OCPD/Conductor Sizing Confusion

kenjsil · Jul 18, 2004

I just read our patron's article called "Conductor Sizing and the National Electrical Code". The link is: http://www.mikeholt.com/technical.php?id=nec/unformatted/necconductorsizing&type=u&title=Conductor%20Sizing%20and%20the%20National%20Electrical%20Code

I thought I understood this topic well, but now I'm confused. Please help me with an example calculation:

Suppose you had to find the correct OCPD and Cu THHN feeder conductors to be run in conduit for a 4-Wire 3-Phase Wye 75C panelboard with a continuous 190 Amp load at an ambient of 30C.

Under 215-3 and 384-16(d), the OCPD must be at least 1.25 x 190 Amps = 238 Amps. From the standard CB ratings in 240-6, the minimum OCPD is 250 Amps.
Under 215-2, the conductors must also be rated for at least 238 Amps, and under 110-14(c), the 75C conductor rating must be used. So table 310-16 gives a conductor size of 250 MCM with a 75C rating of 255 Amps.
As we have 4 current-carrying conductors in the pipe, we have to derate the ampacity using table 310-15(b)(a)(2). Under 110-14(c), we use the 90C rating for this, so the 250 MCM is derated to 290 Amps x 0.8 = 232 Amps.
Under 240-3(b), we can use the next larger standard OCPD to protect the 250 MCM, which is 250 Amps. And this satisfies the minimum OCPD required for the load.

Here's my confusion: Even though the minimum OCPD for the conductors of 250 Amps, and the conductor's 75C ampacity of 255 Amps are adequate, the derated 90C conductor ampacity of 232 Amps is less than the 238 Amps required. Should the conductor be increased to 300 MCM (which would have a derated ampacity of 256 Amps)?

roger · Jul 18, 2004

Re: OCPD/Conductor Sizing Confusion

My take on this is, (per the letter of the code) the conductor after all adjustments must be at or above the load ampacity.

The adjusted number is the conductors ampacity and all table columns are out of the picture.

Now this is where some common sense should be applied, and if I were an inspector I would use 90.4 and allow this installation.

If the question was on an exam, you would have to go with the 300 mcm.

Roger

don_resqcapt19 · Jul 18, 2004

Re: OCPD/Conductor Sizing Confusion

Ken,
There is nothing in the information that you have provided that tells me that we have 4 current carrying conductors.
don

kenjsil · Jul 18, 2004

Re: OCPD/Conductor Sizing Confusion

Ken,
There is nothing in the information that you have provided that tells me that we have 4 current carrying conductors.
don

Don,
You are quite right. I was assuming a nonlinear load, but didn't state that.

OCPD/Conductor Sizing Confusion

kenjsil

Member

roger

Moderator

don_resqcapt19

Moderator

kenjsil

Member