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

1. Originally Posted by Team-MEI
(4) #4/0 and (1) #6 XHHWâ€“compact fits in (1) 2" EMT.
Again why #6... Isn't this a 225 amp breaker?

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Originally Posted by Dennis Alwon
Again why #6... Isn't this a 225 amp breaker?
#6 is based on 200A OCPD size per art. 250. It is a common installation since 4/0 may be specified due to Voltage Drop. The reason why EGC wire is considered is that many engineers and indeed many internal specifications for clients, ranging from universities to defense contractors to military, call for an EGC wires (the merits of which, or lackethereof, have been discussed a considerable number of times on this forum and elsewhere). I'm pretty sure you've been involved in many of those threads so you should have a good idea of both sides of arguments for this practice

3. Originally Posted by tkb
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.
I don't have to figure the percentage, I just can't exceed what is shown in the column of Table 4.

4. Originally Posted by skeshesh
#6 is based on 200A OCPD size per art. 250. It is a common installation since 4/0 may be specified due to Voltage Drop. The reason why EGC wire is considered is that many engineers and indeed many internal specifications for clients, ranging from universities to defense contractors to military, call for an EGC wires (the merits of which, or lackethereof, have been discussed a considerable number of times on this forum and elsewhere). I'm pretty sure you've been involved in many of those threads so you should have a good idea of both sides of arguments for this practice
I understand that but I thought the op was using 4/0 for a 225 amp breaker.

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

5. Originally Posted by iwire
I don't have to figure the percentage, I just can't exceed what is shown in the column of Table 4.
It does exceed Table 4.
Table 4 for 2" EMT = 1.342
Area of conductors = 1.3455

You rounded your calculation down to 1.34

Also as Dennis stated, you cannot use this a 225a feeder without a #4 ground. Needs 2-1/2" EMT
Last edited by tkb; 02-15-11 at 09:07 PM. Reason: additional stuff

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Originally Posted by skeshesh
#6 is based on 200A OCPD size per art. 250. It is a common installation since 4/0 may be specified due to Voltage Drop. The reason why EGC wire is considered is that many engineers and indeed many internal specifications for clients, ranging from universities to defense contractors to military, call for an EGC wires (the merits of which, or lackethereof, have been discussed a considerable number of times on this forum and elsewhere). I'm pretty sure you've been involved in many of those threads so you should have a good idea of both sides of arguments for this practice
If you have up the current carrying conductors for voltage drop you would still have to up the EGC one size so even with 4/0 on a 200 amp breaker you would still have a #4 EGC. so its 3/0 on a 200 with a #6 EGC max for existing conduit, or you use the conduit for the EGC.

But the OP states 3 phase 4-wire which I take as meaning 3 phase conductors and a neutral not an EGC?
Last edited by hurk27; 02-16-11 at 12:44 AM.

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I would agree with the OPer if the load is a motor load, in which he could install 3/0 and have a 225 amp breaker, but then a neutral wouldn't be needed, so I guess more input is needed:grin:

8. Originally Posted by tkb
It does exceed Table 4.
Table 4 for 2" EMT = 1.342
Area of conductors = 1.3455

You rounded your calculation down to 1.34
I did not round anything down, I simply calculated it to two places. How many places does the NEC require?

I still ask ......... where does it say I cannot calculate it this way.

Also as Dennis stated, you cannot use this a 225a feeder without a #4 ground.

I mentioned that way back in post 11. :grin:

9. My opinion is that you use the number of decimals that you have in the code book. So for the wire area you use 4 and for the conduit area you use 3.

10. Originally Posted by iwire
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:
Originally Posted by iwire
I did not round anything down, I simply calculated it to two places. How many places does the NEC require?

I still ask ......... where does it say I cannot calculate it this way.

I mentioned that way back in post 11. :grin:
It looks like rounding down to me.

1.2948+.0507=1.3455
Rounded to two decimal places = 1.35, thus over 40%
If the NEC gives you 4 decimal places that is the accuracy they are looking for.

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