Digging trench for electrical?

[hanly2]

Ok So I am going to bury some 2" pvc for running my elec. to the garage its about 108'. I read that the pvc needs to be buried in a 18" trench. But I am a little unclear. I have dug about 10" for the whole trench and as far down as 22 in some places, but I am getting tired of digging, so I was going to rent a trencher. Thing is the trenchers are 18". So does the trench need to be 18" or does the trench need to be 20" to account for the 2" of pvc pipe?
Thanks.

 

[teamextreme]

18" to top of conduit

 

[pattenp]

You need 18" of cover, so trench needs to be at least 20".

 

[Aceman]

Most of the time I tell people a 24" deep ditch for 18" of cover. If I told them 20", it might start out at 20" but then turn to 18" and then back to 20", etc as the trencher goes over the terrain or hits a rock and bounces.

 

[Rookie2]

Its your property ! do what you want. Its not a new construction.

 

[hanly2]

I guess i will rent the 18" trencher and then just shovel out the final 2". Shouldn't be too bad. Thanks for the info.

 

[Notgrownup]

**** man, just dig 18" and bury the damn thing, it will be fine.

 

[tfi racing]

If its that hard to dig by hand,that trencher will be pretty much useless at this point.Buy some beer and call up a couple of friends to help finish it off.

 

[GYPSY400]

If you use teck90 cable you won't need the conduit .. In my area, the teck90 cable is code for direct burial, with a pressure treated 2x6 on top to prevent shovel damage. You may also need to add caution tape in the trench.


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[MN4x4]

Call a contractor rental place, not a big box or 'weekend' one. The trencher I rented easily dug down 30 inches.

The other benefit is that the contractor places aren't usually open on weekends. Why is that good? I picked up the trencher on Friday afternoon and returned it on Monday morning with a full tank of gas (maybe 3-4 gallons) and they charged me for a half-day (4 hour) rental!

 

[pattenp]

Don't know where the OP lives, but Teck90 seems to be a wire product more used in Canada. I've never seen it around here.

If you use teck90 cable you won't need the conduit .. In my area, the teck90 cable is code for direct burial, with a pressure treated 2x6 on top to prevent shovel damage. You may also need to add caution tape in the trench.


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[Rookie2]

A big hammer drill with a spade (or an electric jack hammer) to loosen the hard areas.

 

[mm08822]

If you started your trench by hand with shovel and pick, a trencher now will start to be a pain in the a$$. Spoils will most likely be in the way of the trencher and the extra width of your hand digging will create troubles removing new spoils out of the trench and you may loose the trencher into the existing trench. Have plywood and 2x4 or 2x6's handy.

 

[hanly2]

^^^^this is exactly what I realized. I borrowed a "trench" shovel and I will finish by hand makes it 10 times easier only digging 4" wide. In the 9" trench. Thanks for all the comments.

 

[BFBOB]

Code question for previous responders - the last time I looked it up (many years ago) depth was 12" in conduit, 24" direct burial. Am I remembering wrong, has it been changed in NEC, or are you quoting local regs?

After all, the last line in the NEC is "Or whatever the inspector says."

 

[hanly2]

Also another question. I am running a 100 amp panel and the elec. said to get #2 copper wire, is that correct or can I get #3 or aluminum wire?

 

[MTW]

Recommendation. Skip the code minimums and put it below the frost line for your area, especially when your installing plastic, if you want a permanent installation. PVC snaps like glass when it's below freezing and the ground heaves from freezing.

For digging it's best to find an operator with a backhoe if you have much to do, a tired body or rocky soils. With a good operator you can dig, place pipe and backfill in a few hours. Trenchers can be a mixed bag if you have a lot of rocks, and usually don't go deep enough. Hand digging is cheap but can be brutal on your spine. 42" frost line = 44" to top of conduit = 46" final depth and amount of backfill.

For those interested in the minimum code depths and rules, see the attachment.
View attachment Burial Rules NEC 2011.pdf

MTW Ω

 

[MTW]

For minimum size 100A wire on a residential feeder would be #4 copper or # 2 aluminum. For commercial work or if you really need the full capacity #3 copper or #1 aluminum.

These would be the minimum, for longer runs it may need to be up-sized for voltage drop resistance.

See the attached charts for reference.





MTW Ω

 

[hanly2]

Its residential, about 125' run.

 

[MTW]

Without doing any calculations for voltage drop, rule of thumb. Go one size larger for every 50' past the the first 100' if you are concerned about voltage drop.

MTW Ω

 

[mark2457]

Recommendation. Skip the code minimums and put it below the frost line for your area, especially when your installing plastic, if you want a permanent installation. PVC snaps like glass when it's below freezing and the ground heaves from freezing.

For digging it's best to find an operator with a backhoe if you have much to do, a tired body or rocky soils. With a good operator you can dig, place pipe and backfill in a few hours. Trenchers can be a mixed bag if you have a lot of rocks, and usually don't go deep enough. Hand digging is cheap but can be brutal on your spine. 42" frost line = 44" to top of conduit = 46" final depth and amount of backfill.

For those interested in the minimum code depths and rules, see the attachment.
View attachment Burial Rules NEC 2011.pdf

 

[MTW]

The 6" depth listed is for GRC galvanized rigid conduit only (heavy wall) schedule 40, not for EMT electrical metallic tubing. EMT is not approved for burial. GRC requires less digging but a lot more money. Your call.

MTW Ω

 

[wyliesdiesels]

For minimum size 100A wire on a residential feeder would be #4 copper or # 2 aluminum. For commercial work or if you really need the full capacity #3 copper or #1 aluminum.

These would be the minimum, for longer runs it may need to be up-sized for voltage drop resistance.

See the attached charts for reference.





MTW Ω

Yes i realize this is an old thread.

I see u listed T310.15(b)(6) which has been changed to t310.15(b)(7). Nonetheless, the OP said this was for his garage. That table is for 3-wire residential dwelling feeders.

Also, we dont know if this going to be a feeder off of his house service or a separate service. If off his house service, then he shouldve ran 4-wire and thus that table doesnt apply. If separate service, then that table arguably still doesnt apply because its for dwelling services and his garage isnt a dwelling. However, since its a residential garage, i can see someone arguing that this table WOULD apply.

We really needed to ask the OP how he was hooking this up first.

Furthermore, that table is no longer used in the NEC starting with the 2014 code cycle so calcs need to be done to see if reduced conductors would work...

Also, your charts are a bit outdated as 310.16 has been changed to 310.15(b)(16)...

 

[Jlarson]

Rent a real trencher, with rock chain if needed.

Don't know where the OP lives, but Teck90 seems to be a wire product more used in Canada. I've never seen it around here.

We have MC-HL here which is similar but it's rare outside of heavy industrial and kinda expensive. It's no where near as popular as Teck is up north.

 

[gtae07]

I think you can get some relief on depth if you put at least 2" of concrete on top. For me, that difference meant another hour or three of sweating and digging with the trencher and a shovel, or $15 in sackrete.

Of course, there's also no such thing as a "frost line" here.