Sub panel 300 + feet away

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Old 05-11-09, 10:45 AM
J
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Sub panel 300 + feet away

I hope I use the right terminology.
I recently built a pole building on the same lot that my house is on and I want to get some power to it. Thinking 3 20 amp circuits and one 15 amp for lights.
I have some long runs and want some advice.

I have a sub panel in the basement that has 100 amp service its 60í from the meter. From there I ran 3 # 6 copper wire another 125í to another sub panel at my well. Fed with a 50 amp breaker. Sub panel at well is grounded to well casing. The well is only used in the summer to water lawn. I have 2 20amp double breakers at the well. One for the well and one for the sprinkler pump.

From the well sub panel I would like to see how much service I can get to my pole building. Its 300í from the well.

I know there will be voltage drop so what size wire should I use and what is the biggest breaker I can use. Since I will be coming off a 50 amp sub panel. I should mention that I used the 3 wire since there are no other wires or metal pathways any where near my runs and I was planning on using conduit.

Thanks Jason
 
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Old 05-11-09, 12:01 PM
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You started out by referring to the "sub panel in the basement". Is this really a "sub" panel?

To summarize, you have 100 amps 60 feet from the meter to the basement.
Then 50 amps 125 more feet to the well.
And ?? amps 300 more feet to the pole building.

The pole building is now 485 feet from the meter. That's a very long ways.

So we have voltage drop to worry about--a lot of voltage drop.
Then we have total power to worry about--how much power is really left over for your pole building considering all the other loads you have in the house, the basement, and the well.

Voltage drop is a function of distance and amps and wire guage. You have spelled out the distances, and some (but not all) of the wire guages. What we don't know is the amps. You told us the amp limits of the breakers, but that doesn't tell us anything about the amps actually used now. You told us what circuits you want in the pole building, but not how much power they will use, nor whether any of that power is needed while the sprinklers are running.

This is a mind-bogglingly complex puzzle. In many cases, we can approximate things and give you a rough idea. But, if this can be made to work at all, you will have razor-thin margins, so the design needs more precision. The other approach is just to wing it, i.e., put something in and see how it works. That, however, can easily turn out to be an expensive mistake.

The first thing to do is to figure out how much power you use now and how much more you need. That cannot be determined from breaker sizes, but must be computed based on actual loads (e.g., appliances).
 
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Old 05-11-09, 07:45 PM
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Let me try to answer. I donít know how to use the quote function

I guess I donít know if itís a sub panel. I have 200 amp service at the meter with two disconnects leading to two panels one in the basement and one in the garage.

I donít know how to determine how many amps I actually use from this basement panel. There is the ac on it (40 amp breaker) the well sub panel and just a few basement outlets that arenít used much. I think there is a lot of power left over at this panel. The well sub panel is the one I am more worried about.

The well sub panel has 2 20 amp double breakers on it. One for the well pump and one for the sprinkler pump. Again I donít know how much they actually draw.

The well sub panel is fed from the panel in the basement panel with 50 amp breaker and 125í of AWG 6 AL wire. I think I could have used a 60 amp breaker here though right?

300í away from the well at the pole building I will have about 1000 watts of light and a garage door opener and a few outlets at least. More circuits if I can get enough amps there.

I guess I want to know what is the biggest size breaker I can use in the well sub panel to feed the pole building panel.

I did run the wire from basement panel to the well panel in 1 ľ conduit so if I have to I could replace that 125í run. I donít want to if I can get enough power from the current setup.

I donít know if I gave enough info for you guys to help. If there is any thing else I can give you please let me know. Thanks
 
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Old 05-11-09, 08:47 PM
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You mention "a few outlets" in the pole building. Outlets don't use any power by themselves. Any idea as to what you might plug in to those outlets?

There's no real limit on the size of the breaker you can use in the well subpanel as long as it doesn't exceed the rating of the panel. But just because you put a large breaker in there doesn't mean you can get that much power.

If you tell us how many watts you need in the pole building, we can compute voltage drop based on distances and wire sizes. This computation would be if absolutely nothing else were running in the house, basement, or well. This will give you absolute best case. You already told us about 1000 watts of light--that uses about 4 amps of 240 volts (assuming you can split the lights on both sides of the panel). The GDO will be about 5 amps at 120 volts, depending on how many horsepower (so how many horsepower anyway?). So what else besides that? Hand tools? Stationary tools? Refrigerator? Heat? Air conditioning? Welder? Kiln?
 
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Old 05-12-09, 11:07 AM
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Ok this may be the better way for me to ask.

Say the well uses 10 amps. I want 40 amp sub panel in the pole building. Can I get that? If not I could get buy with 30 amp service.

How do I find out how much power the well is using? Do you need the H.P. from the motors?

When I built the pole building I had one 10 gauge extension cord 200 feet long. Then a 12 gauge 100' cord. I hooked into a 20 amp 115 volt outlet at the well. From this cord I ran a small 5 hp compressor alone. Circular saw, two 1/2 inch drills at same time. If the compressor kicked on I couldnít use the other power tools.

If the above is all I can get down there then thatís what Ill live with if I have to. I would like to have more though and on there own circuits so if I do trip one then the lights and other things wont trip. I canít really say how much I will use in the future. Now the plan is just storage with 1000 watts of light and garage door openers.

I would like to have as much as I can for unknown future use. I beleave I'm limited by the fact I only ran 50 amp service to my well though. If the well was not used could I get 50 amps to the pole building?
 
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Old 05-12-09, 12:51 PM
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How do I find out how much power the well is using? Do you need the H.P. from the motors?
Yes or look on the motor name plate.
From there I ran 3 # 6 copper wire another 125í to another sub panel at my well. Fed with a 50 amp breaker.
125 feet is border line for #6 and you want to go another 300 feet. Also unless the conduit is metal you need a forth wire for ground under current code.. Assuming a maximum of a 15 amp load at the pole barn, no load at the pump house and #6 wire all the way you would have a 6 volt drop. Bur all you could really use would be a few lights and maybe an electric drill. A circular saw till the first time it pinched in a board. No 5 HP compressor.

You IMHO are going to need to significantly increase the size of the wires to the pump house regardless of if you use it or no. Best plan may be to run new wire larger then #6 all the way from the main to the pole barn. You might even want to check out the price of 480/240 transformers. Bumping up the voltage up for transmission would reduce the size of the wire needed and the cost saving on the wire might offset the cost of the transformers. Plan B would be to use existing wires to the pump house but increase the voltage on them up to 480v. That would almost double the amps available at the pump house if I did the math right. (copper #6 - 125ft - 100a- 480v- 2.6% voltage drop) Of course your only coming off a 100 amp sub panel so the loads on it would limit actual available load.
 
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Old 05-12-09, 02:47 PM
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This project is extremely marginal. I suggest you ask your power company about a separate service to your pole building. Or maybe buy a generator.
 
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