Wire size for 800A feed to MCC

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brentmccabe

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Hello, new to this forum. I am determining wire size from a substation to a 480V, 800A MCC, at a distance of 700'. I believe (using table 310.15(B)(16)) that I can use paralleled 600MCM and a 1/0 ground. My question is, this table is for not more than 3 current carrying conductors. Since I will have 6 current carrying conductors, I derate by 80% (table 310.15(B)(3)(a)), correct? So then, I must use the next size bigger, yes? Thank you.
 
Are you designing it for one conduit or two?

Roger
 
brentmccabe said:
. Since I will have 6 current carrying conductors, I derate by 80% (table 310.15(B)(3)(a)), correct? Yes So then, I must use the next size bigger, yes? Thank you Not necessarily.
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You need to look at the actual load and the nature of the load.
In it's simplest form, you are correct concerning the 80%
If you are using a 90° conductor, you are allowed to apply that 80% to the 90° rating which would give you an allowable ampacity of 760 amps. If you load is 760 or less, you would be allowed to supply your 800 amp service {240.4(B)} meaning you would not need to increase the size and your voltage drop at 700 ft would not be over 3%.
If part of your load consist of relatively large motors, the story may change and engineering input might be prudent.

(Rob put it more simply as I typed :))
 
I didn't think you could round up on 800 amp feeders and above.

JAP>
 
It's 800 and below that it is allowed.

Roger
 
roger said:
Are you designing it for one conduit or two?

Roger
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I changed the design to 2 conduit, so I can use the 600MCM. I do have another question please. I am running this a length of 700 feet and am able to use table 310.15(B)(16), based on temperature and only 3 current carrying conductors. Do I need to de-rate the values given in this table for 700' length? Thank you
 
What I am asking is, am I able to use the value from the ampacity table in NEC or do I need to de-rate the wire further? It is a distance of 700' and the temperature doesn't exceed 100°F.
 
Distance has no impact on ampacity. It only affects voltage drop. I did a quick and rough calculation that told me a 700 foot run of 600 MCM copper in two parallel runs would give you a voltage drop of around 2-3%. You need to decide whether that will serve the project's needs.
 
charlie b said:
Distance has no impact on ampacity. It only affects voltage drop. I did a quick and rough calculation that told me a 700 foot run of 600 MCM copper in two parallel runs would give you a voltage drop of around 2-3%. You need to decide whether that will serve the project's needs.
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May I ask how you came up with that? I am using the method from Ugly's 2017 and I get a different value. Thank you
 
You can use an online VD calculator like this one from Southwire, I used 600 amps:

https://www.southwire.com/support/voltage-drop-calculator.htm

Results

2 conductors per phase utilizing a #600 Copper conductor will limit the voltage drop to 2.86% or less when supplying 600.0 amps for 700 feet on a 480 volt system.
For Engineering Information Only:
420.0 Amps Rated ampacity of selected conductor
0.023 Ohms Resistance (Ohms per 1000 feet)
0.039 Ohms Reactance (Ohms per 1000 feet)
14.399999999999999 volts maximum allowable voltage drop at 3%

13.714. Actual voltage drop loss for each cable at 2.86%
27.42820233240232 volts Total maximum allowable parallel voltage drop loss for the circuit
0.9 Power Factor

**Note to User:All ampacity values are taken from the Section of 310-15 of the NEC. The conductor characteristics are taken from Table 9 of the NEC. The calculations used to determine the recommended conductor sizes for branch circuits are based on 60°C ampacity ratings for circuits rated 100 amps or less or marked for use with #14 AWG - #1 AWG. Circuits rated over 100 amps or marked for conductors larger than #1 AWG are determined using 75°C ampacity ratings. Calculations to determine service and feeder conductor sizes are based on overcurrent device ratings rather than actual expected loads which are conservative and may yield oversized conductors. No calculations take into account temperature correction factors or conductor de-rating.
This voltage drop calculator is applicable only to NEC applications. It does not optimize conductor sizes for several different loads at various points in a circuit. The total combined load and length of the circuit must be used. Consult with an engineer if your application requires more complex engineering calculations.
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charlie b said:
I have a spreadsheet calculator. It does not recognize 600 MCM, so I had to mentally extrapolate.

Click to expand...

No 600 kcmil? That's surprising since it's a very common conductor size for us, probably the most common for large feeders and 400 amp transformer secondaries.
 
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