Power to Detached Pole Barn

[Guest]

Getting lost in the details of ampacity and derating.  I have had advice from the electrical gurus here before, and I seek that counsel again.

I am working on my electrical permit and power for my new pole barn.
I am running from a 100A subpanel in my garage, up into the rafters, across the garage, down the wall in a 2" conduit (EMT or Sch 80 PVC?), through the wall to the exterior via a LB inside and another outside.  2" PVC SCH 80 18" deep at least, for 100' to the new shop. The conduit is turned up into the shop into a new 100A sub panel.  I know I need two ground rods at the pole barn.

I have read I need #1 AL and I also read it should be 1/0 AL for 100A.  I assume since total wire length is 150 feet, I need at least 1/0?  Is that derated enough?
I would prefer to put it in conduit, all the way.  I have been told to use mobile home service wire that has 4 conductors already, but I don't know if that is allowed in conduit.

My queries are:

1a. What size wire do I need for 100A at 150 feet.
1b. If I put an 80A breaker in the sub panel feed in the garage, what wire size would that require? (asking for $ conservation)
2.  For inside conduit, what Insulation can I should it be, USE, SER?
3.  Can I use SCH80 inside the garage and down into the ground or does some portion of the conduit run have to be EMT?
4. It should not need to be in conduit running across the garage rafters, should it?


Thanks for the help.

[TDKPE]

Minimum aluminium conductor size for 100A is 1/0. That's using the 60C column. The 75C column allows 100A on 1-gauge. Since it's conduit, and assuming individual conductors, and since it's underground for part of the run (considered a 'wet' location), you'll be using THWN most likely, and that 75C rating is acceptable as long as the lugs on both ends are also 75C rated, which they should be.

Voltage drop with 1-gauge at 100A is about 7.5V, assuming all of it is through one hot and the neutral only. Half that if 100A from each leg, as the neutral carries nothing then.

Voltage drop with 1/0 is 6V at 100A on one leg only.

It's highly unlikely you'll ever draw 100A on one leg only, if you balance the 120V circuits reasonably well. And the NEC doesn't require a max voltage drop - it's in the fine print notes (fpn's), which are informational only, and not code enforceable.

For 80A, 1-gauge at 60C or 2-gauge at 75C is ok.

And you can't run loose conductors in the rafters - has to be in conduit, or in a cable tray. I would use schedule 40 PVC indoors, but 80 outdoors (required where it comes out of the ground, due to damage exposure), or just sch. 80 everywhere.

[Guest]

Got it TDKPE and thanks for that.

If I use mobile home feeder like

MH Feed with a #2 USE AL for ground
 
and forgo the conduit, except for protection where it transitions underground, can I run it in the rafters? (The rafters are 14' high and not load bearing on the bottom chord, so I would think it is considered inaccessible) Can it be run in conduit in the garage to get from the rafters to underground?

This should get me 100A, and I can't imagine ever needing more than that since it is fed from a 100A subpanel. So I could forego conduit with no upgrade potential.

This would be my feed in the garage subpanel and it looks like it will work fine with 1/0 AL Sq D homeline 100 A Breaker.

Seem reasonable?

[TDKPE]

I don't think so. URD doesn't have an outer sheath, but it is suitable for wet locations and for use in conduit or direct burial. It's intended as service cable, up to the meter, not as service entrance cable. And I don't believe the insulation is rated for indoors.

You need #6 minimum for the grounding conductor at 100A. Probably cheapest to buy THWN of the desired gauge for both current-carrying and grounding conductors. Doesn't need to be color coded at that size (but do mark the ends).

[Guest]

You need #6 minimum for the grounding conductor at 100A.  

Is that # 6 CU or AL?  The inspector approved #1 AL for the feed but I forgot to double check the ground wire size.
And what size do I use from the panel to the ground rods.  I normally see bare CU to the ground rods, is that the standard?
Thanks for the help

[TDKPE]

#6 aluminium and #8 copper equipment grounding conductors are good to 100A per Table 250-95. If you upsize the phase conductors for voltage drop compensation, the grounding conductor also has to be upsized, but that doesn't sound like the case here.

Per Table 250-66, service conductors of 1/0 or smaller require #8 copper grounding electrode conductor, with no splice at the first grounding electrode (rod) in the chain. And bare is typical.

[MauleSkinner]

All ya really need is one 20-amp fuse, provided you switch the lights, air conditioner, table saw, and dust collector on in that order!


No...I'm not advocating that...I did that for a couple of months until I got my garage rewired.

[crokett™]

Do you actually need 100A at your pole barn? I'm asking because I built a shop and ran electrical nearly as far as you were proposing. I used 2-2-2-4 aluminum which was both cheaper and easier to work with than 1/0. Although I did have to derate to 90A that was about twice as much as my worst case power draw was going to be.

[Guest]

Do you actually need 100A at your pole barn? 

Probably not, but it just made sense to overkill than underwhelm.

Worst case, in the dead of night, lights blazing, heat on, welding with my old, big Montgomery Ward buzz box. One or both sons doing who knows what.

Typical load is probably mini-fridge and radio.