Breaker protection for 1 kva Stepdown?

[bobconan]

I'm looking to install a 277v - 120v 1kva direct wired step down transformer that I need to use for powering a security system. My question is how to protect the circuit at such a low amperage? Is it sufficient to just run a 20 amp circuit to the unit with a glass fuse installed in the transformer housing?

Thank You

 

[tom baker]

Your title states 5kva and then the post is 1kva?
It’s much more involved than a glass fuse. You will need secondary ocp, possibly primary ocp, all per code. Start with article 450.
I suggest you have an electrical contractor work with you on this.

 

[bobconan]

Sorry, It is going to be 1kva single phase. I plan on over current protection on the primary and secondary. I'm more asking how I get a breaker that is 3 amps on the primary and 6 amps on the secondary.

 

[infinity]

I fixed the thread title.

 

[bobconan]

I am also considering just replacing the 120v to 24v transformer in the security panel with a 277v to 24v and glass fuse it inside the security panel.

 

[petersonra]

In general, glass fuses are not rated to be used as branch circuit protection and it seems likely what you will be creating here is a branch circuit.

You can get a branch circuit rated circuit breaker with a rating as low as 0.5 amps. Just go to your local electrical house and tell them you want a UL 489 DIN rail mounted circuit breaker and what rating you want. You will likely need a two pole for the primary and a one pole for the secondary.

 

[kwired]

Since you said this is 277 - 120 volts, I am presuming this is two wire in two wire out, if so the secondary can be protected by the primary device if selected in accordance to transformer primary to secondary ratio.

If you have 1kVA transformer that is 3.61 amps on primary at full rating and 8.33 secondary rated amps.

I probably would use 125% rated primary fuse, next common size up is 5 amps, next standard size up according to 240.6 is 6 amps. Though I think T250.3(B) would allow you to go up to 167% in this situation.

Need branch circuit rated devices, use a fused safety switch and RK5 fuse? Those fuses are somewhat readily available, just not at Home Depot.

 

[don_resqcapt19]

Note 1 in Table 450.3(B) will permit the use of a 15 amp circuit breaker or a 6 amp fuse for the protection of the transformer primary.

 

[gar]

220616-1535 EDT

Transformer inrush current can be rather high for ordinary transformer iron (core material). And higher for some core materials, and virtually no air gap.

See my waveforms for a 175 VA transformer using a common transformer core material from 40 years ago. The primary RMS current full load rating is about 175/120 = 1.46 A, or a peak of 2.07 A. My worst measured peak inrush was 40 A. This is 20 times the operating full load peak.

Now assume that transformers ratio about on a VA rating basis, then I might expect a 1 kVA to produce up to about 40 * 1000 / 175 = over 200 A peak current. That would trip a Sq-D QO15 breaker.

You might have a source impedance that would somewhat limit the peak current.

The way you get maximum inrush current is to have no load on the secondary, and turn off excitation to the transformer. Then turn on the transformer so that core flux is increased from its residual flux state. This worst case will randomly occur.

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