SCCR QUESTION

[fifty60]

I have a piece of lab equipment that employs hermetic compressor and nichrome heaters. I have a mains circuit of 480V that is separated from a control circuit of 115VAC by a general purpose transformer. The equipment is marked as having a 5KA SCCR.

I would like to add a small outlet to this equipment to plug in a 24VDC piece of equipment. The SCCR of the outlet is 2KA. I am using circuit breakers for the primary and secondary transformer protection, so the let through current is probably higher than 2KA. Would I have to install a current limiting fuse in line with the outlet in order to increase the SCCR of the outlet to keep the 5KA SCCR of the entire equipment?

Also, since this equipment has a main fuse requirement of 25A, and employs Hermetic Compressors, I am not even sure that it is required to be marked with an SCCR according to NEC 440.

 

[petersonra]

If the outlet is on the secondary side of the control transformer, i would not worry about it as that is probably not considered a power circuit subject to the SCCR requirements of UL508a which is how SCCR is determined by most people for this kind of assembly.

 

[fifty60]

Would the same logic apply for chord connected equipment? I am not certain how to differentiate chord connected equipment from permanently connected mains equipment. In most of the permanently connected equipment I have seen, there is a control transformer to cleanly separate the Power and Control circuits. For plug connected equipment, there is not a transformer so all of the components inside the equipment are at line voltage, or at least half of line voltage depending on how you want to look at it.

120V 20A US and 230V 16A (Europe) are center taps from the supply transformer. Is it not appropriate to consider them "line voltages" or "power circuits"? Would the same rules and considerations apply for SCCR considerations on equipment plugged into these types of supplies?

 

[petersonra]

Would the same logic apply for chord connected equipment? I am not certain how to differentiate chord connected equipment from permanently connected mains equipment. In most of the permanently connected equipment I have seen, there is a control transformer to cleanly separate the Power and Control circuits. For plug connected equipment, there is not a transformer so all of the components inside the equipment are at line voltage, or at least half of line voltage depending on how you want to look at it.

120V 20A US and 230V 16A (Europe) are center taps from the supply transformer. Is it not appropriate to consider them "line voltages" or "power circuits"? Would the same rules and considerations apply for SCCR considerations on equipment plugged into these types of supplies?

I would suggest this.

UL508a SCCR determination applies to industrial control panels built in accordance with UL508a. If it is not built that way, you cannot use the way UL508a determines the SCCR.

UL508a has provisions for cord and plug connected equipment, so that does not matter any.

I don't believe UL508a allows you to have control circuits that exceed 120V so if you have 230V control circuits you can't claim it meets the UL508a standards.

The standard itself tells you how to differentiate between a power and a control circuit. Only power circuits have to be evaluated for SCCR.

Merely putting current limiting fuses in a power circuit does not always "fix" the SCCR problem. It can, but it might not, and it is not easy to explain when it might or might not.

 

[Besoeker]

I have a mains circuit of 480V that is separated from a control circuit of 115VAC by a general purpose transformer.

Do you know what the transformer rating is in kVA?

 

[fifty60]

No, I do not know the KVA of the service transformer. The equipment is marked with the 5KA SCCR and it is up to the customer to go from there. My concern is the 2KA SCCR outlet that I plug an IEEE to RS232 converter into. The power supply in the plug converts the 120V from the secondary of my control transformer to 24VDC for the converter.

So, plug in equipment (120V), an SCCR rating is not a concern or required?

How is SCCR handled in Europe?

 

[fifty60]

I am clear on what a Power circuit is when compared to a control circuit...until we get to chord connected equipment. After reading through NFPA 79 again, I am still confused. Would motors and heaters used inside of equipment that is plug connected to a 20A outlet be considered power circuits? Or would components inside of equipment pluged into a 120V 20A outlet be considered control equipment.

My concern is with the sizing of conductors inside the equipment. NFPA 79 states that 18,16,14AWG can be protected by 20A OCPD in control circuits. Would this apply to all of the wiring to any equipment that plugs into a 120V 20A outlet?

Or, would I expect to see 12AWG wire on a circuit inside 120V 20A plug in equipment that employs a heater?

 

[petersonra]

It is not likely that a piece of lab equipment would be covered by either NFPA79 or UL508a.

 

[fifty60]

I am trying to determine what is acceptable safe. It is still plug in equipment. it is powered off of a 20A 120V circuit. My "Code and standard" experience has always indicated that I would need to use a 12AWG conductor, with exceptions for motor loads.

My research has shown me that this is not always the case, and that extra precaution is given for house circuit and components that people can easily change themselves---hence the NEC small conductor rule.

I see 16AWG in Lab equipment that plugs into a 20A outlet and I cannot find a definite answer as to whether or not is acceptable.

 

[GoldDigger]

Basically if UL allows that size cord to be used (permanently attached as part of the equipment) then the NEC does not have anything to say about it.
Now if the equipment is not UL listed....

Tapatalk!

 

[fifty60]

My concern is not so much the chord attached to the equipment, but the individual conductors inside of the equipment. I get concerned when I see 16AWG on a piece of equipment that plugs into a 120V 20A service.

 

[petersonra]

My concern is not so much the chord attached to the equipment, but the individual conductors inside of the equipment. I get concerned when I see 16AWG on a piece of equipment that plugs into a 120V 20A service.

my clock radio has a #18 cord with no ground and plugs into a 20A outlet.

 

[fifty60]

Yes, I am trying to get an intuition as to why that is allowable. The fuse or circuit breaker that the 20A outlet (or 20A outlet as part of a 20A circuit) does not provide any overload protection, but only short circuit protection...I'm trying to find it written somewhere that the short circuit protection is adequately provided by the 20A circuit...and that you do not have to protect individual circuits inside of the equipment.

Can I treat all 120V circuits as control circuits?