Voltage drop calcs

[greg Forster]

Please walk me thru a voltage drop calculation.

200amp main panel➡️
125' in buried conduit to 100amp subpanel➡️
100' in buried conduit or direct bury to subpanel#2➡️
10' #12/2 to AC unit- "minimum circuit ampacity 25 Ampheres"

what are the wire size options? and allowable voltage drops from main panel to AC load

[Bob Coleman]

Try this: http://www.southwire.com/support/voltage-drop-calculator.htm

[Rollie Meyers]

Cannot use 12/2 NM cable at 25A, NM is limited to the ampacity of the 60° column of 310.15(B)(16), formerly 310.16 in 2011 & earlier editions, by 334.80, use pipe & THHN/ THWN & 12 AWG is fine for 25A for the A/C unit, otherwise 10/2 NM is required. Voltage drop is a footnote & not enforceable in the NEC, but a lot more info is required for VD calc's then has been given.

[greg Forster]

"minimum circuit ampacity 25 Ampheres"
I wrote this from memory and my memory serves me poorly;
I looked today and actually stamped as " min. circuit ampacity 15A"

[Rollie Meyers]

"minimum circuit ampacity 25 Ampheres"
I wrote this from memory and my memory serves me poorly;
I looked today and actually stamped as " min. circuit ampacity 15A"

What is the maximum amperes? With A/C equipment you get to play with different rules which means run the conductor which meets the minimum ampacity, and use the maximum fuse / circuit breaker allowed, no need to size at 125% because the equipment manufacturer has already done it for you.

If the maximum amperes are 25A, then a 25 ampere circuit breaker and 15A minimum circuit ampacity then 14 AWG would be OK, this would not apply to a branch circuit though, the reason it's OK for A/C equipment is that the compressor has overload protection, so a fuse / circuit breaker is just for short circuit protection.

[greg Forster]

Yes , max amperes 25A, 15A minimum
Then 12/2 with 25A circuit breaker?
It's 10' from subpanel to AC disconnect

[Rollie Meyers]

Yes , max amperes 25A, 15A minimum
Then 12/2 with 25A circuit breaker?
It's 10' from subpanel to AC disconnect

12/2 would be fine, since it is A/C equipment.

[Mike Heidrick]

For only 10' why would you not put in 10/2 in case it ever gets used for something else?

[Marty Gulseth]

What are the wire sizes (gauge) for the 125 ft and 100 ft runs to the subpanel? That's a long run, and may create some challenges, particularly on motor startup. Offhand, I'm recalling startup current surge of roughly 4X to 7X run current. With those lengths, and depending on wire size(s), you may get voltage sag that will cause problems on start.

Regards, Marty

[Rollie Meyers]

For only 10' why would you not put in 10/2 in case it ever gets used for something else?



It is already oversized with 12 AWG.

[greg Forster]

I ran 14 for the lights, 12 for everything else; the only thing the elec. inspector had me change was a 3 switch /2 circuit box for the lights. I put part of the lights on one circuit and the remainder on a second circuit. I tied the grounds separate for each circuit, he asked for all grounds in the box tied together -easy 60 second fix.

Now I'm trying to figure out the feeds to the panels

[greg Forster]

Mike, this building is a bit smaller than yours(192sqft to be exact), so the electrical requirements aren't much.330733





I built another larger building for power tools.330734330736