SCCR: Power circuit with custom transformers

I'm struggling to figure out how to calculate the SCCR of a panel I'm building.

The Power circuit is basically this

Incoming 480/3/60 > 100A Disconnect Non-Fused > 6x J-Class Fuse blocks with 50A Class J fuses (each on two legs, staggered to balance load) > Custom-built transformers in field, 480vac primary/200vac secondary, 20kva, single phase > 6x Bussmann PDBFS204 power lugs > Three single-phase SCRs with 100A semiconductor fuses on one leg, 200ka SCCR rating > Single phase Heating elements, 15kW.

Typically I don't include transformers in my SCCR calculation because they're usually for control power, are less than 3 kva, and fused with Class CC fuses on the primary side. This time, however, is a bit different because these transformers are part of the power circuit and are much larger.

I've contacted the manufacturer and they don't know the SCCR. All they've given me is an FLA of 115 on the secondary side, and that the secondary side is wired with Bifilar #5 square HP magnetic wire, and the primary with #6. I've asked for the impedence but haven't received a reply. We're getting them today so hopefully they're on the nameplate like they're supposed to be.

Anyway, does anyone have any idea what I'd need to know in order to calculate the SCCR? Do I even need to do so since these transformers are fully field devices and not attached to or are any part of the control panel? Am I really calculating an SCCR or am I calculating a max let-through?

First of all, based on UL508A SB4.2.1 Exception 1, ANY power transformer is exempt from the SCCR calculation. So regardless of what it is used for, it is exempt.

You make a good point with the transformer location. Since it is not located in or on the enclosure the SCCR is being calculated for, it really doesn't apply.

Keith

kamenges said:

First of all, based on UL508A SB4.2.1 Exception 1, ANY power transformer is exempt from the SCCR calculation. So regardless of what it is used for, it is exempt.

You make a good point with the transformer location. Since it is not located in or on the enclosure the SCCR is being calculated for, it really doesn't apply.

Keith

Click to expand...

I see. I always thought Exception 1 was referring to small power transformers used by power meters since it's listed right along with current transformers. But if my transformers fall under that, then so much the better.

FactoryTalktotheHand said:

I see. I always thought Exception 1 was referring to small power transformers used by power meters since it's listed right along with current transformers. But if my transformers fall under that, then so much the better.

Click to expand...

Think of it this way;
Where is the power for the primary of that transformer coming from? basically if it is coming FROM the line terminals in your panel, it is ALREADY covered in the SCCR calc you did for the primary power. The transformer is not ADDING energy to the system.


Now if you had a transformer in the panel that was fed from somewhere ELSE, that's another story and presents a number of other issues as well.