I plan to install a 400-amp 3-phase service in 4" PVC. I believe that installing parallel service conductors (2 3/0 Cu THWN/THHN per phase) in the 4" raceway would total 6 current carrying conductors (neutral not counted) and require derating to 80%. Even using the 90 deg. column for derating purposes, the ampacity falls below 400-amps. Would 240.4(B) apply, or would I have to upsize to a pair of 4/0 per phase.

I will probably chech with AHJ, but was wondering what you guys thought about the installation.

The required ampacity of service conductors is called for in 230.42(A), and is based on the load calculations (125% of continuous loads plus 100% of noncontinuous loads). The ampacity of the conductors is to be determined from 310.15, which includes 310.15(B)(2)(a). As you have stated: 6 current carrying conductors in one conduit needs to be derated to 80%.

230.42(B) also requires the minimum ampacity for ungrounded conductors to be not less than the rating of the service disconnecting means, which you have given as 400 amps.

Assuming 90 degree insulation, paralled 4/0 copper, or 300 aluminum would be the minimum, as 260 amps x 2 x 80% = 416 for copper, and 255 amps x 2 x 80% = 408 amps for Aluminum.

240.4(B) does not come into play on services.

Earl

#171091 - 11/19/0705:19 PMRe: Parallel 3/0 or 4/0 for 400-amp
[Re: Redsy]

"230.42(B) also requires the minimum ampacity for ungrounded conductors to be not less than the rating of the service disconnecting means, which you have given as 400 amps."

This reference only deals with: "Specific Installations" If a person uses conductors that will carry the load (100% Non continuous and 125% Continuous) and the wire is derated due to raceway fill, he can round up to the next size overcurrent device unless he is over 800a. The "Specific Installations" rule doesn't apply to Redsy"s problem because he is over 100a. and less than 800a.

George Little

#171298 - 11/24/0702:35 PMRe: Parallel 3/0 or 4/0 for 400-amp
[Re: Redsy]

Service wires are only required to be sized to the computed load (230.42) not by the breaker since the service wires are up stream of the main breaker. The service wires are protected by the POCO OCPD at the transfomer. OCPD only protects wires down from the device, not up. Your local AHJ may have other restrictions. What is the computed load?

The calculated load is only about 85 KVA (205 amps @ 120/240 3-phase Delta). The plans, however were drawn by a PE and showed a 400-amp service with 600 kcmil copper. I have decide to parallel conductors instead and the Camden, NJ plan reviewer wants new drawings to reflect the new installation.

The local cost for 600 kcmil is about $9.50 / ft. 3/0 is $3.30 /ft. ($6.60 for 2)

At about 100' per leg, the savings is around $1,200.00.

Thoughts?

Last edited by Redsy; 11/24/0707:06 PM.

#171304 - 11/24/0708:44 PMRe: Parallel 3/0 or 4/0 for 400-amp
[Re: Redsy]

This is a whole new can of worms. You are working off a PE's drawing, you should bounce it off of him/her first. If you are parallelling a pre-sized wire, you must meet the ampacitiy of the original wire. a 600 kcmils is rated at 420 amps at 75 degree rating. 1/2 of that is 210 amps, so a 4/0 would be the minimum which is rated at 230 amps. With the derating of six conductors, you are at 368 amps or 88.3 kva. To meet 100% capacity of the 600 kcmils, 420 x 1.25 / 2 = 263 amps or 300 kcmils. Now you may be looking at a larger conduit(s) In other words, I suggest you contact the PE first. He stamped the drawings so he/she assumes the liability of the design. The PE may have taken in some consideration for some future expansion or some heavy load that you may not be aware of.

Another issue you can run into is whom is paying you to do the work. You are attempting value engineering and the one holding the money may want a piece of the savings.

I may be making mole hills out of ant hills however, I do not know all the details. These kind of things need to be considered prior to deviating from a stamped drawing.

The issue of parallel came up when the original EC decided to run (2) 2 1/2" conduits instead of a 4".

The city plan reviewer told me to have the PE draw it accordingly.

However, the PoCo requires a minimum 4" for a 3-phase 400-amp service, so I have to remove the original pipes and replace with 4".

The Engineering Firm is from out of state and works for the "Family Dollar" store corp. and will re-draw the service based on whatever is required by the locals.

As far as the load calculation, remember it is 3-phase Delta, so 85 kva is only 205 amps at 120/240.

Last edited by Redsy; 11/24/0711:27 PM.

#171347 - 11/25/0710:30 PMRe: Parallel 3/0 or 4/0 for 400-amp
[Re: Redsy]