Motor control transformer fuse ratings

[alaskanassassin]

I have a 1KVA, 480V/120V control transformer and I need to make sure my calculations are correct. I am using 430.3(B). Primary side 1000/480=2.08 amps, 2.08X250%=5.2 amps. Secondary side 1000/120=8.33 amps, 8.33X125%=10.4 amps. Am I doing this right? I plan on using KTK fuses on the primary and using FNM fuses on secondary. What type of fuses do you typically use? Fast or slow acting? Thanks Doug

 

[augie47]

all looks good to me.

 

[Jraef]

KTKs are non-time delay fuses, they will blow almost instantly due to transformer inrush unless seriously over sized. You need TD versions, like FNQ.

 

[alaskanassassin]

Thanks, should I use that type of fuse on both sides of the xfrmer?

 

[augie47]

I bow to Jraef's knowledge on the FNQ or KTK being more applicable due to it's being time dealy.
KTKs are more common in this area and I have sucessfully used them, but, his knowledge in this area far exceeds mine, so it is well worth lisrening.
Unless he sees otherwise, the FNM should be o.k. on the secondary as it is time dealy.

 

[Jraef]

Thanks, should I use that type of fuse on both sides of the xfrmer?

You CAN use KTKs on the primary side, but they need to be over sized to handle the inrush so you end up sacrificing some protection. I prefer to use TD fuses on the primary.

By the way, should have mentioned using "rejection" type fuses (and holders) for the primary so that nobody in the future is tempted to stick a FNM on the primary side because that's all they have in hand. FNM and FNQ (and KTK) are all the same physical size. So better to use FNQ-R (or KTK-R) for the primary side, because you cannot buy FNM-R fuses..


Also by the way, we are being a little unfair using only Bussman numbering, so here is a crossover chart that shows the equivalents from most of the other major manufacturers.

 

[alaskanassassin]

Thanks for the information.