Calc Question

[Alwayslearningelec]

Have a long run from breaker in board to the disconnect switch. Primary side is 277/480.

Curious how the size of the disconnect is calculated here(200AF/100AT)??

Another drawing shows the same disconnect as 100AT/70AF which I think is both written wrong and sized wrong. Sorry I pasted it in funky location next to panel and can't move it.

 

[don_resqcapt19]

The rating of the disconnect is directly related to the required fuse size. The fused disconnect sizes are 30, 60, 100, 200, 400, 600, 800 and 1200 amps, with the number being the largest fuse that will fit in that disconnect. You need a 110 amp fuse so you need a 200 amp disconnect.
Have never seen the "AF" and "AT" used with anything other than breakers before,, but it makes sense.

The 110 amp fuse on the primary of a transformer that supplies a panel with a 250 amp main breaker makes sense. 480/208 * 110 = 254 amps.

For the 100AF/70AT, what does that fused disconnect supply?

 

[Alwayslearningelec]

The rating of the disconnect is directly related to the required fuse size. The fused disconnect sizes are 30, 60, 100, 200, 400, 600, 800 and 1200 amps, with the number being the largest fuse that will fit in that disconnect. You need a 110 amp fuse so you need a 200 amp disconnect.
Have never seen the "AF" and "AT" used with anything other than breakers before,, but it makes sense.

The 110 amp fuse on the primary of a transformer that supplies a panel with a 250 amp main breaker makes sense. 480/208 * 110 = 254 amps.

For the 100AF/70AT, what does that fused disconnect supply?

Those disconnect are the same ones one the primary side. Just think the screwed up on the floor plan when they meant to show the 200a and accidentally showed it as 100at/70af. Yeah that's a good point ...never seen a disconnect labeled like that. Is the af the fuse? Or the at the fuse. Usually for a breaker af would be the frame and at would be the trip.

 

[topgone]

My take:
Your secondary amps should be ~200A (250A/1.25). Looking at full load amps of 480/208V transformers, I guess your transformer should be a 75 kVA. (FLA = 90A primary/ 208A secondary). Given a 90A primary and fusing up to 250%, the amximum allowed OCPD shall be 90 x 2.5 = 225A. But the designer has chosen 110A (110/90 = 122%)
IMO, fuses do not have frame sizes, circuit breakers do.

 

[Alwayslearningelec]

My take:
Your secondary amps should be ~200A (250A/1.25). Looking at full load amps of 480/208V transformers, I guess your transformer should be a 75 kVA. (FLA = 90A primary/ 208A secondary). Given a 90A primary and fusing up to 250%, the amximum allowed OCPD shall be 90 x 2.5 = 225A. But the designer has chosen 110A (110/90 = 122%)
IMO, fuses do not have frame sizes, circuit breakers do.

Thanks. So you think the af in this instance is fuse size?

 

[petersonra]

Thanks. So you think the af in this instance is fuse size?

Thas how I would take it.

Personally, I would have just labeled it as 110 A Class J, or whatever the fuse rating is.

 

[Strathead]

IMO, fuses do not have frame sizes, circuit breakers do.

Technically "frame" isn't the correct word, but disconnects have a size, it is the equivalent of a breaker frame size. I am pretty sure the 200AF/110AT represents a 200A disconnect with 110 amp fuses.

 

[Jraef]

Thas how I would take it.

Personally, I would have just labeled it as 110 A Class J, or whatever the fuse rating is.

Yeah, I see no need to list the size of the switch, the fuse size inherently dictates that.

I suppose you COULD possibly find a way to adapt a 400A disconnect to hold a 110A fuse, but i think it would be funky.

 

[don_resqcapt19]

Yeah, I see no need to list the size of the switch, the fuse size inherently dictates that.

I suppose you COULD possibly find a way to adapt a 400A disconnect to hold a 110A fuse, but i think it would be funky.

There are listed fuse reducers for that purpose.