Correct Wire Size for underground?

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  #1  
Old 12-28-15, 09:02 AM
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Correct Wire Size for underground?

All wire is x3 using two hots and a neutral. Currently I have a service drop to a 200 amp panel on my property that drops only 30 feet from the transformer to a 200 amp panel. it then goes underground into pull box number one only ten feet away. I used 1/0 copper from the service drop/200 amp panel and swept up into pull box number one of three where i tied into 750 mcm Aluminum that goes 500 feet then sweeps up into and out of pull box number two and goes another 250 feet to pull box three. Making a 750 foot run to pull box three using 750mcm Aluminum

In pull box three - tied into the 750mcm I have copper going out to two places.
1. 1/0 copper running 60 feet to a 125amp panel in my well pump house
2. 1/0 copper running 70 feet to an indoor 200 amp service panel in my home.

QUESTION
I want to tie in another copper line to pull box three and take it underground another 250 feet to another 200 amp panel.

What size copper wire will I need?
how deep should the trench be?
I want to add a water line in the same ditch. What's the separation needed and I'll assume that the water would be on top?

I live in Placerville, California
 
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  #2  
Old 12-28-15, 09:19 AM
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Welcome to the forums.

750 mcm aluminum is roughly 400A cable.
1/0 copper is 170A cable.

Your feeding your long service with 170A of power.
What is currently connected at the other end ?

It doesn't sound like you have enough ampacity from the start for what you want to do.
 
  #3  
Old 12-28-15, 10:40 AM
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yes I realize this and when I installed this 15 years ago I had an electrician calc this out for me. He had me use the 1/0 copper because obviously the 750mcm aluminum is too big to fit in the lugs on the 200 amp panel. Perhaps you have another solution I'm not aware of?

Your question - "what is currently connected at the other end"? I'm sorry but I'm not sure I understand exactly. I did say, "In pull box three - tied into the 750mcm I have copper going out to two places.
1. 1/0 copper running 60 feet to a 125amp panel in my well pump house
2. 1/0 copper running 70 feet to an indoor 200 amp service panel in my home."

Are you asking me what's connected to each of those panels?

I've always been my understanding that the 200 amp panel at the service entrance protects the wire. If the load hasn't ever exceeded the 200 amps than what's the problem? In other words the 200amp breaker has never tripped.

I want to know what I need to do to run another 200amp service panel an additional 250 feet from my third pull box. This new panel is going to initially only be using roughly 125 amps but I want to run a 200amp service for future needs so therefore I need to make necessary improvements.

thanks for your reply
 
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Old 12-28-15, 10:56 AM
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My point was that you need to do a load calculation for everything that will be connected to that service. If it exceeds 200A you'll need to increase the service (meter end) size.
 
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Old 12-28-15, 11:05 AM
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gotcha. Ok that makes sense. Wouldn't it be ok to see first if the 200 amp breaker begins to trip? and then deal with increase of transformer, etc. ???
 
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Old 12-28-15, 02:11 PM
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Sure it would be ok to see if the 200A breaker trips.

So to run another copper cable from your last splice point would require 2/0 copper.
Technically 2/0 copper is rated for 195A. You may want to consider 3/0 copper.
 
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Old 12-28-15, 04:17 PM
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Old 12-28-15, 04:24 PM
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I think even at 4/0 there will a larger than normal drop with 750' of 750mcm aluminum fed with a 1/0 copper service.

I don't think the 4/0 will yield much improvement over the 3/0 in this application.
 
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Old 12-28-15, 04:27 PM
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Correction: To provide 150 amps at 240 volts 750 feet away, a 5% voltage drop over the entire run pro-rated is going to require 4/0 copper conductors. At least use the 4/0 for the parts you have not strung yet.

I'm not sure of the equivalent requirement for aluminum which would be fatter and be measured as MCM as opposed to AWG gauge.
 
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Old 12-28-15, 04:31 PM
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Any portion of the run for which you upgrad3e with fatter wire than exists or substitute fatter wire than you planned to extend it with will improve the voltage drop situation for the total run.

You might want to figure out the cost of having the power company install a pole transformer and new service in your property close to where your proposed 750' run ends. Compare that with the cost of 4/0 copper or fatter aluminum cable.
 
  #11  
Old 12-28-15, 04:37 PM
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With just 100' of run from a pole transformer you would need just the 2/O copper for 200 amps.
 
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Old 12-28-15, 06:12 PM
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1/0 copper is 170A cable.
I wouldn't consider the 1/0 copper to be good for more than 150 amps.

I've always been my understanding that the 200 amp panel at the service entrance protects the wire. If the load hasn't ever exceeded the 200 amps than what's the problem? In other words the 200amp breaker has never tripped.
That's the way it is supposed to work, but in order for it to work that way the feeder needs be rated at 200 amps.

Wouldn't it be ok to see first if the 200 amp breaker begins to trip?
If the 200 amp breaker begins to trip the feeder is severly overloaded.


Obviously none of this was ever inspected.
 
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Old 12-28-15, 08:58 PM
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Ok, I'll go with the 3/0 copper, THHN Building Wire. I found it for $2.65/ft
What size conduit would I need to run three wires?
What connectors would you recommend to make the splice?
 
  #14  
Old 12-28-15, 09:06 PM
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It was all inspected indeed... at the service drop by the county inspector and PG&E and the third pull box inspection was inspected by a hired electrician.

I'm not an electrician but I have always done my own work and enjoy doing so however, I always have had an electrician involved to make sure I was doing it correctly. I will say however, electricians - none of them seem to be on the same page when it comes to calc and wire size, etc. hardly any of them agree on a scenario either. I know there's rules to this stuff but it seems that those rules are often bent depending on who you talk to. I don't mean to sound like I'm condescending but look at the answers below between the other two gentlemen. they're different from yours. There is more than one answer it seems every time. This electrical stuff is baffling. I just want to do it right but also do it myself and I need some answers to my original questions and am so thankful to have found this wonderful forum full of knowledgeable people. It's confusing when you get many different answers.
 
  #15  
Old 12-29-15, 05:43 PM
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A competent electrician would never install 1/0 copper as a 200 amp feeder and no competent inspector would ever approve it. Although your feeder was upsized to 750 MCM, the feeder is still only as good as the 1/0 copper that was terminated at the 200 amp breaker on the line end of the feeder.
 
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