standard OCPD size

[binwork91]

Question, sometime when we size OCPD, we need to use one size down. For example, I have a switchboard contain motor and other load.
480V 3P switchboard with all 3p equipment.
400HP motor 1 with VFD: 477A FLA
400HP motor 2 with VFD: 477A FLA
250HP motor 3 with VFD: 302A FLA
60HP motor 4: 77A FLA
other load: 640A
Main OCPD= 477*1.75=834.75 next size up 1000.
1000+477+302+77+640= 2496A, next size down 2000A.
So main OCPD is 2000A.

Question, will it down size too much? i mean it is about 496A different. it more than the different you size up, you only size up 165.25A.

The big standard OCPD have big jump. 2000, 2500, 3000, 4000. will you feel comfortable when you using next size up or down for the big OCPD?

 

[Rock86]

If you're doing the load calc, why would you size your OCPD lower than the value calculated? IMO, I would size up for the security.

 

[Dennis Alwon]

Firstly, you cannot upsize over 800 amps

(B) Overcurrent Devices Rated 800 Amperes or Less. The
next higher standard overcurrent device rating (above the
ampacity of the conductors being protected) shall be permitted
to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a branch
circuit supplying more than one receptacle for cord-andplug-
connected portable loads.
(2) The ampacity of the conductors does not correspond with
the standard ampere rating of a fuse or a circuit breaker
without overload trip adjustments above its rating (but
that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed
800 amperes.

Secondly you need to size the overcurrent protective device at least to the calculated load as stated by Rock86

 

[binwork91]

Firstly, you cannot upsize over 800 amps

So if I have 400HP 480v 3p motor, when I size fuse, 477*1.75=834.75A, I can not go next size up, I only able to use 800 and smaller number, right?

 

[binwork91]

If you're doing the load calc, why would you size your OCPD lower than the value calculated? IMO, I would size up for the security.

I just make up the number of other load to show my example, my bad, I did not realize my load is more than my OCPD.

 

[Dennis Alwon]

So if I have 400HP 480v 3p motor, when I size fuse, 477*1.75=834.75A, I can not go next size up, I only able to use 800 and smaller number, right?

Well for a motor over 800 amps I am not sure if 240.6 applies but for motors you can use 250% for inverse time breakers.

 

[binwork91]

Well for a motor over 800 amps I am not sure if 240.6 applies but for motors you can use 250% for inverse time breakers.

I know i can use breaker for 250%, but it is same. after I size 250%, Can i step up to next large size?

(B) Overcurrent Devices Rated 800 Amperes or Less. The
next higher standard overcurrent device rating (above the
ampacity of the conductors being protected) shall be permitted
to be used, provided all of the following conditions are met:

for motor feeder is:477*1.25=596.25A, next size should be 600A. I think it does not apply. I think I just not step up. step down one size is allow as long as it large than 125%of FLA.

 

[don_resqcapt19]

Well for a motor over 800 amps I am not sure if 240.6 applies but for motors you can use 250% for inverse time breakers.

For motor circuits 240.4(G) applies and that says you can use the rules in 430 for the motor circuits. The rule in 240.4(B) does not apply.

 

[don_resqcapt19]

So if I have 400HP 480v 3p motor, when I size fuse, 477*1.75=834.75A, I can not go next size up, I only able to use 800 and smaller number, right?

Exception #1 to 430.52(C)(1) permits going up to the next standard size. The rule in 240.4(G) says that the rules in 430. and not those in 240, apply to the overcurrent protection of motor circuit conductors.