Calculation Check

Status
Not open for further replies.
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician (retired)
I was checking over some engineered drawings and found that they have specified #4/0 Cu for a run about 250' with a 100 amp apartment panel and a 100 amp OCPD ahead of the feeder. In using the Southwire voltage drop calculator I come nowhere near needing #4/0 Cu even with a 3% maximum drop and a full load of 100 amps. Voltage is 102/208, 1?. Some shorter runs 175'-200' are calling for #2/0 Cu. Seems like overkill to me unless the calculations are incorrect. Anyone want to take a stab at the EGC size for #4/0?
 
Mike's 'free stuff' calculator on a 250 ft run of cu at 100 amps, 240v single phase calls for a 1/0 cu for a 3% voltage drop (actual 6.1 volt drop)
 
For 208V, 2/0AWG seems to be the value.

Normally from the 75 deg column, you waould have had #3, so the EGC would need to be ~2x the cross sectional area of the regular EGC #8.
 
If you run the calc with 100 amps on only one phase conductor at 120 volts, I come up with a 2.8% drop using 3/0 at 250'. This is not realistic, but a 100% loading on a single phase conductor with no loading on the other phase conductor is the worst case.

As far as the EGC size if you use 4/0, it would be the cross sectional area of the 4/0 divided by the cross sectional area of #3, and that result times the cross sectional area of #8.
(211600/52620)(16510) =66,391. This is the minimum cross sectional area of the EGC. #2 is 66,360 so you will have to use a #1 EGC with the 4/0 phase conductors on the 100 amp circuit.
 
I would guess that the 100 amps is probably not even close to the connected load of a small apartment.
 
NEC Demand Load calculations for an apartment complex I am in the middle of. Numbers are for each unit.

1 br: 107 A

2 br: 112 A

3 br: 113 A

Mains are 125 A

So maybe the 4/0 isn't too far off.

But, as we all know the NEC is ultra conservative. Just think of the diversity factor the utilities use for services. But, again, we are bound to the NEC.

RC
 
infinity said:
Well there have been questions about value engineering and it appears that most of these feeders were designed with overkill in mind. :)
Click to expand...

I have never liked the misnomer of value engineering.

Either bid to the specs or bid to something else but I hate it when someone comes back after the fact and wants to cheapen things.
 
petersonra said:
I have never liked the misnomer of value engineering.

Either bid to the specs or bid to something else but I hate it when someone comes back after the fact and wants to cheapen things.
Click to expand...

"value engineering" typically comes down from the client level, not the person bidding the job. What clients do you know of that are excited about a ton of money on MEP systems? Whoever they are please introduce us.
 
don_resqcapt19 said:
As far as the EGC size if you use 4/0, it would be the cross sectional area of the 4/0 divided by the cross sectional area of #3, and that result times the cross sectional area of #8. (211600/52620)(16510) =66,391. This is the minimum cross sectional area of the EGC. #2 is 66,360 so you will have to use a #1 EGC with the 4/0 phase conductors on the 100 amp circuit.
Click to expand...
I came up with the same calculated numbers. But I would be willing to go with a #2. The manufacturing tolerances of wire would easily overwhelm the significance of the 0.05% difference between 66,391 and 66,360.

 
charlie b said:
I came up with the same calculated numbers. But I would be willing to go with a #2. The manufacturing tolerances of wire would easily overwhelm the significance of the 0.05% difference between 66,391 and 66,360.
Click to expand...
Charlie...that would clearly be a code violation and the building would burn down...I'd have to give you a red tag:)


From a real world and a practical standpoint, I agree...the #2 would be fine.
 
Status
Not open for further replies.
Top