copper feeder size for 3-phase 4-wire 225 amp OCP use 3/0 or 4/0?

[chase15]

3/0 XHHW is rated 200 amps @75c.
4/0 XHHW is rated 230 amps @75c.

per NEC 240.4B & 240.6 is it allowable to use 3/0 on a 225A ckt bkr? (for a non-continuous rated load).

my colleague says you cannot use 3/0 since there is an OCD rating of 200A so 3/0 cannot exceed this established OCD rating. the argument is that the next OCD rating of 225A must use 4/0 since 3/0 cannot go up to the next higher rating because it is locked in at 200A.

this is a big deal since there is an existing 2 inch conduit that can be utilized and the smaller 3/0 conductor would help the installation. (this feeder is not continuously rated.)

 

[skeshesh]

Unfortunately I have to agree with your colleague per 240.2(B)(2). Is it less cost effective to replace the 225A OCPD with a 200A?

 

[raider1]

3/0 XHHW is rated 200 amps @75c.
4/0 XHHW is rated 230 amps @75c.

per NEC 240.4B & 240.6 is it allowable to use 3/0 on a 225A ckt bkr? (for a non-continuous rated load).

my colleague says you cannot use 3/0 since there is an OCD rating of 200A so 3/0 cannot exceed this established OCD rating. the argument is that the next OCD rating of 225A must use 4/0 since 3/0 cannot go up to the next higher rating because it is locked in at 200A.

this is a big deal since there is an existing 2 inch conduit that can be utilized and the smaller 3/0 conductor would help the installation. (this feeder is not continuously rated.)

I agree with your colleague. 240.4(B) (2) states that to use the next size up rule the ampacity of the conductors must not correspond to a standard sized breaker or fuse. 3/0 copper has an ampaciity of 200 amps which is a standard sized breaker.

Chris

 

[iwire]

I agree with the others but will add that we run 4-4/0 and an EGC in 2" all the time.

 

[skeshesh]

I agree with the others but will add that we run 4-4/0 and an EGC in 2" all the time.

Can you also fit a #6 EGC in there?

 

[cadpoint]

XHHW is dual listed I'll assume that this will not help since the breakers are @75c?

 

[Dennis Alwon]

Can you also fit a #6 EGC in there?

Depending on the conduit. If it is emt no egc is required however a #6 egc will fit. PVC is another story-- won't get a #6 in there and be compliant. The op may be able to drop the size of the neutral if there are a lot of 3 phase load.

 

[steve66]

With the numbers I'm using, THHN wires, (4) 4/0, and (1) #6 ground, I get that it barely fits in rigid steel conduit.

For EMT, I get a fill of 40.04%, with the limit being 40%.

Are we allowed to round off the .04% , or does a maximum fill of 40% mean just that??

 

[skeshesh]

With the numbers I'm using, THHN wires, (4) 4/0, and (1) #6 ground, I get that it barely fits in rigid steel conduit.

For EMT, I get a fill of 40.04%, with the limit being 40%.

Are we allowed to round off the .04% , or does a maximum fill of 40% mean just that??

That's why I brought it up. One of our senior engineers told me he specified 2" (emt) on a project before where client's internal specs required EGC wire. The contractor refused to pull due to the fill ratio and inspector agreed with him. Apparently they argued over it for a while but the rounding down got shut down at every point by the inspector and his higher-ups. I guess it was one of the "expensive lessons to learn" (every experienced engineer, technician or contractor seems to have a few of those!). But I bet you could get away with it if you know the inspectors in the area and/or if they are not very strict.

 

[Dennis Alwon]

With the numbers I'm using, THHN wires, (4) 4/0, and (1) #6 ground, I get that it barely fits in rigid steel conduit.

For EMT, I get a fill of 40.04%, with the limit being 40%.

Are we allowed to round off the .04% , or does a maximum fill of 40% mean just that??

Op is using xxhw

 

[iwire]

With the numbers I'm using, THHN wires, (4) 4/0, and (1) #6 ground, I get that it barely fits in rigid steel conduit.

For EMT, I get a fill of 40.04%, with the limit being 40%.

Regarding the EMT.

With 4 - 4/0 and 6 THHN you should be under 40%, check your figures.

With 4 - 4/0s and 4 AWG (the required EGC for 225 amp feeder) you will be just a touch over.

 

[skeshesh]

Regarding the EMT.

With 4 - 4/0 and 6 THHN you should be under 40%, check your figures.

With 4 - 4/0s and 4 AWG (the required EGC for 225 amp feeder) you will be just a touch over.

I forget what size EGC the guy I mentioned used and I did not do the calcs - but what do you think about Steve's question regarding compliance?

 

[iwire]

I forget what size EGC the guy I mentioned used and I did not do the calcs - but what do you think about Steve's question regarding compliance?

As far as I know there is no rounding allowed ........ of course bare EGCs are also allowed and that would stay under 40% with the others being THHN, I have not run the numbers with XHHW.

 

[tkb]

Regarding the EMT.

With 4 - 4/0 and 6 THHN you should be under 40%, check your figures.

With 4 - 4/0s and 4 AWG (the required EGC for 225 amp feeder) you will be just a touch over.

Bob, I think you should check your calculations.

4-4/0 & 6 THHN in 2" EMT = 40.09%
4-4/0 & 4 THHN in 2" EMT = 41.04%

4-4/0 & 6 XHHW in 2" EMT = 39.86%
4-4/0 & 4 XHHW in 2" EMT = 40.53%

I may be wrong, but I think my calculations are correct.

 

[iwire]

Bob, I think you should check your calculations.

They are sitting on my desk at work.

From Table 4:

2 EMT area for more than 2 'wires' 1.342 sq in.

From Table 8:

4/0 THHN = 0.3237 * 4 = 1.2948

6 THHN = 0.0507

1.2948+.0507=1.34. :grin:

 

[tkb]

They are sitting on my desk at work.

From Table 4:

2 EMT area for more than 2 'wires' 1.342 sq in.

From Table 8:

4/0 THHN = 0.3237 * 4 = 1.2948

6 THHN = 0.0507

1.2948+.0507=1.34. :grin:

Correct so far. Table 5 is for the conductors.

1.3455 / 3.356 (100% area of 2" EMT) = 40.09%

 

[iwire]

Correct so far. Table 5 is for the conductors.

1.3455 / 3.356 (100% area of 2" EMT) = 40.09%

Sorry table 5.

But that said, what makes your way more right than my way?

There is nothing in the NEC that tells us how to make the calculation and nothing in my calculation is false.

 

[Dennis Alwon]

Why are we using an EGC and why are we using #6. If it is a 225 amp breaker it must be #4- which I originally missed- and no egc is even needed assuming emt.

We are all making a lot of assumptions. Type of conduit- op says xxhw which is smaller than thhn. Even with #4 and xhhw in emt it would be .017 sq. in over.

 

[tkb]

Sorry table 5.

But that said, what makes your way more right than my way?

There is nothing in the NEC that tells us how to make the calculation and nothing in my calculation is false.

Your calculation only calculated the area of the conductors and not the fill of the conduit.
You need to divide the conductor area by the conduit area to get the percentage of fill.

 

[Team-MEI]

(4) #4/0 and (1) #6 XHHW?compact fits in (1) 2" EMT.