voltage drop question

[Stevenfyeager]

Feeding a barn from a house, I ran #1 AL from a panel to a j box (20 feet) on a 100 amp breaker. From there I will be going 184 feet underground. I am tempted on increasing the size of the underground conductors to cut down voltage drop. But would that do any good since my first 20 feet is #1 AL SER ? (SER is now inside a wall and not easy to replace) Thank you.

 

[Dennis Alwon]

Sure it will do some good. Often times the cable is reduced to connect under the lugs of the breaker. The 20 feet may have some drop or none at all depending on what you upsize to on the 184'

 

[infinity]

Since your 20' of SER is in the wall does it need to be used at it's 60° C ampacity?

 

[gar]

190501-0802 EDT

Stevenfyeager:

Quite obviously using much heavier wire for the long distance portion will make a big difference.

See http://www.paigewire.com/wire_resistance-prop.aspx?AspxAutoDetectCookieSupport=1
Aluminum at room temperature is about 0.2 ohms/1000 ft. You have a 40 ft run of #1 or about 100 ft or 0.02 ohms. At 100 A this is a drop of about 2 V.

Another 190 ft or 380 ft of loop for #1 aluminum you add about 0.4*0.2 = 0.08 ohms. At 100 A this is another 8 V drop.

Going to 900 MCM aluminum the added resistance for the 400 ft loop portion will drop to about 1/10 of #1. Or an 0.8 V drop at 100 A for that portion of the total loop. Work it out for whatever is a reasonable balance of cost vs voltage drop.

Check my math on the above.

.

 

[Dennis Alwon]

Of course, 100 amp feeder is unlikely going to see 100 amps. VD is based on load not breaker size. If you really think the load will be up at the higher end then you may need to use 90 or 100 amps.

Also remember section 250.122(B)

 

[gar]

190501-0838 EDT

I should add that if you have moderate size motors in the barn, like 10 HP, then their starting current can be in the many 100s of amps. Additionally, if they have a high inertia load, like a large saw blade, then starting current time can be long, and is extended by high voltage drop.

If nothing is bothered a great deal by voltage drop, like lights, then go with #1 all the way.

.

 

[kwired]

190501-0838 EDT

I should add that if you have moderate size motors in the barn, like 10 HP, then their starting current can be in the many 100s of amps. Additionally, if they have a high inertia load, like a large saw blade, then starting current time can be long, and is extended by high voltage drop.

If nothing is bothered a great deal by voltage drop, like lights, then go with #1 all the way.

.

Tricky balance to the long run and high inertia load, or even high surge loads at startup. The voltage drop of the long run has a soft starting effect to some degree and isn't always totally undesired. Voltage effects on other loads on the same feeder may not always be desired though.

 

[kwired]

Feeding a barn from a house, I ran #1 AL from a panel to a j box (20 feet) on a 100 amp breaker. From there I will be going 184 feet underground. I am tempted on increasing the size of the underground conductors to cut down voltage drop. But would that do any good since my first 20 feet is #1 AL SER ? (SER is now inside a wall and not easy to replace) Thank you.

There is a voltage drop on that first 20 feet, but it may be pretty insignificant compared to the additional 184 feet of the run.