Determining SCCR with a Control Transformer

[mazdarc]

Hi,

I am working on a new panel design and figuring out the SCCR. It is a 460V panel comprising a VFD, and a 50VA 480/24 transformer with some controls components downstream of this. There are also fuses to protect the VFD and primary and secondary sides of the transformer.

I believe the control transformer does not affect the rating. Am I correct?

The VFD is rated for 100kA short circuit capacity. So the overall panel SCCR is 100kA?

Thanks in advance for your help and advice.

 

[Jraef]

Hi,

I am working on a new panel design and figuring out the SCCR. It is a 460V panel comprising a VFD, and a 50VA 480/24 transformer with some controls components downstream of this. There are also fuses to protect the VFD and primary and secondary sides of the transformer.

I believe the control transformer does not affect the rating. Am I correct?

The VFD is rated for 100kA short circuit capacity. So the overall panel SCCR is 100kA?

Thanks in advance for your help and advice.

The secondary side of the transformer is irrelevant. The primary side depends on the fuses you use on the primary. If you are not fusing the primary you will need to know the SCCR rating of the transformer without fuses.

The overall SCCR rating of the panel is a tested or calculated value, there are specific rules set out in Article 409 of the NEC telling you haw to go about it.

 

[mazdarc]

Jraef, Thanks for your response.
The primary side of the transformer will have 1/2A fuses on both poles, rated for 200kAIC at 600V.

I have the NEC on order and want to get a head start before it arrives. However, for calculating the SCCR for the transformer I think I need the impedance. I cannot find any SCCR info or impedance info on the transformer (a Square D 9070T).

 

[Jraef]

Jraef, Thanks for your response.
The primary side of the transformer will have 1/2A fuses on both poles, rated for 200kAIC at 600V.

I have the NEC on order and want to get a head start before it arrives. However, for calculating the SCCR for the transformer I think I need the impedance. I cannot find any SCCR info or impedance info on the transformer (a Square D 9070T).

No, if you have 200kAIC fuses in front of the transformer, you are done with that part of the circuit.

As you await your NEC copy, there are plenty of white papers from various manufacturers on NEC Article 409 and SCCR ratings. Google is your friend.

http://tinyurl.com/5vvn9jb

 

[mazdarc]

Jraef,

I still have a hang up over the primary coil on the transformer. I am using extracts from various white papers covering the UL50A supplement SB.

My understanding is this?
Following the decision making process SB4.3 my class CC fuse is good for limiting through current to the transformer primary winding. Then referring to Table SB4.2, I cannot find a reference to a 1/2A class CC fuse. The nearest is 15A fuse and this has a let-through current of 3kA for a 100kA threshold. Using this nearest value, I need to justify the transformer primary winding has an SCCR of at least 3kA. But I have no information on the transformer and asking the manufacturer, they say I don?t need to do this.

Am I missing something? I am trying to get a rating of 100kA for best compatibility so need to provide a calculation to support this.

Thanks.

 

[Jraef]

The problem is, you are thinking of the CPT primary as a "power circuit components", when it fact it is considered a LOAD. It is not a switching or interrupting device, the primary fuses are. So all you need to consider are the fuses themselves. If you had ANOTHER switching device, such as a relay on contactor, BETWEEN the primary fuses and the transformer primary windings, then THAT device would have to be factored in based on the let-through of the fuses. But I think I had you are over-interpreting here. I don't have my copy of UL508A with the SB supplement here, but somewhere I believe it defines power components for you and expressly excludes things like transformers, CTs, capacitors etc. that are not switching or interrupting devices.