Sizing Multi wire Branch Circuit for Voltage Drop and Bu

[tenee]

Here's my question. I have a street lighting system. The lights are equipped with receptacles on top and 2- 100W lights. My service is 3 ph 4 wire 120/208Y vac. The run is about 600' with lights poles distributed about every 75 to 100'. The lights have a multitap ballast and are being fed 1 phase 208 vac. The receptacles are 120 vac @ 20 amps. The lights are fed with one 15 A two pole breaker. The receptacles are fed from 3 20A single pole breakers (3 receptacles to a breaker).

Question #1: To avoid voltage drop on receptacle circuits from exceeding 3% I have oversized the conductors over the length to maintain a less than 3% drop at the furthest receptacle. My wire starts at #1/0 and is reduced in size (between the respective lights) as the length of the run becomes shorter. All 3 receptacles circuits are fed with a common neutral. Am I required to oversize the neutral because of the larger potential load (in addition to the oversizing required to accomodate the voltage drop)? Furthermore am I required to derate the oversized conductors to 80% to account for the bundling (2 ungrounded lighting conductors and four ungrounded receptacle conductors) in addition to the voltage drop upsizing already performed?

Question #2:

Are GFCI receptacles required to have a separate neutral wire (one per receptacle) per the code? to work correctly?

Are GFCI Breakers required to have a separate neutral wire (one per breaker)per the code? to work correctly?

Question #3:

For multiwire branch circuits is there any NEC Code requirements that specify a minimum of 1 neutral for every three multiwire branch circuit conductors?

Your thoughts are appreciated.

 

[roger]

Re: Sizing Multi wire Branch Circuit for Voltage Drop and Bu

What are these receptacles doing? to spend the money to start this branch circuit with 1/0 seems extreme in the least.

For an answer to question #1 you must derate the conductors per Table 310.15(B)(2)(a)

#2 No

#3 No, read 225.7.

Roger

[ December 15, 2004, 08:46 PM: Message edited by: roger ]

 

[ira]

Re: Sizing Multi wire Branch Circuit for Voltage Drop and Bu

You do not give the loads on the receptacle circuits - but for a 600ft run with 16 amps I found two references that give a wire size of #2.

The neutral wire will not have to be increased in size above that of the phase wires unless there are harmonic producing loads on the receptacles. Non harmonic loads will not have a neutral current greater than the phase current.

You will have to derate the wires per code due to bundling, but 80% of the wire ampacity will be well above your load.

Don't forget that you will also have to increase the lighting circuit wire size for voltage drop.

With the length of the circuits you should consider using GFCI receptacles rather than circuit breakers to minimize nuisance tripping.