Fusing a Motor Starter

[gilldawg79]

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,

 

[itsmoked]

50VA / 480V = 0.1A
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

 

[gilldawg79]

Is there any way to determine this by the info on the Name Plate? With some calculations of course. Which calculations would you need to use to determine how to fuse the control circuit with namplate info.

 

[rbulsara]

Control transformers generally have inrush currents of 25 to 40 times their rated current. You need to account for that say for 0.1 sec. Sizing fues just above rated current is not a good way to size fuses for control transformers or solenoids etc.

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.

 

[jraef]

Since you brought up 480V we can assume you are in North America somewhere, most likely the US. The rules for fuse sizing with CPTs is specifically laid out in the NEC, articles 430.72 (C)(1) through (4), article 450.3(B) and article 725 (if it is a class 1, 2 0r 3), as well as UL508. Nothing else matters really.

 

[btrapani]

Why not just use the suggested values by the manufacture?

 

[itsmoked]

Probably a moot point after half a year...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com