Feeder conductor size/voltage drop

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ammklq143

Senior Member
Location
Iowa
Occupation
Electrician
I have a bid I'm working on for a customer that's building a big shed. I originally sized the conductors at 250MCM aluminum because the conductors were going to be about 400 feet long. Now, he says it's ok to put the panel on the other end of the shed, so the total length would be around 300 feet. It's a 200 amp service, but most likely the load would probably never exceed 100 amps. Mostly lighting. 20-6 lamp flourescents, an air compressor occasionally, a welder occasionally, etc.

Here's my question. If I use the 3% voltage drop for 120 volts, I can have a drop of 3.6 volts, so that would put the acceptable voltage at 116.4 volts. Ok....so his true voltage is around 124 volts. Do I use the 116.4 volts as the threshold? I would think so, because it's the acceptable level. When I do my calculation for conductor size, can I use 7.4 volts instead of 3.6 volts.
 
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Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Voltage, Nominal is listed under definitions

300' @ 100 A @ 240V with 250MCM AL 20 deg C is about 1.7%

The distribution voltage would be 240v & 3% of that would be 7.2 v.

IMO don't worry about your actual applied voltage unless it is 5-10% low before a load is applied.

Remember VD is a fine print note.
 

ammklq143

Senior Member
Location
Iowa
Occupation
Electrician
Voltage, Nominal is listed under definitions

300' @ 100 A @ 240V with 250MCM AL 20 deg C is about 1.7%

The distribution voltage would be 240v & 3% of that would be 7.2 v.

IMO don't worry about your actual applied voltage unless it is 5-10% low before a load is applied.

Remember VD is a fine print note.


Ok, so would this be right?

Max distance for a conductor = (cmil X voltage drop) / (2 X K x I)

(250,000 cmil x 7.2) / (2 x 21.2 x 100 Amps) = 424.5 feet or would I use the 3.6 volts for the voltage drop since I'm muliplying the denominator times 2? If I use 7.2 as the value for voltage drop, I could use 4/0 for a length up to 359 feet.

I thought I should use one 120 volt leg of the conductor and the neutral for the calculation and that would mean I would use 3.6 for the voltage drop number?????
 

ammklq143

Senior Member
Location
Iowa
Occupation
Electrician
Use 120v if you have all the load on one leg of your panel. Most likely you will have a close to balanced load if you do it correctly with only the imbalance using the neutral.


It will be balanced as closely as possible, so I'd use the 240 volts and the 7.2 for the max. voltage drop, right? So, dose that mean I should be able to use 4/0 to feed the building up to 359 feet away?

(211,600 cmil x 7.2) / (2 x 21.2 x 100 amps) = Max Distance of 359.32 feet

I'd like to know what everyone else would do. The load in the shed is 20 - 4ft 6 lamp flourescents, a 30 amp air compressor that will kick on occasionally, a 30 amp pressure washer, and a 50 amp welder that will be used very little. Other than that, it's just small shop tools that would most likely be used.
 

tryinghard

Senior Member
Location
California
It will be balanced as closely as possible, so I'd use the 240 volts and the 7.2 for the max. voltage drop, right? So, dose that mean I should be able to use 4/0 to feed the building up to 359 feet away?

(211,600 cmil x 7.2) / (2 x 21.2 x 100 amps) = Max Distance of 359.32 feet

I'd like to know what everyone else would do. The load in the shed is 20 - 4ft 6 lamp flourescents, a 30 amp air compressor that will kick on occasionally, a 30 amp pressure washer, and a 50 amp welder that will be used very little. Other than that, it's just small shop tools that would most likely be used.
I would determine the calculated load per Art 220, it might take a bit but once done its good to live by and compliant and then you'll know what your feeding. This is not to mean your not already doing this,:) use Annex D examples to help like D3 or D3(a)
 

Electric-Light

Senior Member
What's the specs on the compressor? What's the NEMA code letter? (which determines kVA/hp in locked rotor state @ name plate voltage)

http://www.engineeringtoolbox.com/locked-rotor-code-d_917.html

What's the source power? two phases from 208Y/120 or 120/240 single split phase?

The biggest issue is that there is enough starting voltage available for the compressor, whatever the size it is. If the voltage at locked rotor kVA becomes too low, the motor won't develop the starting torque the compressor is design to have and depending on how much pressure it is starting up against, it might not start.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
It will be balanced as closely as possible, so I'd use the 240 volts and the 7.2 for the max. voltage drop, right? So, dose that mean I should be able to use 4/0 to feed the building up to 359 feet away?

(211,600 cmil x 7.2) / (2 x 21.2 x 100 amps) = Max Distance of 359.32 feet

I'd like to know what everyone else would do. The load in the shed is 20 - 4ft 6 lamp flourescents, a 30 amp air compressor that will kick on occasionally, a 30 amp pressure washer, and a 50 amp welder that will be used very little. Other than that, it's just small shop tools that would most likely be used.
Go with the 4/0 AL and sleep well at night. That's what I would do.
 
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