Shed sub panel questions.

[psychob0b1977]

Hi all,

I have an 8x20 shed that I use as a workshop of sorts. I currently use extension cords for power, but I want to run electric out there for ease of use.

I have all the materials needed to run a 20amp sub panel.

My question involves how many circuits can I run on the 20amp sub panel? My plan is to have a 110v 20 amp circuit for outlets in the shop for things like saws and drills, only used one at a time or maybe 2 at a time if a buddy is helping. A 110v 15amp circuit to run lighting, 4 64 watt fixtures. that totals just under 2.5 amps in lights. but I'm planning on 5amps for lighting incase I want to run a drop light at the same time. That totals 25 amps. I am thinking of purchasing a 60 gallon air compressor that uses a 220v 15amp circuit. Would all if that be safe to run on a 20 amp sub panel? Our will I have to run a separate 220v run when I get the air compressor?

Thanks in advance!

 

[tylernt]

Sub panels are usually sold by the number of spaces they have for breakers, not the amperage. For example a 6-space panel is commonly used for detached structures and is usually rated for incoming power up to 60A.

Note that the amperage coming in doesn't have to match the amperage of the breakers going out. You could have a 20A feed and install 120A of breakers if you wish, no problem. As long as you only plan to use 20A at a time, of course.

As for the compressor, you need to look at the nameplate to see how many amps it draws in operation (if it asks for a 15A circuit, that means it probably actually draws 12A or less).

Once you get that number, you can decide how many amps to feed your shed with. For example, you might decide to run 10/3 UF from a 30A breaker in the house, and install a 15A breaker for lights, a 20A breaker for receptacles, and a 20A 2-pole breaker for a 240V compressor receptacle (a 20A 240V receptacle will serve both 15A and 20A compressors for future flexibility).

Note that if there is a long distance from house to shed, you may need to upsize your feeder wire to 8ga for a 30A feed, or 10ga for a 20A feed. Check out the voltage drop calculator at http://www.electrician2.com/calculators/wireocpd_ver_1.html

 

[Zeke]

You will always benefit from running a 240v capability. However, if you want to wire up what you have, do it for now and secure the extra wire. I don't understand the 20A max at present, but you know what you have in the main better than I.

Using the 20A panel as you describe is fine except for the 15A breaker for the compressor. Again, I don't know what your compressor is, but most that size need a 20A circuit. And I'd put it in so you have the two 20A and the one lighting circuit at 15A.

The worst that can happen is that the compressor starts when you both are bearing down on some big tools like skillsaws. That will overload the panel's protection back at the main.

So, why limit yourself if you're going to install permanent wiring? I'd at least run 10/3 w/grd off a 30A and then divide it up as mentioned.

 

[psychob0b1977]

The 20a limit its because I have hundreds of feet of 12 gauge wire. Keeping my copper cost to zero.

the 60 gallon compressors I was looking at were under 3hp, and used about 12amps, so they call for a 15 amp circuit. i was planning on using a 20 amp single pole breaker for the outlets, a 15amp single pole breaker for switched ceiling outlets for lighting. And a 20 amp double pole breaker for the 220v compressor, so that if I end up getting a compressor with a 5hp motor it will still be sufficient.

Does that make sense?

 

[tylernt]

Gotcha, I like zero copper costs too.

Still run that voltage drop calculation -- if you're a long ways away you may experience excessive voltage drop with a big compressor. If the voltage drops too much the compressor motor can stall at startup and smoke itself. I believe 5% voltage drop is the limit for branch circuits, not sure about feeders though.

 

[pattenp]

Just casually running this through my head the numbers don't work. A 20A circuit with #12 should have no more than a 80% continuous load which is 16A. And 16A load on #12 at hundreds of feet will have way too much of a voltage drop. You should really be at no more than 100ft.

Edit: You need more power to the sub-panel for a bigger compressor. NEC doesn't allow a second power feed to be run to a detached structure.

 

[psychob0b1977]

I ran the voltage drop calculator and it came back with just over 2% drop.

The sub panel will not be hundreds of feet from the panel, only about 65-75 feet. when i said hundreds of feet of 12 guage wire, i meant that i have a lot of 12guage that is allready been paid for and is left over from other projects/jobs.

So this should be good to go then?

As far as how much is used at any one time, i cant see the compressor ever being used at the same time that other heavy load items are also being used. i would shut off the compressor when not in use, no ned to waste electricity compressing air that will probably just leak out before its used anyway.

 

[tylernt]

So this should be good to go then?

I don't see a problem.

Just curious, is your free wire UF or THWN? NM/Romex is a no-go for burial, regardless if inside or outside of a conduit.

 

[psychob0b1977]

I have a bunch of thwn for the conduit that im going to run under ground to the shed. and i have a ton of romex for in the walls and from the panel to the conduit. but i might just buy some more conduit and go from panel to sub panel with all thwn. that depends on if i have any 12-3 that is long enough.

Thank you for all the help, i tend to over think everything and make mountains out of mole hills, or i second guess myself.

 

[pattenp]

I now see what you said about having hundreds of feet of #12. I misread your post.

I ran the voltage drop calculator and it came back with just over 2% drop.

The sub panel will not be hundreds of feet from the panel, only about 65-75 feet. when i said hundreds of feet of 12 guage wire, i meant that i have a lot of 12guage that is allready been paid for and is left over from other projects/jobs.

So this should be good to go then?

As far as how much is used at any one time, i cant see the compressor ever being used at the same time that other heavy load items are also being used. i would shut off the compressor when not in use, no ned to waste electricity compressing air that will probably just leak out before its used anyway.

 

[brewchief]

Only thought I have is to oversize the conduit so if you find yourself undersized in the future it will be easy to pull larger wire without digging up the yard.

 

[psychob0b1977]

I am way ahead of you on the oversize conduit. I have 1 1/4 conduit. So if in the future I find it to be a problem I can easily pull bigger wires.