Need check on wire size for Voltage drop calcs

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alphabraxon

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Done a VD clac and need some one to advise.

208/120 Volt 3 phase 4 wire 60 Hz

I= 1600A D= 300 FT. VD =3% Copper in PVC conduit 3 ft. under ground.

ambient is about 90 deg F.

Came up with 1 785.36 MCM

4 500MCM per line only does about 1540 Amps

5 400MCM per line is 1675 amps.

Assuming I dont have to be NEC compliant, which, if any, is right?

appreciatte any help.
 
alphabraxon said:
208/120 Volt 3 phase 4 wire 60 Hz

I= 1600A D= 300 FT. VD =3% Copper in PVC conduit 3 ft. under ground.
5 400MCM per line is 1675 amps.
Click to expand...

5 400 kcm VD = 3.8%
4 600 kcm VD = 3.9%
5 500 kcm VD = 3.4%
5 600 kcm VD = 3.1%

Do you need to add 25% for continuous load?

Assuming I dont have to be NEC compliant, which, if any, is right?
Click to expand...
What does this mean?
 
Thanks for the replys.

Yes , we are not governed by NEC or other "local" AHJs. However as a E.E. with 20+ yrs of design/build experience have NEC found it to be very helpful.

Max 3 conductors/line?

Basic calc from 2008 NEC

cmils=1.73*12.9[ohms]*1600[amps]*300[ft]/6[volts]

= 1750 M[cmils] more or less

Am I doing this right?

My doubt is that ampacity of cables and MCMs do not add up.

thanks for your help.
 
Is 1600A the OCP or the actual load. If the actual load then what is the OCP? Can't properly size the conductors without this info.
 
The OCPD is 1600 Amps.

Our present design load is below this, but the max is going to be 1600amps.

Clients engineers [telecom comapny] have requested this.

Thanks for your patience.
 
alphabraxon said:
The OCPD is 1600 Amps.

Our present design load is below this, but the max is going to be 1600amps.

Clients engineers [telecom comapny] have requested this.

Thanks for your patience.
Click to expand...
If the main breaker is 1600 amps then the conductors must match or exceed this rating per the NEC. If you can only install 3 conductors per circuit, then you can not get the ampacity required unless you use 1000 kcm @ 545 x 3 = 1635.
 
My last reply probably confused the issue.

Is the NEC requirement that no more than 3 conductors per line?

I dont have to follow the NEC so that was just a general question.

Basically what I need is confirmation that Ive used the required equation correctly.

What would be the recommended wire size based on 1600 amps and parameters as previously provided.

also if some one could explain the confusion in difference between the ampacity of the cables as taken from the tables and the calculated Mcmils from the equation from NEC assuming I did those calcs correctly.
 
What some are trying to tell you is that the actual load is what is considered in voltage drop. If you calculate a load of 1200 amps on a 1600 amp service then you would use 1200 amps for the calculation. If you think the load could be full then I guess you could use the 1600 amp load. Generally a service is not loaded to capacity.

Alpha-- here is an online site for voltage drop
 
alphabraxon said:
My last reply probably confused the issue.

Is the NEC requirement that no more than 3 conductors per line?
Click to expand...
No. You can have as many as you need.
Basically what I need is confirmation that Ive used the required equation correctly.
Click to expand...
The equation you used is for voltage drop and not ampacity. Check the NEC table 310.16 for ampacity. Use the 75C col.

What would be the recommended wire size based on 1600 amps and parameters as previously provided.
Click to expand...

Suggest you use 4 ckts of 600 kcm copper. You can choose other conductors
but you would need additional circuits.

Also if some one could explain the confusion in difference between the ampacity of the cables as taken from the tables and the calculated Mcmils from the equation from NEC assuming I did those calcs correctly.
Click to expand...

Your formula gives the minimum conductore for 3% VD and not ampacity.
Ampacity is from the table 310.16.
 
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