Fusing a Motor Starter
Fusing a Motor Starter
(OP)
What formula do you use to properly fuse the control circuit in a Motor Starter....480 VAC for an example.
This is the formula that I have....
Primary side: VA of control transformer .050KVA / 480 (Primary side)= Fuse size
50VA/480= .10+25% =.13 or 1/10
Fuse on Primary side should be larger due to Inrush and use a time delay fuse?
Secondary side: VA of transformer .050KVA / 120 (Secondary Side)= Fuse size
50VA/120=.42 or 4/10 or smaller .25 amp
Fuse on Secondary side should smaller than the formula with a Fast Acting Fuse?
Is this correct?
Thanks,
This is the formula that I have....
Primary side: VA of control transformer .050KVA / 480 (Primary side)= Fuse size
50VA/480= .10+25% =.13 or 1/10
Fuse on Primary side should be larger due to Inrush and use a time delay fuse?
Secondary side: VA of transformer .050KVA / 120 (Secondary Side)= Fuse size
50VA/120=.42 or 4/10 or smaller .25 amp
Fuse on Secondary side should smaller than the formula with a Fast Acting Fuse?
Is this correct?
Thanks,





RE: Fusing a Motor Starter
You will need two fuses on the primary. As a failure to ground on an unfused side would be.. um.. unpleasant?
They will need to be delayed fuses preferably like
with fuse holders to match. The screwy end then prevents a Bozo from replacing one with a fuse that can't handle 480V which again would result in unpleasantness.
I just did a job where I'd ordered a 500VA control transformer and the fuses math-ed out as above but subsequently was supplied a 750VA transformer. I figured no sweat I only need about 100VA. Sorry. On site I flip the switch to 480 for the first time. Pfft. Both primary fuses toasted. The inrush won't permit much, if any, under-sizing of the primary fuses.
Once on the secondary side you can use about anything. Set the fuse to whatever you need. You have to assess the target loads. If you have any large power supplies their starting inrushes are going to likely demand you use a slow blo type fuse or you may get nuisance blows.
If you are just running a bunch of lights and switches a fast blow may be fine.
Generally in your example:
50VA/120V = 0.42A
I'd probably try a 0.315 commonly called a 315mA fast blow. Measure your actual full running load. See where you are.
If it looks good proceed with testing.
If it toasts in less than 5 power-ups switch to a 315mA slo-blo. If it doesn't blow then look closely at the element if it's visible during a power up. If it glows you need a slo blow as it will fail soon.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Fusing a Motor Starter
RE: Fusing a Motor Starter
In fact to me O/L is less of a concern than the short circuit protection and even 5A or 10A fuse is safe enough for SC protection. Low amp or fractional amp fuses are recipe for nuisance fuse operation.
RE: Fusing a Motor Starter
RE: Fusing a Motor Starter
RE: Fusing a Motor Starter
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com