100% Rated Breaker to Add Load?

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elecshop

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Electrical Engineer
I have a 1200A regular service entrance breaker, so I cannot load it up to more than 80% of its rating.
If in the future the client wants to add load, can they replace it with a 100% rated 1200A service entrance breaker? Provided the service entrance conductors are 4 sets of 350 kCMIL.
This might be a silly question, but I wanted to triple check.
Thanks folks!
 
If you think you could be getting close I'd say check demand history to see how close you really are. If you have a majority of load that is constant you maybe are close but many times the load varies enough you aren't drawing as much as you might think.
 
Bottom line is that if you're adding load to the service you need to go back and do a new load calculation. That will tell you if the 1200 amp service is large enough.
 
You probably know this but if you used a 1600A switch size with 1200A fuses they could run at 100%.
 
tortuga said:
You probably know this but if you used a 1600A switch size with 1200A fuses they could run at 100%.
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So the idea is that the fuses are 100% rated, but the switch is not, so you use a 1600A switch at the normal 80% (or less) and the fuses themselves at 100%?

Are fused switches ever 100% rated?

1600A would bump up to BPS territory for what that's worth
 
electrofelon said:
So the idea is that the fuses are 100% rated, but the switch is not, so you use a 1600A switch at the normal 80% (or less) and the fuses themselves at 100%?

Are fused switches ever 100% rated?

1600A would bump up to BPS territory for what that's worth
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Yeah thats the idea, still have 75C terminations.
 
tortuga said:
1600A * 80% = 1280A
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That document doesn't support your claim that you can use 1200A fuses at 100% in any 1600A rated switch. It states the switch would need to be 100% rated.

And if you think about the issue, that makes sense. The concern is that the fuse testing is done in free air, and that an enclosure could trap enough heat to elevate the temperature enough to change the fuse behavior. Thus the switch needs a 100% rating to indicate that the assembly including the enclosure is sufficiently large/ventilated that the thermal environment is close to free air and the fuse may be used at 100% of its rating.

Cheers, Wayne
 
elecshop said:
I have a 1200A regular service entrance breaker, so I cannot load it up to more than 80% of its rating.
If in the future the client wants to add load, can they replace it with a 100% rated 1200A service entrance breaker? Provided the service entrance conductors are 4 sets of 350 kCMIL.
This might be a silly question, but I wanted to triple check.
Thanks folks!
Click to expand...
You can load a "regular" 1200A breaker to 1200A of non-continuous load forever. Very few occupancies (if any) would have an entire building that would be considered continuous load.
 
ron said:
You can load a "regular" 1200A breaker to 1200A of non-continuous load forever. Very few occupancies (if any) would have an entire building that would be considered continuous load.
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And since this a service the service size would be determined by an Article 220 load calculation. No other load consideration would be required.
 
wwhitney said:
That document doesn't support your claim that you can use 1200A fuses at 100% in any 1600A rated switch. It states the switch would need to be 100% rated.
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Then how do you calculate the continuous load rating of a 1600A switch?
 
tortuga said:
Then how do you calculate the continuous load rating of a 1600A switch?
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If it's the switch is 100% rated, then it's equal to the size of the fuse in the switch. If the switch is not 100% rated, it's equal to 1/125% of the size of the fuse in the switch.

The last sentence of your document says: "Therefore, if the switch is rated/listed for 100%, fuses can be applied at 100% of their ampere rating, too." The converse holds as well.

Cheers, Wayne
 
So If you put 1600A fuses in a 1600A fused disconnect switch it can carry 1280 Amps continuously.
but if you put in 1200A fuses that same 1600A switch can only carry 960A continuiously ?
 
I go off the marked rating per article 404 (404.14 in 2023) (404.20 in the 2020 NEC), not the fuse in the switch.
 
tortuga said:
So If you put 1600A fuses in a 1600A fused disconnect switch it can carry 1280 Amps continuously.
but if you put in 1200A fuses that same 1600A switch can only carry 960A?
Click to expand...
Yes for the fuse/switch combination.

You could put in dummy fuses and I think the switch itself wouldn't be harmed by carrying 1600A continuously, you just wouldn't have any OCPD. So it's all about what the fuse can handle in a particular thermal environment with sufficient confidence that it won't blow below its rating.

At least that is my understanding.

Cheers, Wayne

P.S. The document you referenced indicates that fuses are tested (in free air) to hold at 100% of their rating for up to 600A in size, and 110% of their rating for 601A and above. Which would suggest that for 601A fuses and above, even in a non-100% rated switch, it should be fine to run 110%/125% = 88% of their rating through them continuously. Except that there's no such provision in the NEC, and the 125% factor is somewhat arbitrary anyway, so there's no particular reason to expect it to work out like that.

I mean, I bet in 20C ambient the risk of ignoring the 125% factor for continuous loads on a breaker or fuse and getting an OCPD trip below the nominal OCPD rating is probably pretty low to begin with, since circuit breaker free air testing is done at 40C ambient, and I assume the same for fuses.
 
tortuga said:
I go off the marked rating per article 404.14 (404.20 in the 2020 NEC), not the fuse in the switch.
Click to expand...
But aren't we talking about fused disconnect switches, in an application where the OCPD is required, and the OCPD is the limiting factor?

Cheers, Wayne
 
wwhitney said:
But aren't we talking about fused disconnect switches, in an application where the OCPD is required, and the OCPD is the limiting factor?

Cheers, Wayne
Click to expand...
I think if I buy a 1600A fused disconnect switch with a UL listing for 1600A my calculation for how much load the terminations in the switch can carry continuously would be the rating of the switch per article 404 X 1/125% or (1600 X 80%) = 1280 Amps.
 
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