250 or 300

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-=PEAKABOO=-

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Location
Fort Worth, Texas
Quick question but I am away from a code book and needed a quick answer.
If you have 2 parallel conduits that are about 60' long each feeding a 600 amp main panel 277/480 from the poco pad mounted xfmr do I need to run 300 KCMIL or can I run 250 KCMIL??? These conductors would be THHN. I know that 250 has a rating of like 290 amps but could not remember for sure.

Thanks
 
2 conductor(s) per phase utilizing a 350 kcmil Copper conductor will limit the voltage drop to 0.58% or less when supplying 600 amps for 60 feet on a 277 volt system.

For Engineering Information:

310 Amps Rated ampacity of selected conductor
0.038 Ohms Resistance (Ohms per 1000 feet)
0.04 Ohms Reactance (Ohms per 1000 feet)
8.31 volts maximum allowable voltage drop at 3%
0.58 Actual voltage drop loss for each cable at 0.58%
3.22 volts Total maximum allowable parallel voltage drop loss for the circuit
0.9 Power Factor
 
I don't think this was a voltage drop question.

-=PEAKABOO=- said:
I know that 250 has a rating of like 290 amps but could not remember for sure.
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The rating of 290 amps applies at 90C. However, we are not permitted to use the 90C rating unless all terminations are also rated for 90C. That seldom happens. So we most frequently limit a 250 MCM copper conductor to its 75C rating of 250 amps. In order to get a total ampacity of 600 from two parallel conductors, you need to use 350 MCM, as it has a rating of 310 amps.
 
You could use parallel 300 kcmil Cu conductors if your connected load is 570 amps or less. (75 deg C, 285 amps * 2 = 570 amps)
 
-=PEAKABOO=- said:
I personally have never used 300 MCM but I have numerous times noticed it in the code book. How common is it?? I will ask my supply house tomorrow if they stock it.
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I would assume that it's as readily available as 350 or 400 kcmil. For larger conductors we tend to deal with a wire and cable supplier who seems to have just about everything.


Adjusted for 40C ambient?
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I'm not sure where you got the 40 degrees C?
 
The code would permit the use of parallel 250s if the calculated load is less than 510 amps, assuming a 600 amp or smaller OCPD on the supply end of the conductors.
Don
 
infinity said:
I would assume that it's as readily available as 350 or 400 kcmil. For larger conductors we tend to deal with a wire and cable supplier who seems to have just about everything.

I'm not sure where you got the 40 degrees C?
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According to Graybar 300kCMil is a rare bird, not unlike 3AWG.

The poster is in Texas, so unless is a project in Alaska the ambient temperature correction factor is applicable as per the Table under 310.16.
 
weressl said:
The poster is in Texas, so unless is a project in Alaska the ambient temperature correction factor is applicable as per the Table under 310.16.
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Do you know that the poster is running outside?
 
iwire said:
Do you know that the poster is running outside?
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I don't know if he is a jogger or not:roll: , but it is natural to assume - unless stated otherwise specifically - that the installation is subject to the area's climatic conditions. That is a primary assumption that one may take when the data is not available. Eg. it is what is "natural", most common data for the area. For all I know he is installing in a cyrogenic facilty for superconductors, BUT if he does not say so assumptions will be made.

As you may note I have posed that as a question not an admonishing statement on the work. The answer could be:
- I have not,
- It is not necessary because the area is airconditioned (although in that case he may choose to adjust downward as airconditioned areas are seldom kept at 30C)
- Yes, I did.
 
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weressl said:
I don't know if he is a jogger or not
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:D

but it is natural to assume - unless stated otherwise specifically - that the installation is subject to the area's climatic conditions.
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I guess that is just a matter of opinion, my work is mostly large retail, large office etc. almost none of the wiring is subject to the climatic conditions.

Let me ask this, if he was installing this feeder outside in Alaska would you be so quick to apply the 1.04 allowed?
 
iwire said:
:D



I guess that is just a matter of opinion, my work is mostly large retail, large office etc. almost none of the wiring is subject to the climatic conditions.

Let me ask this, if he was installing this feeder outside in Alaska would you be so quick to apply the 1.04 allowed?
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It is not a question of opinion, but what is the data that is available.

When installing anything, anywhere, one must establish the base conditions before starts designing. My company has basic design data for each major location. Average temperature is just one of those. Most of my design involves feeders and branch circuits originating in a conditioned environment, but then the majority is run on the outside. We have warehouses that have a maximum air-temperature of 45C. We have production units where we needed to take 50C as the base data for equipment and wire selection for lighting that was installed close to dryer units, at ceiling levels with poor ventilation. We have cyrogenic enclosed units with ~-10C operating temperature. The adjustment factors are OK to ignore where it's colder, but when it is warmer you may get into trouble.
 
You have to consider the derating factors. If your ambient temperature is more than 30C, then you have to apply the temperature adjustment factor below table 310.16 and multiply that to your ampacity on 90c Column. If the major portion of your load is nonlinear, then you have to consider your neutral as current carrying conductor and that will lead you to 4 conductors which will further result to 0.8 adjustment on you ampacity on 90c column. If the adjusted ampacity is more than the ampacity shown in 75c column, then you have to use the ampacity on 75c column. If it's less, then you have to use the adjusted ampacity.
 
You have to consider the derating factors. If your ambient temperature is more than 30C, then you have to apply the temperature adjustment factor below table 310.16 and multiply that to your ampacity on 90c Column. If the major portion of your load is nonlinear, then you have to consider your neutral as current carrying conductor and that will lead you to 4 conductors which will further result to 0.8 adjustment on you ampacity on 90c column. If the adjusted ampacity is more than the ampacity shown in 75c column, then you have to use the ampacity on 75c column. If it's less, then you have to use the adjusted ampacity.
 
weressl said:
It is not a question of opinion, but what is the data that is available.
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No, it was your opinion based on what you see as the facts available.

We can add this to the list of things we see differently.
 
iwire said:
No, it was your opinion based on what you see as the facts available.

We can add this to the list of things we see differently.
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So I guess I missed that "fact" when I was reading his post. Darn it, I keep re-reading it and it just does not seem to be visible. Maybe I need to have specially trained "moderators" eyes to see it.;)
 
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