Help on sizing feeds out of existing MCC

[masher]

I'm spec'ing three feeds to be run out of an existing MCC, each feed will be fed seperately from its own bucket containing a 3phase breaker.

1. I need 1 feed from the MCC for a 15KVA, 1phase lighting transformer (31.2A primary @480v)

2. 1 feed from the MCC to supply 150A/480v/3ph to supply some general motor loads fed from existing combination starters. The total motor load is much less than the feed size (around 50A), but feed will be available for future use.

3. 1 feed from the MCC to supply 100A/480v/3ph to supply several fans w/ existing fusible disconnects. Total fan load is around 50a maximum.

So, my questions is twofold:

1. I understand the conductor size to be calculated by multiplying the total load by 1.25, so to provide a 100A feed, my wire size
would have to be good for 125A, or #2AWG@ 90C per table 310.16, correct?

2. What is the correct way (by code) to size the breaker protecting the feeder?

Thanks!!

 

[bob]

So, my questions is twofold:

1. I understand the conductor size to be calculated by multiplying the total load by 1.25, so to provide a 100A feed, my wire size
would have to be good for 125A, or #2AWG@ 90C per table 310.16, correct?

2. What is the correct way (by code) to size the breaker protecting the feeder?

Thanks!!

1. 90C rating can not be used. You must use the 75C rating
2. Size the conductor and then select the breaker by the NEC rules.

 

[billsnuff]

1. You are allowed to derate at the 90C column, for temp and no. of conductors, but the final derated ampacity cannot exceed either the 60C or 75C rating of the terminals, so I agree with Bob.

2. See Article 240 Overcurrent Protection.

also, for motor feeders and ocpd see 430.24 and 430.62(A), respectively.

 

[augie47]

1. I need 1 feed from the MCC for a 15KVA, 1phase lighting transformer (31.2A primary @480v)

Depending on how you protect your transformer secondary, you are probably looking at a 40 amp circuit

1. I understand the conductor size to be calculated by multiplying the total load by 1.25, so to provide a 100A feed, my wire size
would have to be good for 125A, or #2AWG@ 90C per table 310.16, correct?

Unless it's a continuous load, there is no need to use the 125% multiplier
on the total load and as Bob states, the 90? column can only be used for derating. {see NEC 110.14(C)}
Calculate the load (or desired capacity)
size the conductor per 310.16, 110.14
apply any derrating factors as needed
select the overcurrent device accordingly

 

[masher]

Thanks for the advice!

Thanks for the advice!

Thanks for the advice! Very helpful