Wye delta fire pump- motor conductor derating

Status
Not open for further replies.
G

greenspark1

Senior Member
Location
New England
We'll have 6 conductors going from the controller to the fire pump. Per 430.22(C) this means our conductors must be 72% of our full load current (FLC). What this article and 695 fail to mention is any derating of the conductors based on >3 current carrying conductors. Don't we need a 80% derate (310.15(B)(3)(a))?
 
Good question. I would guess that since the conductors are already sized at 125% the derating factor of 80% is built in?
 
gadfly56 said:
That's how I'd see it, FWIW.
Click to expand...

I appreciate the replies, but not sure I follow. Article 310.15 reduces the allowed ampacity of a conductor. 695.6 (via 430.22) requires conductors rated for 72% of FLC. So if my allowable cable ampacity is reduced by 80%, that adjusted value would have to meet 695.6.
The FLC is 124A so 72% = 89A = #3 @ 75C
But #3 derated 80% is 110*.8 = 88A.
 
The actual current on the motor conductors is 58% of the rated load current, to that you multiply by 125% which = 72% of the rated load current which give you the minimum conductor size.

For example:
100 amp RLC * 58% = 58 amps * 125% = 72 amps
100 amp RLC * 72% = 72 amps minimum conductor size

I'm guessing that for these Wye start-Delta run motors you they would not require any derating because it's already built in. But I'm just guessing maybe it actually is required but I've never derated for the 6 conductors in one of these installations based on 430.22(C).
 
Good to hear, thank you.
I also realized my 2005 table 310.16 has been updated. #3 at 95C is now good for 115A (previously 110A). This apparently changed in 2014. I see a couple others that have shifted up and down. It's not marked in 2014 as having changed and I thought these ampacities had been set it stone decades ago. Any ideas why they have been edited?
 
Status
Not open for further replies.
Top