Ampacity

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roshan1430

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Los Angeles, CA
Hi Guys, I have an unique situation here. I am Designing a 2400V cable run from Xmer to MCC through 100ft long duct-bank. My FLA comes to 902.2 and after 125% it gives me 1127.65 amp. I am planning to have 3 parallel run. So if I use 500 MCM cable I get 375 amp each conductor or 1125 amps for my 3 parallel run(NEC table 310.60(C)(77). It puts me 2.65 amp below my required ampacity. Is there a NEC code which help me deal with the situation. I am trying avoid to go to next up 750 MCM which will be too big unnecessarily. Am I referring to correct table ? Thank you.
 
petersonra

petersonra

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Location
Northern illinois
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engineer
roshan1430 said:
Hi Guys, I have an unique situation here. I am Designing a 2400V cable run from Xmer to MCC through 100ft long duct-bank. My FLA comes to 902.2 and after 125% it gives me 1127.65 amp. I am planning to have 3 parallel run. So if I use 500 MCM cable I get 375 amp each conductor or 1125 amps for my 3 parallel run(NEC table 310.60(C)(77). It puts me 2.65 amp below my required ampacity. Is there a NEC code which help me deal with the situation. I am trying avoid to go to next up 750 MCM which will be too big unnecessarily. Am I referring to correct table ? Thank you.
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where is the 125% coming from?

This table says it is for UG duct banks.

personally, I would look real close at the loads and see if I could shave off 3 A somewhere.

BTW, what is the 125% for? is that something for MV conductors?
 
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roshan1430

Member
Location
Los Angeles, CA
I will have 3 conduit, so only 3 CCC per conduit. we dont need to adjust upto 3 conductor per raceway, for 4 above adjustment factor is 80%.
125% continuous load factor.
Thank you
 
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Ingenieur

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Location
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the fact that you have calculated a load >900 A to 0.x precision would indicate to me take a good look at the load calcs
why the 125%?
what size xfmr feeding the 2400? or is a feeder off a common swbd?
 
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Ingenieur

Senior Member
Location
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roshan1430 said:
Bus Rating : 1200 Amps
CB: 1200 Amps

only that 2.6 amps (1127.6-1125) are bothering me.

Thank you so much.
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use the 3 x 500 and set cb toc at 1100 or 1000
likely a vcb with relaying
that will still allow 122% of rated xfmr capacity and < conductor ampacity

I would set cb at 115% x xfmr fla ~1000 A, maybe less depending on actual load
what is the mcc connected continuous load?
should be no more than 90% xfmr rated fla imo ~ 810 A
 
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