Are programming receptacles allowed in a UL 508A control panel?
Yes, receptacles are allowed inside a UL 508A listed control panel. An example is shown in Figure 1. This is convenient for powering a laptop used for programming the Programmable Logic Controller (PLC) or retrieving technical data to support troubleshooting and logistics.
Observe that the receptacle marking stipulates a 2A current limit and the intended use. In this case the receptacle is intended for the service technician to charge or power their mobile devices.
Figure 1: Programming receptacle included in an industrial control panel. It is current limited to 2A.
What could go wrong
Alice and Bob received a request to include a 120 VAC receptacle in their latest UL 508A certified panel. Since they already had a 208/120 feeder, they thought they would be clever and wired the receptacle between line and neutral. To keep cost low, they included a UL 1077 circuit protector. It was a low-cost solution that appeared to work well and was greatly appreciated by their customers.
And then the UL inspector arrived for their quarterly panel shop inspection.
The Short Circuit Current Rating (SCCR) calculation was performed on the panel. Instead of the 50 kA target, the panel’s SCCR had dropped to 0.2 kA. To make matters worse, the inspector emphatically stated that the previously shipped panels were no longer listed with UL and the panel shop’s certification was in jeopardy. This was a costly mistake.
How should the convenience receptacle be powered?
Before answering this question, let’s first explore the ten-thousand-dollar mistakes made by Alice and Bob to determine why they nearly lost their UL panel shop certification.
-
They mistakenly used a low-cost UL 1077 supplementary protector in place of a proper UL 489 listed circuit breaker or properly sized fuse. By the UL 508A standard, a supplementary protector is rated for 0.2 kA (untested) which is inappropriate and woefully inadequate for branch protection.
-
They connected directly to the feeder. According to Table SB4.1, the receptacle is rated for a 10 kA SCCR. This may be acceptable in some situations. However, let’s assume Alice and Bob had otherwise designed for a 50 kA SCCR. The direct-coupled receptacle compromised their otherwise excellent panel.
Best practices for powering a UL 508A convenience receptacle
There are two potential solutions for powering the in-panel receptacle including fuse protection or use of an isolation transformer. For this 208/120 system both approaches are equally valid. However, had the feeder been 480/277, a step-down transformer would be required.
Fuse protection of the receptacle
To maintain SCCR, the fuse must be the weakest link in the chain. Low current fuses are certainly available with up to 200 kA SCCR. However, there will be a temptation to change the fuse in the future. For example, ten years down the road a novice technician may power a vacuum cleaner from the receptacle. Not knowing anything about SCCR, they may “solve the problem” by replacing the fuse with something larger such as a 30 A time delay. While well-intentioned, the panel no longer UL-compliant and now harbors a latent hazard.
Transformer protection of the receptacle
A potentially safer long-term solution is to use an isolation transformer to isolate the feeder from the receptacle. Recall that a transformer has a natural current-limiting action that limits downstream SCCR fault current. An example is the Siemens MT0300A transformer as shown in Figure 2. The transformer is UL listed with file number E172116.
We still need to provide upstream protection to maintain Alice and Bob’s desired 50 kA SCCR. We would need to investigate the fuse holders installed on the unit. Depending on our research, it may be desirable to use a circuit breaker to minimize the chances of incorrect fuse installation. As for downstream protection, the transformer provides natural SCCR protection even if the future technician installs incorrect fuses.
Figure 2: Image of a Siemens isolation transformer suitable for isolating a convenience receptacle.
Tech Tip It takes a certain amount of work to locate a collection of parts to provide the convenience outlet functionality. For example, unless you know where to look, it can be difficult to verify the UL E172116 file number. In addition, we need to locate related circuit breakers, fuses, fuse holders, wire, receptacle, and a metal enclosure to house the receptacle. After that Alice and Bob need to document everything and demonstrate compliance to the inspector.
Given the technical and admin overhead, Alice and Bob decided to make a recipe for the convenience receptacle. Since nearly every panel has either a 208 or a 480 VAC feeder, they centered on the Siemens dual-voltage transformer. A single transformer meets their needs for either feeder.
Best wishes,
APDahlen
Related information
Please follow these links to related and useful information:
About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.