voltage drop calculation assistance

champion · Sep 9, 2004

Have a small commercial building, 120/208 3ph.
Am attempting to power six parking lot lights on three different support poles, each light is rated 1080va per light at 208v and each pole will support two lights.
The poles are mounted distances apart.
Building to top of 1st pole is 200feet and from 1st to second pole is 110 ft and from second to third pole is 200 feet.
The total distance from the building to the third pole is 510ft.
Am attempting to power these lights with 208v 3ph ckt protected with 30a brkr.
Each light will be connected accross the phases balaced. The last two lights in the string are giving me some trouble since only two lights will be connected between phase a-b, a-c.
At 501 feet total:
1080va x 6 =6480
6480va/(208 x 1.732)=17.9 amps.
Am attempting to use no. 6 conductor.
vd=1.732x12.9x501x17.9/26240 = 7.63
Which exceeds 3% allowed by code.
Since the last pole is not really drawing 17.9 amps. Would it be better for me to calculate between pole 2 and 3 then pole 1 and 2 and use the remainder for building to pole 1?
If I calculate the current between pole 2 and 3, connecting pole 3 lights between phases a-b, a-c and each light is rated 5.2 amps at 208v, would I use the 3ph formula or use the single phase formula for this drop.?
Confused because the third phase will not be run to pole 3.
or---
Should I use the resistance table 8, applying 5.2amps as my current draw? (two 5.2 amp lamps split between two phases).
Thank you in advance for feedback.

charlie b · Sep 9, 2004

Re: voltage drop calculation assistance

This is a special case, and a precise calculation would require some fancy math. But you don?t need to get that fancy, because you don?t need to be precise. You can calculate everything as a 3 phase system, as follows:

Start with the run from pole 2 to pole 3. You have 6 amps of current flowing a total of 200 feet. That gives you a 1.02 volt drop along that distance.

Next consider the run from pole 1 to pole 2. You have 12 amps of current flowing a total of 110 feet. That gives you a 1.12 volt drop along that distance.

Finally, consider the run from the building to pole 1. You have 18 amps of current flowing a total of 200 feet. That gives you 3.07 volt drop along that distance.

The total voltage drop is 5.21 volts, or 2.5% of a 208 volt system. That is acceptable.

By the way, having a voltage drop over 3% does not violate the NEC. The related NEC statement is in a Fine Print Note, and is therefore an unenforceable recommendation. Also, the recommended limit is 3% along a feeder, and 5% along the combination of a feeder and branch circuit.

voltage drop calculation assistance

champion

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charlie b

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