what size fuses

[solo618]

Had a situation today where we had a 200amp transfer switch coming off a buss duct riser. The building engineer told us we could only use up to 175 amp fuses because of the 80%. i'm no math wiz but i thought 80% of 200 is 160. I think he was wrong but i'm just an indian not the chief so i keep my mouth shut. Would the 200 amp fuses be okay to use but then be our responsibility not to load it over 80%. kind of like a 200amp main breaker in a panel or a 20 amp breaker for that matter.

 

[iwire]

Had a situation today where we had a 200amp transfer switch coming off a buss duct riser. The building engineer told us we could only use up to 175 amp fuses because of the 80%. i'm no math wiz but i thought 80% of 200 is 160. I think he was wrong but i'm just an indian not the chief so i keep my mouth shut. Would the 200 amp fuses be okay to use but then be our responsibility not to load it over 80%. kind of like a 200amp main breaker in a panel or a 20 amp breaker for that matter.

Thats funny as you can only load that fuse to 80% so now the continuous load on that feeder must remain under 140 amps.

 

[augie47]

I would think you would need to look at the "purpose" of the fuse.
From one aspect it is protecting the conductor and in that case you would look at 240.4 and protect the conductor at it's ampacity (taking the provisions into account ie: next larger size, etc.)
You would also need to look at the what type of distribution or utilization equipment is being supplied to assure you meet the requirements for that.

If the conductors and transfer switch are rated at 200 amp, I see no problem with the 200 amp fuses.