Notices

Confused about sizing wire

Old 08-31-2011, 10:03 AM
  #1  
Senior MemberCaptains Club Member
Thread Starter
 
Join Date: Jul 2006
Location: RI
Posts: 6,357
Default Confused about sizing wire

I have buried conduit that I would like to run wire through to bring power to my barn. i would like a 50 amp subpanel. i would like 50 amps available for 240 volts, or 12000 watts. The run is approximately 300 feet, with total circuit length 600 feet

When I go to various online wire size calculators and enter 50 amps, 300 feet and 240 volts I get a wire size of #2. when I change the volts to 120, the wire size jumps to 2/0. If I have to run 3 wires of 2/0 (presumably only 1/0 required for the neutral) I will not run for a 50 amp panel, way too expensive. Even at 30 amps, I would need #4 and #2 for 240V/120V, respectively. I guess 30 amps is more than enough as I will only be powering lights and a welder or compressor from time to time, but I typically oversize things like this to account for future changes like a small AC or electric heater in the workshop.

for wire sizing, is the presumption that the amperage load is based on a 120 volt circuit? Having trouble with this one.
BACKTOTHESEA is online now  
Old 08-31-2011, 11:50 AM
  #2  
Admirals Club Admiral's Club Member
 
Join Date: Nov 2008
Location: Somewhere in the middle of Michigan
Posts: 10,369
Default

Wire in power systems is sized based on the allowable voltage drop at the rated current. Let's say the allowable voltage drop is 3%. The voltage drop is equal to the resistance of the wire multiplied by the current in the wire, or V=IR

So, for example, a length of a certain gauge wire drops 4 volts at 50 amps. In a 240 volt system, that is a 1.67% drop and acceptable. The same wire in a 120 VAC system also drops 4 volts, which is now a 3.3% voltage drop, and is not acceptable. The 120 VAC system requires a larger diameter (smaller gauge number and lower resistance) wire in this example.
yarcraft91 is offline  
Old 08-31-2011, 01:02 PM
  #3  
Senior MemberCaptains Club Member
Thread Starter
 
Join Date: Jul 2006
Location: RI
Posts: 6,357
Default

Ah, that makes sense.

So when sizing the wire, I should be sizing for amperage at 110 volts, giving me the same amperage at 220 volts as I am running two separate hots. So in a 30 amp circuit I would run two #2's plus neutral. I have an uncle who is an electrician that I'll call before running anything.

Originally Posted by yarcraft91 View Post
Wire in power systems is sized based on the allowable voltage drop at the rated current. Let's say the allowable voltage drop is 3%. The voltage drop is equal to the resistance of the wire multiplied by the current in the wire, or V=IR

So, for example, a length of a certain gauge wire drops 4 volts at 50 amps. In a 240 volt system, that is a 1.67% drop and acceptable. The same wire in a 120 VAC system also drops 4 volts, which is now a 3.3% voltage drop, and is not acceptable. The 120 VAC system requires a larger diameter (smaller gauge number and lower resistance) wire in this example.
BACKTOTHESEA is online now  
Old 08-31-2011, 09:28 PM
  #4  
aln
Senior Member
 
Join Date: May 2004
Location: West America
Posts: 3,640
Default

Don't forget the green wire....
aln is offline  
Old 09-01-2011, 04:54 AM
  #5  
Admirals Club Admiral's Club Member
 
Join Date: Nov 2008
Location: Somewhere in the middle of Michigan
Posts: 10,369
Default

Originally Posted by BACKTOTHESEA View Post
I have an uncle who is an electrician that I'll call before running anything.
That should get you the best answer.
yarcraft91 is offline  
Old 09-01-2011, 08:23 AM
  #6  
Admirals Club Admiral's Club Member
 
Join Date: Aug 2009
Location: VA
Posts: 5,932
Default

Considered getting the utility company to run it from the pole/transformer and putting a separate meter base on the barn? In my area, they do that very reasonably, and you can get 100A or whatever you want. I have three meters at the homestead--house, workshop and garage.
Buoy Scout is offline  
Old 09-01-2011, 08:53 AM
  #7  
Senior MemberCaptains Club Member
Thread Starter
 
Join Date: Jul 2006
Location: RI
Posts: 6,357
Default

Originally Posted by Buoy Scout View Post
Considered getting the utility company to run it from the pole/transformer and putting a separate meter base on the barn? In my area, they do that very reasonably, and you can get 100A or whatever you want. I have three meters at the homestead--house, workshop and garage.
Not an option. Closer to 400' from the nearest pole.
BACKTOTHESEA is online now  
Old 09-01-2011, 11:32 AM
  #8  
Admirals Club Admiral's Club Member
 
Join Date: Aug 2009
Location: VA
Posts: 5,932
Default

did they want to put in another pole? I think they limit the run to 300 feet from the pole, right? They wouldn't do that? Not sure where you are, but in Virginia at least, the utilities bend over backwards to get you power where you need it. Sales, you know.
Buoy Scout is offline  
Old 09-01-2011, 12:24 PM
  #9  
Senior MemberCaptains Club Member
Thread Starter
 
Join Date: Jul 2006
Location: RI
Posts: 6,357
Default

Originally Posted by Buoy Scout View Post
did they want to put in another pole? I think they limit the run to 300 feet from the pole, right? They wouldn't do that? Not sure where you are, but in Virginia at least, the utilities bend over backwards to get you power where you need it. Sales, you know.
They would need at least one, maybe two, and they would be in my back yard. i live on a peninsula of land nearly 500' deep and around 400' wide with power on the south side and the barn on the north side. Behind me is reservation land.
BACKTOTHESEA is online now  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are Off


Thread Tools
Search this Thread