I have a buried stretch of 16' along a driveway that I need to upsize the existing conduit for a sub panel. Basically the conduit comes out of the side of the house in an LB, goes down about 2 feet, 90 bends to a 16' straight section, then 90's back up for 2' to another LB that feeds into the garage. The old conduit is 3 years old 1" rigid steel and is covered with rust where it was buried, it did not have any wrap on it. Can aluminum rigid conduit be buried? Or is using steel rigid and wrapping it a better way to go to prevent corrosion. the area soil is clay, the conduit trench will be backfilled with sand/small gravel, with brick pavers on top.
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Burying Rigid Conduit
- Thread starter Paulski
- Start date
Innovate1
Well-known member
Why not use PVC? You can use schedule 80 for more durability if you think it is needed.
alfredeneuman
Well-known member
NoCan aluminum rigid conduit be buried?
PVC needs an 18" burial depth. Rigid=6"Why not use PVC?
Zeke
Well-known member
What size wire are you planning on pulling to the garage? You can pullup to #4 in clean conduit and 3 years old is nothing.
I should add that #4 would be copper.
I should add that #4 would be copper.
Last edited:
mcbane
Well-known member
PVC is way more durable than rigid steel, so if you have sufficient depth of cover, go with PVC. If you must use rigid, tape wrap it first. For added protection if you have corrosive soils, attach one or more zinc anodes and bury them at least a few feet away from the conduit.
The new wire is going to be #4 in 1 1/2" conduit, the old wire was #12 in a 1" conduitWhat size wire are you planning on pulling to the garage? You can pullup to #4 in clean conduit and 3 years old is nothing.
I should add that #4 would be copper.
I can't get the PVC 18" deep without ripping out more driveway for access - there is water lines crossing through the trench at around 14" deep. The trench is only 4" widePVC is way more durable than rigid steel, so if you have sufficient depth of cover, go with PVC. If you must use rigid, tape wrap it first. For added protection if you have corrosive soils, attach one or more zinc anodes and bury them at least a few feet away from the conduit.
rlitman
Well-known member
Threaded conduit is galvanized inside and out, but there is no galvanizing on the threads, so those will start rusting within minutes of getting wet underground. You can spray the threads with cold galvanizing compound. Then wrap the whole pipe, and finish by coating in an asphaltic coating. That will buy you a few more years. Properly done anodes may buy some years (improperly done ones add nothing). Still, it WILL eventually rust through.
However, here's a thought. 16' is still a reasonable length to shoot pipe through without trenching. Especially if you dig a pit in the middle and split the length in half.
However, here's a thought. 16' is still a reasonable length to shoot pipe through without trenching. Especially if you dig a pit in the middle and split the length in half.
So go under the water lines. I dig underneath stuff like that all the time, it's no big dealI can't get the PVC 18" deep without ripping out more driveway for access - there is water lines crossing through the trench at around 14" deep. The trench is only 4" wide
they're 2 water lines that are spaced out and run diagonal through the trench, and the trench ends at a wall on both ends so I can not dig it longer, I would have too dig way far down (~16'??) to be able to rotate a straight piece of 16' long conduit to get underneath both of them.So go under the water lines. I dig underneath stuff like that all the time, it's no big deal
It seemed like it would be tight, but I just checked with an online calculator and it is 31% fill with 3 #4s and a #8.#4 will fit pretty easy in 1" conduit with only (2) 90* bends.
A chart I have from my city says for "MAXIMUM NUMBER OF TW CONDUCTORS IN CONDUIT OR TUBING" for a 1" conduit is 3.
Does the #8 ground wire count as conductor? if not, than I will give it a try.
sparky 1971
Well-known member
It's not TW. I'm not sure if that's even made any longer. With three #4 THHN/THWN or XHHW and a #8 THHN/THWN you are at 30.86% full. It will be ok.It seemed like it would be tight, but I just checked with an online calculator and it is 31% fill with 3 #4s and a #8.
A chart I have from my city says for "MAXIMUM NUMBER OF TW CONDUCTORS IN CONDUIT OR TUBING" for a 1" conduit is 3.
Does the #8 ground wire count as conductor? if not, than I will give it a try.
wyliesdiesels
Well-known member
thats an old chart. throw it out. nowadays THWN is used for underground....they're 2 water lines that are spaced out and run diagonal through the trench, and the trench ends at a wall on both ends so I can not dig it longer, I would have too dig way far down (~16'??) to be able to rotate a straight piece of 16' long conduit to get underneath both of them.
It seemed like it would be tight, but I just checked with an online calculator and it is 31% fill with 3 #4s and a #8.
A chart I have from my city says for "MAXIMUM NUMBER OF TW CONDUCTORS IN CONDUIT OR TUBING" for a 1" conduit is 3.
Does the #8 ground wire count as conductor? if not, than I will give it a try.
yatg
Well-known member
Does the #8 ground wire count as conductor? if not, than I will give it a try.
You could use the RMC for the ground.
I am using 2" flex from the main panel -> to a 18" pull box -> 1" flex -> 1" rigid -> sub panel
The main and sub both have 2 ground rods
Can I count on all the conduits being interconnected together and count it as the ground? do I need to use bushings with a ground? I guess I can also verify continuity from end to end.
The main and sub both have 2 ground rods
Can I count on all the conduits being interconnected together and count it as the ground? do I need to use bushings with a ground? I guess I can also verify continuity from end to end.
sparky 1971
Well-known member
Flex can only be used as a ground if the length is 6' or less and the circuit is 20 amps or less.