Sizing Fuses

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Little Bill

Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
I am in the process of wiring battery chargers for fork lifts. These have multi-tap transformers. When changing them to the correct voltage, the fuses also have to be changed. There is a chart for the fuse sizes depending on the voltage.
Example: 480V with 36A/120V model, the fuse size is 10A, For 208V with 36/120 the fuse size is 25A. I assume the 36/120 is the DC output. These are 3 ph chargers. No problem with these.

The problem is I have to get fuses before going back and I just realized that one of the units is single ph. The voltage will be the same. I haven't looked at the chart for this one and wondered if the fuse size for a single ph would be the same as 3 ph.
 
I think your best bet is to see if you can get the manual on the web and figure out what fuse size you actually need rather than guessing.
 
petersonra said:
I think your best bet is to see if you can get the manual on the web and figure out what fuse size you actually need rather than guessing.
Click to expand...
Probably going to end up making a trip back to the supply store after going to the jobsite and looking at it. I was just trying to avoid having to make an extra trip.
 
Little Bill said:
The problem is I have to get fuses before going back and I just realized that one of the units is single ph. The voltage will be the same. I haven't looked at the chart for this one and wondered if the fuse size for a single ph would be the same as 3 ph.
Click to expand...
Certainly not if the input power is the same. For the same input power with the same L-L voltage, the input current on the single phase L-L unit would be sqrt(3) time as great as on the three phase unit.

Cheers, Wayne
 
wwhitney said:
Certainly not if the input power is the same. For the same input power with the same L-L voltage, the input current on the single phase L-L unit would be sqrt(3) time as great as on the three phase unit.

Cheers, Wayne
Click to expand...
Well I lucked out. The single ph charger had already been changed to 208V. Also, the chart showed the same 25A fuse and that's what was in it. Not doubting your math, it's just what was spec by the mfg so that's what I had to use.
 
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