# Parallel 400 KCMIL for 800 amp service



## Southeast Power (Jan 18, 2009)

I have an existing service that is, 3 sets of 400 kcmil, now feeding a 1000 amp breaker. We will be intercepting the conductors between the pad mounted transformer and the 1000 amp breaker and installing an 800 amp fused disconnect then feeding an ATS and then refeeding the 1000amp MCP. 
The load on the building is only 112 kw at its historical maximum.
All of this was professionally designed and all involved thought it was 3 sets of 350s.
I am building what I want at this point and the job will be finaled per the asbuilts.
If I use the 90degree column, 400s are good for 380 amps and one parallel set of 400s would be allowed as the service is 800amps or under.
If I use the 75 degree column at 335 amps, the ampacity would be 670 amps and would put me back to 700amps.

The AHJ will allow whats in the book.

Question is this. Can I use the 90 degree column for THWN-2 or will the ampacity be determined by the rating of the fused switch terminations?

Much thanks for all comments.


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## Anathera (Feb 16, 2016)

Terminations dictate the maximum allowable temp rating, if it's not 90c then you'll have to use the 75c column or replace the termination lugs

Sent from my SM-G900R4 using Tapatalk


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## electricalwiz (Mar 12, 2011)

you can only use that 90 degree column if the terminals are rated at 90 degree
Most are not but you may be able to special order 90 degree rated terminals


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## RePhase277 (Feb 5, 2008)

You can only use the 90 degree ampacity for derating purposes. For the ampacity, you'll need to use the 75 degree column. But since you have 3 sets, totalling 1005 amps, what's the problem?


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## Southeast Power (Jan 18, 2009)

InPhase277 said:


> You can only use the 90 degree ampacity for derating purposes. For the ampacity, you'll need to use the 75 degree column. But since you have 3 sets, totalling 1005 amps, what's the problem?


I want to just buy one wire size and wanted to just use parallel 400s on the load side.
I might just go with 500s on the load size and be done with it.

The feeder from the generator is only parallel 350s
I'm getting into 3 different sizes now


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## wildleg (Apr 12, 2009)

I don't know if I'm reading your post wrong, or dividing wrong, but it sounds like you could just use a 700a breaker and swap the 1000 for a 700 and all is good, using 2 sets of 400 (?)


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## Southeast Power (Jan 18, 2009)

wildleg said:


> I don't know if I'm reading your post wrong, or dividing wrong, but it sounds like you could just use a 700a breaker and swap the 1000 for a 700 and all is good, using 2 sets of 400 (?)


The existing 1000amp main in the MDP is just going to be a nice big GFI breaker.

The line up will be

800 amp disconnect
800 amp rated ATS
Existing MDP

The plans show three sets of 350s from the utility transformer underground to the existing MDP 1000 amp breaker.

I looked closer and found that it is 3 sets of 400s 
I dug up the underground lateral and plan to cut the PVC outside and turn them up in a 90 outside and feed the new 800 amp fused disconnect outside.
I can then nipple out of the back of the switch through the block wall into the new ATS.
I'll switch to 3sets of 350s for all of the rest of the work.
The feed from the generator, with its 600 amp breaker, calls for parallel 350s.


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## chicken steve (Mar 22, 2011)

all _'formerly T310.16,but we changed it because that's what we do_' math aside, i'd be focused more on a AIC for that 800A serv rated switch JRannis

~CS~


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## Southeast Power (Jan 18, 2009)

chicken steve said:


> all _'formerly T310.16,but we changed it because that's what we do_' math aside, i'd be focused more on a AIC for that 800A serv rated switch JRannis
> 
> ~CS~


I just checked,

The fuses are type L rated at 200,000.


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