OCPD Rating

horsegoer

Senior Member
Location
NJ
I'm a little confused by this. #1 tag is 2 sets of 4 #350 so that makes sense as the load is a 600A disconnect switch. The disconnect switch feeds three other disconnect switches( 2 - 400A and 1- 100A with 250a & 100A fuses). These are fed with tag #2 which is 3 sets of 4-500 which is good for 1,140 amps at the 75 degree column THHN.

So the OCPD is suppose to protect the conductors so how could you have a 600A disconnect switch protecting 3 sets of 500 mcm?
 

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Npstewart

Senior Member
I'm a little confused by this. #1 tag is 2 sets of 4 #350 so that makes sense as the load is a 600A disconnect switch. The disconnect switch feeds three other disconnect switches( 2 - 400A and 1- 100A with 250a & 100A fuses). These are fed with tag #2 which is 3 sets of 4-500 which is good for 1,140 amps at the 75 degree column THHN.

So the OCPD is suppose to protect the conductors so how could you have a 600A disconnect switch protecting 3 sets of 500 mcm?

Post a screen shot of the wire sizes if you can.


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d0nut

Senior Member
Location
Omaha, NE
Without responding to the "why" of your particular design, it is always permissible to use conductors with a higher ampacity than the overcurrent device protecting them. It is the vice versa that can be an issue.
 

horsegoer

Senior Member
Location
NJ
Without responding to the "why" of your particular design, it is always permissible to use conductors with a higher ampacity than the overcurrent device protecting them. It is the vice versa that can be an issue.
Right. So if you had a OCPD that was higher rating than the wire ampacity, that could cause the wire to overheat as the breaker would allow higher current than the conductor could handle???
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Since the screen shot refers to "temporary boilers," maybe feeder 1 and the 600A disconnect are temporary, while feeder 2 is the permanent wiring for a future load.

Cheers, Wayne
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Right. So if you had a OCPD that was higher rating than the wire ampacity, that could cause the wire to overheat as the breaker would allow higher current than the conductor could handle???
Well, yes, with the caveat that you can use the next higher standard rated OCPD to protect conductors of a given appropriately derated ampacity as long is it is 800A or less.
 

Npstewart

Senior Member
I really think there’s just an error in the feeder schedule and it’s supposed to be Aluminum conductors. In any case, I would write an RFI to the GC for the Engineer to respond to.


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horsegoer

Senior Member
Location
NJ
I really think there’s just an error in the feeder schedule and it’s supposed to be Aluminum conductors. In any case, I would write an RFI to the GC for the Engineer to respond to.


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Why do you say an error? Ampacity of conductors much higher than load serving? BTW, do you take the size of the switch or the fuse size when sizing conductors?
 
Last edited:

Npstewart

Senior Member
Why do you say an error? Ampacity of conductors much higher than load serving? BTW, do you take the size of the switch or the fuse size when sizing conductors?

I say “error” because if there’s no reason to oversize the conductors then it probably wasn’t the original intent. Also, if the material was supposed to be aluminum then the conductor would be perfectly sized at 620A.


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horsegoer

Senior Member
Location
NJ
Looks like they upsized for voltage drop how long is the run of 500's?
Only 50'.

NP I'm confused. So it show s2 sets of #350 for feeder #1 form the main bus to the 600A switch. So conductors should be sized for 600A. #350 copper is good for 310A so two sets makes sense. Aluminum conductor would not give enough ampacity.

Feeder #2 is 3 sets of #500 from the 600A disconnect to (3) disconnect switches fused at 250,250 & 100A. Shouldn't the conductors be sized for 600A as well? 3 sets #500 CU would be 1140A and AL would be 930A. NP which AL conductors are you referring to that would be sized perfectly?

Do both 600A loads( the first 600A disconnect and the other three disconnects) have to be multiplied by 1.25. They are serving continuous loads.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Only 50'.

NP I'm confused. So it show s2 sets of #350 for feeder #1 form the main bus to the 600A switch. So conductors should be sized for 600A. #350 copper is good for 310A so two sets makes sense. Aluminum conductor would not give enough ampacity.

Feeder #2 is 3 sets of #500 from the 600A disconnect to (3) disconnect switches fused at 250,250 & 100A. Shouldn't the conductors be sized for 600A as well? 3 sets #500 CU would be 1140A and AL would be 930A. NP which AL conductors are you referring to that would be sized perfectly?

Do both 600A loads( the first 600A disconnect and the other three disconnects) have to be multiplied by 1.25. They are serving continuous loads.
It's muddled enough that it would be best to ask.
 

horsegoer

Senior Member
Location
NJ
Not sure where it's muddled.

On a side note when you see a load on drawings such as these disconnects does one need/should increase that by 1.25( if continuous) to get the conductor ampacity needed?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Totally agree to this comment. Just asking the question. If you have an electronic trip unit that is adjustable, if im not mistaken, conductor has to be rated for frame size. Why isnt this the same case for switch fuses?
Under the guidelines in 240.6, you can size the conductor on the adjustable setting of a breaker.
 
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