What size conductors

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  #1  
Old 07-19-06, 12:10 PM
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What size conductors

I have a garage and office 450 ft. from the SE panel and want to put a sub panel in the garage to run a A/C unit (50amp-240V) and lighting 40 amps-120V). Total load 70 amps 240V. Run in 3" PVC conduit.
What size conductor is needed for about 2% voltage drop?
A/C is an inductive load (compressor Motor). Can I reduce the neutral one size from the phase wires? Much thanks, I appreciate your help. Big Bob
 
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Old 07-19-06, 12:25 PM
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No receptacles? What are you going to do sit there in the cool air and read?

I calculate 3/0 copper.

By the way, there are numerous voltage drop calculators on the Internet. Just google "voltage drop calculator" and choose one. Or better yet, choose several and verify that the results agree.
 
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Old 07-19-06, 01:24 PM
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Thanks Racraft! Wow! What a quick reply. I figured about 2/0
or 3/0. Have you seen the price of copper lately? I guess that
I will have to take a 2nd morgage out on the place. Big Bob
 
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Old 07-19-06, 02:24 PM
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Why a 2% drop? That is cuirously low.
The NEC's recommendation is 5% for feeders.

Also, 40 amps for lighting?!? That's 4800 watts!!!
You are not simply adding two 20 amp breakers to get the 40 are you? That is not how it is calculated.
 
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Old 07-19-06, 03:03 PM
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Originally Posted by IBigBob
Thanks Racraft! Wow! What a quick reply. I figured about 2/0
or 3/0. Have you seen the price of copper lately? I guess that
I will have to take a 2nd morgage out on the place. Big Bob
Yes, I have!! Copper is getting more pricey by the day! When I did my 200A service, 2/0 copper was 1.40/ft @ HD if I remember correctly. Luckily, my conductors were only 12ft long from socket to box. That was awile ago, how much is it now? At 450ft, you'll go broke stringing those fat-boy copper lines. Maybe AL might work for you?

Joe Michel
 
  #6  
Old 07-19-06, 03:22 PM
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IBigBob,

My comment, "What are you going to do sit there in the cool air and read," was partially serious.

My concern is that you failed to calculate the load for receptacles for the office, or that they are on another circuit.

If you failed to calculate the load then you need to do so.

If they are on another circuit, then you have a problem. You are only allowed a single circuit from one building to another. If you already have one circuit from the main panel to this outbuilding, you cannot add a second circuit. You must replace the first circuit. This means that whatever loads it serves must be added to the calculation for the new circuit.

Why don't you shed some light on the project and tell us ALL that is involved.
 
  #7  
Old 07-19-06, 04:30 PM
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Thanks, Speedy Petey, You guys are wonderful. A 4% voltage
drop was in my consideration. I figured a 20 amp recpt.circuit
and a 15 amp lighting circuit, too. The reason that I thought that a 2% VD is the first choice is that A/C compressor motors don't run as well on a lower voltage as on a higher one. Another consideration is that a 3/0 wire doesn't fit into a 70 amp 2 pole breaker lugs. Any suggestions about making that work? Maybe a smaller wire and a split bolt?

Racraft, what I was thinking was that 50 amps 240 v would go to the A/C. This
would allow 20 amps left across each bus. One 20 amp recpt. circuit on one bus and one 15 amp lighting circuit on the other bus. That is basically the load.

Thanks again,
for the terrific advice. Big Bob
 

Last edited by IBigBob; 07-19-06 at 04:40 PM.
  #8  
Old 07-20-06, 01:19 AM
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Originally Posted by Speedy Petey
Why a 2% drop? That is cuirously low.
The NEC's recommendation is 5% for feeders.
FPN No. 2 to article 215 (Feeders) says conductors for feeders “will provide reasonable efficiency of operation” if sized to prevent a voltage drop of 3 percent or less at the furthest outlet. The 5% limit recommendation is for the furthest outlet of both feeders and branch circuits combined.
 
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Old 07-20-06, 04:50 AM
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What's your point?
 
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Old 07-20-06, 05:22 AM
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Originally Posted by IBigBob
...what I was thinking was that 50 amps 240 v would go to the A/C. This
would allow 20 amps left across each bus. One 20 amp recpt. circuit on one bus and one 15 amp lighting circuit on the other bus. That is basically the load.
BigBob,

without knowing more about your layout, it's hard to make exact recommendations, but you might want to consider separating out the load into more circuits.

An office, to me, implies some office equipment. A computer or two, and other items. I would certainly install at least two circuits for this office. As for lighting, you might want to split that too. Having two circuits for lighting means you can balance the load better across the 240 volt service.

As for the garage part of this (if the garage is different than the office), don't forget circuits for receptacles there (needing GFCI protection), as well as for lights and garage door opener(s).
 
  #11  
Old 07-20-06, 10:58 AM
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Originally Posted by Speedy Petey
What's your point?
The NEC recommends the voltage drop on a feeder not exceed 3%. Your statement implies a 5% drop is desired.

My point, simply, was clarification.

Some feel the NEC’s FPN of 3 and 5% are too high. 2% for a feeder is not an unreasonable goal or, IMO, curiously low.
 
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