Fuses or Circuit Breakers on Control Transformers????

[Kilowatt#76]

Hi,

I'm going to be using a 1000VA control transformer (480 x 120). What is recommended, fuses of circuit breakers? I was planning on using miniature circuit breakers, Class D? I will be protecting the primary and secondary of the transformer. Primary: 1000/480 = 2.08 then 2.08 * 2.5 = 5.20 per UL508A table 35.2 and rounded up to 6 amps.

Secondary: 1000/120 = 8.33 then 8.33 * 1.67 = 13.91 per UL508A table 35.2 and rounded up to 15 amps.

Am I good to use a 6A, Class D on the primary and a 15A class D on the secondary?

Thanks!

 

[petersonra]

Hi,

I'm going to be using a 1000VA control transformer (480 x 120). What is recommended, fuses of circuit breakers? I was planning on using miniature circuit breakers, Class D? I will be protecting the primary and secondary of the transformer. Primary: 1000/480 = 2.08 then 2.08 * 2.5 = 5.20 per UL508A table 35.2 and rounded up to 6 amps.

Secondary: 1000/120 = 8.33 then 8.33 * 1.67 = 13.91 per UL508A table 35.2 and rounded up to 15 amps.

Am I good to use a 6A, Class D on the primary and a 15A class D on the secondary?

Thanks!

You can't round up on the primary side. UL508a has the same rules as the NEC on that.

By class D do you mean trip curve D? If so, I would suggest it. But I use trip curve D for all my miniature CBs.

If you use fuses put transformer fuses on the primary side like fnq-r.

 

[Jraef]

Be careful here when using Mini Circuit Breakers (MCBs) with 480V. There are pitfalls involved. They are usually ONLY rated for 480/277V systems, they cannot be used on 480V delta, and many people have no idea what that means or what they have. In addition, most of these small IEC MCBs are only rated for 10kAIC, maybe 18kAIC, which is problematic on many 480V systems. That's why most people use Class CC fuses on the primary side. Primary protection for a CCT is required to be considered in the overall SCCR rating of the panel, so if you use MCBs, you may be drastically limiting the maximum SCCR you can list it at.

 

[D. Castor]

Just my $0.02: If it is important to you that these protective devices are selectively coordinated, fuses will be a much better choice than circuit breakers, especially on the secondary side of the CPT. MCCBs or MCBs have instantaneous trip elements, and will often not fully coordinate when used in series. If you are more concerned with convenience, then breakers are probably the right choice.

 

[Tip DS]

I have to agree with the fuse crew. They are better for SCCR and usually for cost. For your application, you can use the FNQ-R-3-2/10 on the primary side only (both phases), OR you can use a pair of FNQ-R-5 on the primary along with a single FNQ-R-12 on the secondary.