1. Junior Member
Join Date
Apr 2007
Location
Mpls, MN
Posts
19

## VD to Motors

I'm running 3 fans at a distance of 365ft from an MCC.

Fan are:
480, 10HP, 3PH

Given:
14amps from table 430-152
14 * 1.25 = 17.5a for wire size; could run 14THHN, but voltage drop.

Here's my calculation:
total distance I could run = (CM * vd)/(K * I * 1.73)
CM = 10380 from table 8
vd = 480 * .03 =14.4v
K = 12.9 for copper
I= 14a
and 3ph 1.73

Multiply all that out and you get about 465ft.
I'm using #10THHN wire rated at 90C

Here's my derating:
3 fans total for 9 wires = .7
Ambient Temp = .89
#10 THHN wire 90C = 40 x .7 x .89 = 24.92a; more than enough to handle 17.5a at that distance.
I'm running all this in 1" conduit. 3 conductors per motor and a GEC.

So why do I feel uneasy about this, everything checks out I think,,,, I've run the numbers many different ways and still it all adds up to being OK.
What do other people think?, have experienced?

2. Looks ok to me. Using an online calculator I get a voltage drop of 2.1% for #10s and 3.4% for #12s with 14 amp load at 365 feet and an ambient temperature of 40°C.
Where did the .89 correction factor come from? I don't see that in the code book.

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