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Sub Panel feed calculation

Pt_Ranger_v8

Pt_Ranger_v8

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Looking into the future, I'm building a storage shed ~50-60' from the house.

I want to trench a 220v/30 amp cicuit to run the air compressor, with a secondary usage of backfeeding the house with a generator in the even of a power outage.

Generator is 220v/30a as well.

Thought is

Main breaker -- 30a breaker to shed | sub panel with 2 110v breaker and one 30a 220v breaker.

This page:


says 10 gauge copper is sufficient. I was thinking 6 gauge minimum.

Am I high?
 
10 copper is sufficient for 30a, you’ll have a little bit of voltage drop of course.

extra margin if you go 8ga more than makes up for voltage drop

6 is overkill if you’re doing 30, it would give you the option for 60A though.

Its ok to run a 30a genny back on a 60a circuit, btw
 
WutCudGoWrong? said:
10 copper is sufficient for 30a, you’ll have a little bit of voltage drop of course.

extra margin if you go 8ga more than makes up for voltage drop

6 is overkill if you’re doing 30, it would give you the option for 60A though.

Its ok to run a 30a genny back on a 60a circuit, btw
Click to expand...


it's a 12x16 storage shed 50' from the house :laughing:

I'm just putting the compressor out there to free up garage space. Having usable storage space close to the house is a big bonus as well.

A couple of lights and a couple of outlets will be plenty. I don't know if it's even getting tyvek :laughing:
 
WutCudGoWrong? said:
You’ll be fine with 10ga

With 3” you can pull romex if you wanted.
Click to expand...
With 3" he could pull 2-2-2-4 and have 100A out there...
May run into inspection issues running the PEX in the same conduit as the power.
If it's just going to be 10 gauge 1" would be plenty and you could direct bury the heavy wall black HDPE water pipe along side it for your air.
If you go that route and ever have plans to do anything more than storage out there, I would also drop a piece of 3/4" PVC conduit in the trench as far away from power as you can so you could run low voltage wiring in it (Ethernet, or?).

Aaron Z
 
Romex isnt approved for underground since it isnt wet rated.... if we're getting technical.

Nice thing about armoured cable is it wont interfere with cat5 - even strapped to it (not to code tho).
 
If your compressor load is 20A or more I would run a minimum of 40A to the shed so that if you ever are using the outlets out there for anything it’s not competing with the compressor. Voltage drop at 50’ shouldn’t be a concern. You can run #8 copper for the 40A feed.

I would be concerned about the pex line rubbing against the wire in the conduit and eventually causing a short and would run two separate conduits.
 
u2slow said:
Romex isnt approved for underground since it isnt wet rated.... if we're getting technical.

Nice thing about armoured cable is it wont interfere with cat5 - even strapped to it (not to code tho).
Click to expand...

I thought about mentioning that.

As far as “is it safe?” It will be fine, no heating problems in a 3” conduit.

Assuming he knows how to glue pvc enough to be watertight.

And if he wants (or is forced) to change it out for a bigger or proper conductors, the romex is his pull cord. And Sell the 10/2 on Craigslist.
Tin Roof said:
If your compressor load is 20A or more I would run a minimum of 40A to the shed so that if you ever are using the outlets out there for anything it’s not competing with the compressor. Voltage drop at 50’ shouldn’t be a concern. You can run #8 copper for the 40A feed.

I would be concerned about the pex line rubbing against the wire in the conduit and eventually causing a short and would run two separate conduits.
Click to expand...

And yes, drop the pex in the trench, not inside the conduit.

Honestly if it were me i would be dropping (2) 2” on either side of a 24” wide trench and (3) 1” in the middle like 9” apart but only 18” deep. And i would be using some sort of flex for the 1”ers, like irrigation pipe, same PE shit they use for sleeves when line boring. Maybe 1.25”

One of the 2” for power, the other 2” is a spare or you could use it for water or gas in the future.

The 1” is just sleeves for data or air or beer from the kegerator or whatever. Also they are kind of like warning tape with a purpose.

I’m kind of ocd so i would be putting tracer wires in but hes trying for a budget.

Come to think of it, 2 runs of 1.25” irrigation pipe is all he really needs in a narrow trench if he’s just pulling 30A and an air line.
 
WutCudGoWrong? said:
Assuming he knows how to glue pvc enough to be watertight.
Click to expand...
It's the low point in a system. That conduit will fill with water unless it somehow had air flowing through it, no matter how good the glue joints are. Or had drain holes and was above the water table.

Even the condensation that would happen because the ground is cool will eventually fill the conduit.

If it's one thing I've learned in my years is that you cannot keep water out. Best you can hope for is to drain it away.
 
Muckin_Slusher said:
It's the low point in a system. That conduit will fill with water unless it somehow had air flowing through it, no matter how good the glue joints are. Or had drain holes and was above the water table.

Even the condensation that would happen because the ground is cool will eventually fill the conduit.

If it's one thing I've learned in my years is that you cannot keep water out. Best you can hope for is to drain it away.
Click to expand...

The gospel truth
 
So, I''m looking at 1x 2" pvc or something similar for an 8/3 cable run to act as the sub panel.

And a second 2" pvc for my air line (1" poly or pex) in that. Think I could get away with running a shielded cat 6 cable in the same 2" conduit?

The whole thing will be ~2' in the ground - which is lower than my septic tank, since there's not much of a frost line here.
 
I would run URD (stranded AL) or THWN (stranded copper) rather than 8/3 as it will pull easier.
Additionally, 8/3 (unless it's the grey direct burial stuff) isn't made to go in conduit underground.

Aaron Z
 
aczlan said:
I would run URD (stranded AL) or THWN (stranded copper) rather than 8/3 as it will pull easier.
Additionally, 8/3 (unless it's the grey direct burial stuff) isn't made to go in conduit underground.

Aaron Z
Click to expand...


ok, sbustitute "appropriate 8 ga conductor x 4 (including the ground) where I said 8/3.

Does that make more sense? :laughing:


www.homedepot.com

Cerrowire 100 ft. 8 Gauge Green Stranded Copper THHN Wire 112-4005CR - The Home Depot

This green 100 ft. 8-Gauge Stranded THHN cable is designed for indoor use. Its 19 conductor strands are insulated with PVC and jacketed with nylon for effective, safe performance. Wire is used in commercial
www.homedepot.com

I'm thinking 4 rolls of that (in different colors)
would do the trick. I expect I'll be about 80' or so by the time I get done.

And yes, if 8awg wire is enough, I'd prefer to feed the storage building with 40/50a. I'm literally only having 2 outlets, and 2 led's out there, besides the compressor and generator hookups.
 
Last edited:
Pt_Ranger_v8 said:
ok, sbustitute "appropriate 8 ga conductor x 4 (including the ground) where I said 8/3.

Does that make more sense? :laughing:


www.homedepot.com

Cerrowire 100 ft. 8 Gauge Green Stranded Copper THHN Wire 112-4005CR - The Home Depot

This green 100 ft. 8-Gauge Stranded THHN cable is designed for indoor use. Its 19 conductor strands are insulated with PVC and jacketed with nylon for effective, safe performance. Wire is used in commercial
www.homedepot.com

I'm thinking 4 rolls of that (in different colors)
would do the trick. I expect I'll be about 80' or so by the time I get done.

And yes, if 8awg wire is enough, I'd prefer to feed the storage building with 40/50a. I'm literally only having 2 outlets, and 2 led's out there, besides the compressor and generator hookups.
Click to expand...
That should be fine for 40 amps all day. I would get Black, Red and White Green in 8AWG, or you could get all black in one roll and tape the wires with Red, White or Green tape if your run is long enough to be cheaper to buy a 500' spool)
Run it into a small panel like this: Square D Homeline 70 Amp 2-Space 4-Circuit Indoor Surface Mount Main Lug Load Center with Cover HOM24L70SCP - The Home Depot

With a breaker like this: Square D Homeline 2-20 Amp Single-Pole 1-30 Amp 2-Pole Quad Tandem Circuit Breaker HOMT2020230CP - The Home Depot

You will need at least 3/4" conduit to be legal with , I would probably use 1.25", or 1.5" (random fill chart for PVC conduit, the numbers in each row are how many wires of that size you can put through that size conduit): Elliott Electric Supply - Lower Cost, Quality Electrical Supplies, and Personal Service
Screenshot_20231203-215039-716.png




Aaron Z
 
Two more things:
1. Chamfer the inside of the male ends of all your pieces of PVC, something like this works slick: Husky Deburring Tool 80-531-111 - The Home Depot
2. Put a box of some kind (an LB for example: Cantex 1 in. Type LB Conduit Body R5133665 - The Home Depot ) right where conduit goes into the house and shed, then once you have all the wires run, fill the side where the wires go down to go underground with silicone, great stuff or something else to impede critters (or moisture) from making their way in.

Aaron Z
 
Thanks aczlan

Most of what you mentioned is what I had in mind.

It'll be a hot minute before I get to this shed, but I've got the general idea in my head now, and I can keep looking for deals on stuff I need for it - like wire, boxes, and breakers.
 
Muckin_Slusher said:
It's the low point in a system. That conduit will fill with water unless it somehow had air flowing through it, no matter how good the glue joints are. Or had drain holes and was above the water table.

Even the condensation that would happen because the ground is cool will eventually fill the conduit.

If it's one thing I've learned in my years is that you cannot keep water out. Best you can hope for is to drain it away.
Click to expand...
I pulled about 100ft of 1" air hose last summer I installed in conduit in 2011. It was bone dry, no signs of ever being wet.
Conduit is buried about 4ft.

Pulled because new home owner didn't want it. I had a 60 gallon compressor in the shed.
 
AKnate said:
I pulled about 100ft of 1" air hose last summer I installed in conduit in 2011. It was bone dry, no signs of ever being wet.
Conduit is buried about 4ft.

Pulled because new home owner didn't want it. I had a 60 gallon compressor in the shed.
Click to expand...

Regardless, the NEC defines buried conduit as a wet location.
 
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