UL 508a transformer and power supply sizing

[AJElectric]

New to 508a here in our shop. I know how to do basic load calculations, but I'm wondering if there are any requirements by UL for sizing these components for control wiring.

Control Transformer - I can't find any sizing requirements (only circuit conductor sizing, bonding, and OCP requirements)
As an example, currently working on a project where the sum of all 120VAC loads is about 850W; plus two devices nameplate 3.0A (but in reality they're just a timer & solenoids total 50W max); plus a 120V convenience outlet with 3A OCP.

The loads are all controlled by a PLC and would never all operate at the same time. The max would be maybe 500W. I intend to use a 750VA transformer with 8A MEN fuse on the secondary, and 14AWG conductors on all secondary circuits. Would this meet UL 508A standards?

Power supplies - 42.2.3 max 50% load unless it meets UL61010 etc. and then 42.2.3.2 sizing the overcurrent protection. This is all pretty clear that if the sum of all loads is above P.S. rating then you need OCP. Sounds like it would be OK to have qty. 50 of 10-watt relays and indicator lights on a 120W power supply, as long as there is proper OCP on the circuit(s).

A similar but separate question - 49.2 wording "sum of all current ratings...that are able to be operated simultaneously..." does this mean mechanically interlocked to prevent running at the same time? Or hard-wired relay logic? Or is PLC control adequate?

 

[petersonra]

UL 508a does not provide any requirements for sizing transformers. Just like the NEC does not.

There is no further clarification in UL 508a about what the simultaneous operation might mean so IF the only way a load can be operated is via the PLC and it is programmed so it can't happen than I would say you can take advantage of this provision. However, I would point out that many times while it might appear that the PLC prevents simultaneous operation, sometimes the programming does not, especially if there is a manual or test mode for outputs that might be used by an operator.

Keep in mind some power supplies have built in OCP.

My suggestion is not to try and save five cents by being as cheap as possible. It will always come back to haunt you at some point.

 

[AJElectric]

Thanks Bob, I was pretty sure that would be the case but just needed the re-assurance.

Yeah, we're not trying to save $ but just want to use transformers and DC power supplies that are sized sensibly. Sounds like UL is on the same page as the NEC in that regard.

Definitely don't want to err on the low side, and leaving some "breathing room" for future expansion can also be a factor - but grossly over-sizing does mean more wasted energy, more non-linear load, and higher AFC.

On this project there is zero chance of future expansion, and sizing by the sum of all loads would have me about 3x larger control transformer than we would normally use. Also there is no significant risk of danger or damage if a person does somehow manipulate the system to run too many loads at once - the highest cost would be a blown MEN fuse and a little downtime.