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- Retired Electrical Contractor
Again why #6... Isn't this a 225 amp breaker?
copper feeder size for 3-phase 4-wire 225 amp OCP use 3/0 or 4/0?
- Thread starter chase15
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Again why #6... Isn't this a 225 amp breaker?
skeshesh
Senior Member
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- Los Angeles, Ca
#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
- Location
- Massachusetts
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
#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.
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
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hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
#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.:roll:
But the OP states 3 phase 4-wire which I take as meaning 3 phase conductors and a neutral not an EGC?
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hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
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:
- Location
- Massachusetts
- Location
- Illinois
- Occupation
- retired electrician
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.
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.
skeshesh
Senior Member
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- Los Angeles, Ca
I don't really see a reason in doing that. The current your conductor may carry is still limited by the OCPD and so is the fault current should a fault occur. Can you provide a reason why you would upside your EGC conductor in such a case?
jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
250.122 Size of Equipment Grounding Conductors.
(B) Increased in Size. Where ungrounded conductors are
increased in size, equipment grounding conductors, where
installed, shall be increased in size proportionately according
to the circular mil area of the ungrounded conductors.
skeshesh
Senior Member
- Location
- Los Angeles, Ca
I stand corrected. Now that I think about it, I guess the impedance of the circuit will decrease when upsizing the conductor for VD and will allow for higher fault current. I read through 250 again recently and can't believe I missed this...
I forgot about the 225 A breaker: I should have used a #4 ground.
But I never thought about using a bare ground. That might just be enough to make it fit without any rounding.
One of my concerns is I think wire sizes (and conduit sizes also) change a little from one manufacturer to another. I'm always afraid if I specify 4/0 in a 2" conduit, some inspector or electrician is going to do the calculation using actual wire and conduit sizes, and with my luck, they wouldn't fit.
Bob:
From an installation perspective: is it much harder to pull 4/0 into a 2" conduit? Does it seem like you really have to force the wire in, or is it pretty much like pulling 3/0 ?
STeve
But I never thought about using a bare ground. That might just be enough to make it fit without any rounding.
One of my concerns is I think wire sizes (and conduit sizes also) change a little from one manufacturer to another. I'm always afraid if I specify 4/0 in a 2" conduit, some inspector or electrician is going to do the calculation using actual wire and conduit sizes, and with my luck, they wouldn't fit.
Bob:
From an installation perspective: is it much harder to pull 4/0 into a 2" conduit? Does it seem like you really have to force the wire in, or is it pretty much like pulling 3/0 ?
STeve
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