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Old 08-08-2011, 03:54 AM   #1
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Unusual project, questions regarding proper cable and ampacity


This is a little complicated so I'll do my best to explain.

From the main breaker box (200 amp) I've got a 60 amp double breaker at 240 volts with #8 gauge cable going underground for about 150 feet until it reaches a pump house. At this pump house the cable connects to a sub-panel (sub-panel #1). On the sub-panel there is a 30 amp breaker to a circuit to run a well pump.

In this same sub-panel in the pump house I'd like to wire two 15 amp 110 volt circuits, one for two wall outlets and one for two overhead lights both with #14 gauge cable.

Also from this same sub-panel I'd like to put in a 30 amp single breaker that runs #10 gauge cable out of the pump house underground for about 250 feet to another shed. This would connect to yet another sub-panel (sub-panel #2) where again two 15 amp circuits would be wired, one for two lights, one for two outlets.

My main questions are, is the #8 gauge wire from the main to the pump house sufficient for the load? Is it appropriate to have a 60 amp breaker on the #8 gauge wire? Is the #10 gauge wire for the 250 foot span from the pump house (sub-panel #1) to the shed (sub-panel #2) sufficient for its load? The outlets will be used for small appliances and power tools only occassionally.

Please shoot me your advice and recommendations. Thanks.

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Old 08-08-2011, 07:32 AM   #2
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Unusual project, questions regarding proper cable and ampacity


My read of the allowable ampacity table in NEC (table 310-16) is that #8 direct burial wire (I assume you have UF wire, you do not state what type of wire you have) has an allowable ampacity of 40 amps, not the 60 you apparently are allowing.

As for using #10 wire to the pump house, the tables allow for 30 amps for direct burial #10 wire. However, the voltage loss at 30 amps would be 7.5 percent at 240 volts, which would exceed recommended practice. You would need #6 wire to hold the voltage loss to 3 percent. Of course, you don't state what the load is, it could be much less than 30 amps, and that would affect the computations. I have a shed about 100 feet from my house, and it is wired with #10 wire, just a few lights and an outlet on it, it works fine.

By the way, see other posts on this site for information regarding grounding needs at subpanels in disconnected buildings, proper technique to separate equipment ground from neutral. You are running four wire cable I assume so you get a neutral?

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Old 08-08-2011, 09:29 AM   #3
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Unusual project, questions regarding proper cable and ampacity


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Originally Posted by pad2383 View Post
From the main breaker box (200 amp) I've got a 60 amp double breaker at 240 volts with #8 gauge cable going underground for about 150 feet until it reaches a pump house. At this pump house the cable connects to a sub-panel (sub-panel #1). On the sub-panel there is a 30 amp breaker to a circuit to run a well pump.


In this same sub-panel in the pump house I'd like to wire two 15 amp 110 volt circuits, one for two wall outlets and one for two overhead lights both with #14 gauge cable.

Also from this same sub-panel I'd like to put in a 30 amp single breaker that runs #10 gauge cable out of the pump house underground for about 250 feet to another shed. This would connect to yet another sub-panel (sub-panel #2) where again two 15 amp circuits would be wired, one for two lights, one for two outlets.

My main questions are, is the #8 gauge wire from the main to the pump house sufficient for the load? Is it appropriate to have a 60 amp breaker on the #8 gauge wire? Is the #10 gauge wire for the 250 foot span from the pump house (sub-panel #1) to the shed (sub-panel #2) sufficient for its load? The outlets will be used for small appliances and power tools only occassionally.

Please shoot me your advice and recommendations. Thanks.
Why do you need a sub at the second building? Two lights and two receptacle could be fed from the first using a 20 amp breaker and #10 wire. You would need a disconnect switch in the second building but that is easy to do. If you run to a sub panel you will have to follow all the rules for a sub panel.
Your #8 is undersized for a 60 amp breaker.
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Old 08-08-2011, 12:19 PM   #4
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Unusual project, questions regarding proper cable and ampacity


With no subpanel you must have a main panel breaker pair for the entire pump house feed no more than 20 amps.

This 20 amps @ 240 volts must be shared among the pump, lights, and other things you may want to use.

If the cable is 10 gauge or larger (to combat voltage drop for the distance) then the continuous versus intermittent rule does not affect you here; you can use the full 20 amps continuous.
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Last edited by AllanJ; 08-08-2011 at 12:23 PM.
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Old 08-08-2011, 12:31 PM   #5
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Unusual project, questions regarding proper cable and ampacity


Allan,
If you are commenting on what I wrote, I was referring to the second building which the OP wants to wire. The pump house has a sub panel now, maybe not correctly wired, but it is in place.
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