Feeder Conductor Sizing to Industrial Control Panel

S

SoonerEE

Member
Location
Edmond, OK
Occupation
Electrical Engineer
I have a 480V, 3-phase triplex pump with a listed "Enclosed Industrial Control Panel" containing VFDs. The panel is marked with FLA = 88.8A. There is no MOCP marking, and panel does not have a "main" OCPD or disconnect (field wiring terminates to a power distribution block).

Question 1 - How do we determine the upstream OCPD for this circuit? I have seen the suggestion that it be calculated based solely on the marked FLA with a 125% factor.
88.8A * 1.25 = 111A, round up to 125A circuit breaker

Question 2 - How do we size the circuit conductors to the panel? I have seen the suggestion that ampacity be calculated based solely on FLA with a 125% factor.
88.8A * 1.25 =111A @ 75C -> #2 copper.

Question 3 - The sizing rules in Article 409 depend heavily on knowing the individual loads downstream of the control panel. Is the intent of Article 409 to dig into the panel shop's design, categorize each load, and calculate OCPD and conductor size? Or does the panel shop take all of these calculations into account when determining the panel FLA, and we should not apply a 125% factor and size OCPD and conductor using the FLA value directly (i.e. 90A breaker and #3 conductor @75C)
 
I was trying to to give you a hint. The nec feeder conductor ampacity requirements are the same for control panels as any other load. 125% of the largest motor FLA plus 100% of the rest of the load. So 89 Amps plus 25% of whatever the charts say for a 20 HP motor.
 
What is interesting is that seems to be how they calculated the panel FLA already. 125% largest motor, plus 100% of other two motors (plus a little 100VA control transformer).

But for NEC purposes, we only look at nameplate FLA and largest motor… and calculate like a typical feeder?

In general, are ICPs also required to be marked with total heating load so we can apply 125% factor to that as well?
 
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