABS plumbers tape wrapping and encasing the metal sweeps (UPC listed)
-Any pipe out of the ground if it is not traffic rated required EMT minimum
-Ground bond bushings required at the end runs of the under ground conduit (10AWG bare is to remain continous to point of bussing or contacts)
-Utility requires 20" of depth minimum top of conduit, however you can get away with the freeze line which is 18" top of conduit since it is not utility
-Use slurry or sand to pack around the pvc pipe -
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The call before dogging. ... Should take a few days or one week. My utility took several weeks before they sent somebody out.
Make sure to call early just in case.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
.... love it!call before digging ?
yup, the line was NOT exactly where they said it was.... up here the operator of the excavator usually carries a detector to get a fairly good idea of where not to dig or maybe it was supposed to be deeper... my Nat. gas line is mapped as being 18 inches deep but it varies between 4 and 16 deep... discovered when putting in underground sprinklers...
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I also lay several, larger diameter, pipes (I am cheap, I use ABS sewer pipe and use underground rated cable pulled in pipe--not legal, I know--But to be honest, I never checked the pricing of legal plastic conduit--It is just black sewer pipe is available in large diameters at the local home center). Dig once, and you can/update wiring later (add networking cable, some AC wiring back out, etc.). Leave a cord in the pipe so you can pull other wiring later.
-Bil
PS: I also use two 45 degree bends instead of a short 90 degree bend. Easier to get wiring through (also, for sewer/outside storm/water drains, easier to snake).
Actually, as long as the pipe is a protective sheath (physical protection and not metallic) and the cable is already rated for direct burial the NEC would have no problem with what you have done. The only code issue would be the bends. You can get a wide sweep instead of a plumbing 90 and it will be much better than two 45s.
They are often used in drain plumbing to reduce clogging and make it easier to RotoRoot (TM)SMA SB 3000, old BP panels. -
Actually, as long as the pipe is a protective sheath (physical protection and not metallic) and the cable is already rated for direct burial the NEC would have no problem with what you have done. The only code issue would be the bends. You can get a wide sweep instead of a plumbing 90 and it will be much better than two 45s.
They are often used in drain plumbing to reduce clogging and make it easier to RotoRoot (TM)
Or if you want to really keep it cheap. Db120.
There are long and short sweeps in Db120
PGE uses Db120 all the time. -
Actually, as long as the pipe is a protective sheath (physical protection and not metallic) and the cable is already rated for direct burial the NEC would have no problem with what you have done. The only code issue would be the bends. You can get a wide sweep instead of a plumbing 90 and it will be much better than two 45s.
They are often used in drain plumbing to reduce clogging and make it easier to RotoRoot (TM)
What about using black poly pipe as the conduit, if it is THWN-2 cable? Would NEC allow that? I'm trying to avoid, if possible, using rigid conduit. It's not a straight shot for the trench down a hill, and poly pipe allows me the ability to snake around large rocks if needed (and cheaper, too, I believe). -
What does the NEC (2011) and IRC dictate about appropriate conduit for buried THHN wire? I'm thinking of doing an expanded array 400' from a new cabin I want to build, and want to run 6 awg THHN wire from the array to the cabin. I'd like to run the wire in black 1-1/2" poly pipe. Would NEC and IRC allow for this as appropriate conduit? How deep would it have to be buried? Note this would be an off-grid installation. Could I fit 3 runs in each pipe (3 DC +, 3 DC -), or would only 2 be best?
Typical burial depth for most applications is 18 inches. See NEC table 300.5. I assume this is the same system you were discussing here: http://forum.solar-electric.com/foru...ss-impact-of-3 ? If you were planning on using aluminum conductors as I recommended, they are commonly available in type use-2 which is suitable for direct bury and thus any "conduit" you choose. Copper USE-2 will be rare to find in stock any where but a supply house can certainly order it in. OR use a NEC approved conduit system and then you can just use THHN or XHHW (Prettty much all THHN is also type THWN-2 these days). Regarding conduit fill, use the tables in the back of the NEC or an online calculator.SolarPowered wrote: »1" PVC (electrical grey), IF you want 1.5" you may do so but it costs more. NEC requirement for pack and fill doesn't exceed 60% with 1" conduit, if it is (2) 6AWG, and 1 bare copper 10AWG, ground does not count in pack and fill.
-1" rigid long sweeps to come out of the ground, and require ABS plumbers tape wrapping and encasing the metal sweeps (UPC listed)
-Any pipe out of the ground if it is not traffic rated required EMT minimum
-Ground bond bushings required at the end runs of the under ground conduit (10AWG bare is to remain continuous to point of bussing or contacts)
-Utility requires 20" of depth minimum top of conduit, however you can get away with the freeze line which is 18" top of conduit since it is not utility
-Use slurry or sand to pack around the pvc pipe
Most of this is not correct/required, at least per NEC. I cant speak for local rules, what a rogue inspector "wants", or what some may consider "good practice". See chapter 9 table 1 for percentage of conduit fill. It is 31% for 2 wires, and 40% for over 2 wires, and grounding and bonding conductors DO count. You may be thinking of nipples 24 inches or less as they can be filled to 60%. PVC may emerge from ground, some inspectors will want schedule 80 where it transitions and/or an expansion coupling. Bonding bushings would not be required unless it is an isolated section of metal raceway or the system voltage is over 250 volts to ground and the fitting terminates on concentric knockouts. -
Ethan Brush wrote: »
Most of this is not correct/required, at least per NEC. I cant speak for local rules, what a rogue inspector "wants", or what some may consider "good practice". See chapter 9 table 1 for percentage of conduit fill. It is 31% for 2 wires, and 40% for over 2 wires, and grounding and bonding conductors DO count. You may be thinking of nipples 24 inches or less as they can be filled to 60%. PVC may emerge from ground, some inspectors will want schedule 80 where it transitions and/or an expansion coupling. Bonding bushings would not be required unless it is an isolated section of metal raceway or the system voltage is over 250 volts to ground and the fitting terminates on concentric knockouts.
It's correct method. It's over engineered, to not follow NEC bare minimums.
I don't advise bare minimums to novices. If they don't know they can do the extra work, or read the NEC themselves.
Really 3/4" PVC is all that is required.
How ever on a 400' run would you rather pull through smaller pipe or pipe that has wiggle room, and room for expansion.
Rigid long sweep 90's I've been called out to use more than once, even in scenarios that weren't traffic rated, but questionable.
If this is a DIY method, saving costs on professional labor, spend the extra money as needed to have additional safe guards. I never advise bare minimums.
Ground conductors are not recognized as carry continuous loads. Never been called out on fill factors for it. Never.
OP has no design, or engineering method. The thread is arbitrary, I won't advise bare minimums. -
What about using black poly pipe as the conduit, if it is THWN-2 cable? Would NEC allow that? I'm trying to avoid, if possible, using rigid conduit. It's not a straight shot for the trench down a hill, and poly pipe allows me the ability to snake around large rocks if needed (and cheaper, too, I believe).
THWN-2 is rated for use in wet areas, but is not rated for direct burial or for exposed wiring. It can only be used in a raceway that is part of a class defined by the NEC. That includes Rigid (metal pipe with pipe threads), and Intermediate conduit; Electrical Metallic Tubing (what we generally call "conduit"; and several types of flexible metallic and non-metallic tubing, either liquid tight or not.
Black poly pipe is not one of those described and UL rated options. Sorry.....
If you are going to use some sort of non-electrical pipe to provide protection from rocks, digging, etc. or to make it possible to pull more cable after the ditch if filled, you have to use for your conductors a cable type that is also allowed for direct burial without the pipe. Sorry....
SMA SB 3000, old BP panels. -
Thanks, Inetdog. That was the kind of response I was looking for. Sounds like poly pipe isn't the way to go. Does PVC conduit qualify, or does it have to be metallic? I'll likely have 3 runs in each conduit (6 wires of 6 AWG), so I'd probably go with 2" to make it easier to pull.
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Thanks, Inetdog. That was the kind of response I was looking for. Sounds like poly pipe isn't the way to go. Does PVC conduit qualify, or does it have to be metallic? I'll likely have 3 runs in each conduit (6 wires of 6 AWG), so I'd probably go with 2" to make it easier to pull.
Can you combine at the solar array prior to conduit to reduce the amount of wires to pull?
Do you have a one line drawing? -
Thanks, Inetdog. That was the kind of response I was looking for. Sounds like poly pipe isn't the way to go. Does PVC conduit qualify, or does it have to be metallic? I'll likely have 3 runs in each conduit (6 wires of 6 AWG), so I'd probably go with 2" to make it easier to pull.
You can use PVC conduit (the gray stuff that has a UL rating and which generally has a bell end at one end of each 10' or 20' section. ) No need for it to be metallic. And in fact for many soils even rigid conduit (the thickest) will rust away within 10-20 years.
SMA SB 3000, old BP panels. -
And in fact for many soils even rigid conduit (the thickest) will rust away within 10-20 years.
This is correct. Pipe wrap is required for most underground, and concrete poured encasement for metal conduits/pipe, including copper pipe in UPC.
I know it's outlined in UPC, I haven't seen it in NEC. -
SolarPowered wrote: »
Ground conductors are not recognized as carry continuous loads. Never been called out on fill factors for it. Never.
Grounding and bonding conductors are not counted as a current carrying conductor for derating purposes, but ARE counted for raceway fill.Thanks, Inetdog. That was the kind of response I was looking for. Sounds like poly pipe isn't the way to go. Does PVC conduit qualify, or does it have to be metallic? I'll likely have 3 runs in each conduit (6 wires of 6 AWG), so I'd probably go with 2" to make it easier to pull.
Schedule 40 PVC conduit would be code compliant and just about the cheapest thing you can find. Here is a trick/suggestion: Instead of trying to pull the wires and paying a lot more for larger conduit to be able to do so, just glue up 30 foot sections and sleeve it over the conductors. Thats what I did for my 600 foot run to my well. IF sleeving, 1 inch should be fine.
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That is a very practical suggestion, but is a technical violation of the NEC which requires a raceway run to be complete, end to end, before conductors are added to it. If all you are doing is using a sleeve for mechanical protection (which would NOT allow you to use THWN wires) then you can put it together any way you want.Ethan Brush wrote: »Here is a trick/suggestion: Instead of trying to pull the wires and paying a lot more for larger conduit to be able to do so, just glue up 30 foot sections and sleeve it over the conductors. Thats what I did for my 600 foot run to my well. IF sleeving, 1 inch should be fine.
Plumbing type Schedule 40 is a great material for sleeving but is not NEC acceptable as a raceway.
SMA SB 3000, old BP panels. -
SolarPowered wrote: »This is correct. Pipe wrap is required for most underground, and concrete poured encasement for metal conduits/pipe, including copper pipe in UPC.
I know it's outlined in UPC, I haven't seen it in NEC.
You can use PVC conduit (the gray stuff that has a UL rating and which generally has a bell end at one end of each 10' or 20' section. ) No need for it to be metallic. And in fact for many soils even rigid conduit (the thickest) will rust away within 10-20 years.
I have often heard than even from fellow electricians, but, at least in my neck of the woods, I really just dont buy it. Maybe everyone else has super acidic soils or lots of acid rain or something, but I have seen underground RGS that has been in the ground for 75 years very frequently in service upgrades I have done and it is totally fine. I have seen ground rods frequently of the same vintage and they are fine. I have some 150 year old foundations on my property and I go digging for artifacts and find all sorts of metal stuff that is not corroded away - even thin barrel hoops. There isnt much oxygen underground the galvinization is pretty stout. Just my experience.That is a very practical suggestion, but is a technical violation of the NEC which requires a raceway run to be complete, end to end, before conductors are added to it. If all you are doing is using a sleeve for mechanical protection (which would NOT allow you to use THWN wires) then you can put it together any way you want.
Plumbing type Schedule 40 is a great material for sleeving but is not NEC acceptable as a raceway.
I will give you that. NEC 300.18. Out in the field sometimes we have to do what we have to do, and in some situations, I would say conductors are more likely to be damaged from an extreme pull than laying them out and sleeving. -
Ethan Brush wrote: »
I have often heard than even from fellow electricians, but, at least in my neck of the woods, I really just dont buy it. Maybe everyone else has super acidic soils or lots of acid rain or something, but I have seen underground RGS that has been in the ground for 75 years very frequently in service upgrades I have done and it is totally fine. I have seen ground rods frequently of the same vintage and they are fine. I have some 150 year old foundations on my property and I go digging for artifacts and find all sorts of metal stuff that is not corroded away - even thin barrel hoops. There isnt much oxygen underground the galvinization is pretty stout. Just my experience.
For concrete, yes, high PH level, as well as expansion/contraction of concrete.
Regardless of in ground, all metal conduit/pipe according UPC requires it. Galvanize rigid is allowable for "exterior" use, the intent was never for direct burial. It became UPC because gas lines that were direct burial and were galvanized would leak into the soil, also for electrical conduit would rot out an there are other issues such as rodent contamination.
Scotch tape prevents these issues, by slowing down the depriciation processes.
Ground rods are not recognized by UPC, unlike copper pipe which is. Most copper rods are comprised of Iron/Nickel with a coating of copper platting.
All NEC states in its language is to recognize all other applicable codes.
The UPC code requires the UPC stamp to be visible on the tape to recognize mil.Thickness, and its application. Attachment not found.
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$76 for a 100' roll!
When we get into megawatt projects though it doesn't even equate to pennys on the dollar. Here are some examples.
The smaller pipes in the picture are not up to code because they are not using the $76 tape we ran out of so we fooled the inspector and used electrical tape. This is not the correct practice because it doesn't meet the spec. Attachment not found.
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This pic shows the long sweep RGS 90's implemented and taped. Attachment not found.
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