What size fuse do I need?

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patricknola

Electrical contractor/Generac Dealer
Location
new orleans, La, United States
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Electrical contractor.
I got a call from a business this morning with half thier lights are off. They have 3 phase power and I turns out the C phase fuse is blown. The labels on the fuses are all worn off. Its a Gould ITE 400 amp 3 phase disconnect. The B phase (208V) says Bussman R 225 stamped on it. The data label on the door of the disconnect deosn't give a fuse recomendation. How many amps should the replacement fuse be?
 

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I don’t run across fuses much around here. The one currently is a 225 amp fuse. It sure looks like the wires are smaller that a 4/0 copper. So I would fuse it to whatever your wire size is.
 
A 225 Amp fuse is the lowest rated fuse that will fit a 400 A switch, short of finding some kind of adapters. That might be why they used that size.

Personally, I would replace all three, even if two of them are still working. They have been there a long time and the owner has gotten their money's worth.

AC\DC said:
I don’t run across fuses much around here. The one currently is a 225 amp fuse. It sure looks like the wires are smaller that a 4/0 copper. So I would fuse it to whatever your wire size is.
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patricknola said:
I got a call from a business this morning with half thier lights are off. They have 3 phase power and I turns out the C phase fuse is blown. The labels on the fuses are all worn off. Its a Gould ITE 400 amp 3 phase disconnect. The B phase (208V) says Bussman R 225 stamped on it. The data label on the door of the disconnect deosn't give a fuse recomendation. How many amps should the replacement fuse be?
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I can see writing on the conductors but can't see the size. But you should be able to see the size in person. Size the fuse according to what size the conductors are.

ETA: I see now that petersonra pointed out the size and posted while I was writing.
 
The 350Cu conductor is rated higher than 225 amps but you need to see what the conductors are supplying if you plan to install a higher rated fuse..
For example, if they are supplying a 225 amp MLO panel then a 225 fuse would be correct.
 
We should always follow the NEC 70E and PPE for safe practices as a reminder. Thank you

Assuming that the fuse did not blow due to a short circuit, which you should try to test it before powering it up.
The first opportunity I get, I would monitor the each phase with an amp meter on to determine the actual current load. Assuming that the you will capture the full utilization of the system at its peak. If there are other loads other than the lighting, such as motors kicking in etc. This should give an idea of the fuse type and rating.
It is always good to do a load calculations if possible.
 
The fuse cap says right on it “Bussman FRN-R 225”, so it’s a 240V 225A time delay fuse. Unless you know everything about what it is feeding, I wouldn’t go messing with the size or type, just replace like-for-like.

As was said, replace all 3 fuses because when one fuse blows, the others were stressed, increasing the chances of nuisance clearing. But also so that the next person doesn’t have to go though this. If they complain, tell them they can keep the old fuses as emergency spares (then write the type and size on them with a Sharpie).
 
Jraef said:
The fuse cap says right on it “Bussman FRN-R 225”, so it’s a 240V 225A time delay fuse. Unless you know everything about what it is feeding, I wouldn’t go messing with the size or type, just replace like-for-like.

As was said, replace all 3 fuses because when one fuse blows, the others were stressed, increasing the chances of nuisance clearing. But also so that the next person doesn’t have to go though this. If they complain, tell them they can keep the old fuses as emergency spares (then write the type and size on them with a Sharpie).
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These are available around $100 each. Not like it is going to break the bank to replace all three.
 
There are already two different fuse types in that switch. I agree don't make it three types. Change all of them, especially with their apparent age and weathering.
 
Check for VD across all the connections when you get it going. Line to load. You will need a meter capable of mv reading. Or scan them with thermal device after they have carried a load for a while. Identical fuses will help with VD comparison with all else being equal.
 
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