Power Circuit Definition in an Industrial Control Panel

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ClemsonME

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Hello All,

I am new to the forum (first post). I have a question regarding Power Circuits in an industrial control panel as it relates to NFPA79 and UL508A. I primarily deal with control panels for burner controls. The only devices powered from these panels that could be considered power circuits are gas valves (motorized and solenoid), ignition transformers, and some 120VAC actuator motors. Are these types of devices considered power circuits or control circuits? I can't decide if they fit the definition of a power circuit in NFPA79 and would like some opinions from others. I would think since they are 120VAC devices they would be considered control circuits, but they are used for "productive operation" as defined in NFPA 79. My reason for being concerned with this is primarily due to determining whether or not I need to apply an SCCR value to the panel.

Thanks in advance to any that can offer insight.

BR
 
IMO they are control circuits.

However, by the definitions in UL508a I don't see anyway to prove it one way or the other.

2.11 CONTROL CIRCUIT – A circuit that carries the electric signals directing the performance of a
controller, and which does not carry the main power circuit. A control circuit is, in most cases, limited to
15 amperes.

2.40 POWER CIRCUIT – Conductors and components of branch and feeder circuits.

However, the diagram Ul508a that illustrates various concepts such as what constitutes a power circuit versus a control circuit (Figure 6.1 Description of terminology) clearly shows solenoid valves as being in control circuits and that they are also class 1 circuits.

i think if the circuits are class 1 circuits pretty much they have to be considered as control circuits.
 
Thank you for the response. I feel as though they are control circuits as well (at least the valves are). The UL definitions don't really help, but as you stated, the example drawing clearly shows the solenoid valve as a control circuit.

I also found another statement in UL508A that supports the valves as control circuits. In the Flame Control section of UL508A, Article 82.4.1 states "The output from a primary safety control to the main gas valve shall not have interposing components that are able to switch or isolate the control voltage". Keyword in this statement being "control". This further implies to me that the valves are control circuits.

I'm still uncertain on how to consider the ignition transformer. These are 120VAC and are typically only 0.25KVA. It will be fused with a Type CC on the primary, as overcurrent protection is a requirement per Article 82.6.1. This states "The primary winding of an ignition transformer shall have overcurrent protection in accordance with Section 42, Overcurrent Protection - Control Circuits." Having the Type CC fuse will allow me to maintain the SCCR of the panel. But is the ignition transformer a power circuit? If it is, then I will have to wire it with 14 gauge per Article 29.6.1. Using the 14 gauge wire doesn't seem like a big deal until you factor in that the load for the primary side of this ignition transformer is carried across all the safety limit switches of the burner management system. Now suddenly I have a lot of 14 gauge wiring in my panel eating up a lot of wireduct space.

Anyone have any thoughts on whether an ignition transformer is considered a power circuit?
 
ClemsonME said:
Thank you for the response. I feel as though they are control circuits as well (at least the valves are). The UL definitions don't really help, but as you stated, the example drawing clearly shows the solenoid valve as a control circuit.

I also found another statement in UL508A that supports the valves as control circuits. In the Flame Control section of UL508A, Article 82.4.1 states "The output from a primary safety control to the main gas valve shall not have interposing components that are able to switch or isolate the control voltage". Keyword in this statement being "control". This further implies to me that the valves are control circuits.

I'm still uncertain on how to consider the ignition transformer. These are 120VAC and are typically only 0.25KVA. It will be fused with a Type CC on the primary, as overcurrent protection is a requirement per Article 82.6.1. This states "The primary winding of an ignition transformer shall have overcurrent protection in accordance with Section 42, Overcurrent Protection - Control Circuits." Having the Type CC fuse will allow me to maintain the SCCR of the panel. But is the ignition transformer a power circuit? If it is, then I will have to wire it with 14 gauge per Article 29.6.1. Using the 14 gauge wire doesn't seem like a big deal until you factor in that the load for the primary side of this ignition transformer is carried across all the safety limit switches of the burner management system. Now suddenly I have a lot of 14 gauge wiring in my panel eating up a lot of wireduct space.

Anyone have any thoughts on whether an ignition transformer is considered a power circuit?
Click to expand...

they are essentially telling you it is a control circuit. this is clearly a class 1 circuit anyway, at least the line side, and the load side are field conductors that are not part of the ICP.
 
"The primary winding of an ignition transformer shall have overcurrent protection in accordance with Section 42, Overcurrent Protection - Control Circuits."

And Bob beat me to it lol! Although, the load side conductor on an ignition transformer is probably a specialized ignition wire. While the conductor is fairly small (maybe 16awg) the insulation is quite bulky. I think the ignition wire I have for our boiler is 50,000 V rated(could be wrong, I havent actually laid eyes on it in a couple of months)!
 
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Johnnybob said:
"The primary winding of an ignition transformer shall have overcurrent protection in accordance with Section 42, Overcurrent Protection - Control Circuits."

And Bob beat me to it lol! Although, the load side conductor on an ignition transformer is probably a specialized ignition wire. While the conductor is fairly small (maybe 16awg) the insulation is quite bulky. I think the ignition wire I have for our boiler is 50,000 V rated(could be wrong, I havent actually laid eyes on it in a couple of months)!
Click to expand...
I'll just hike over there and have a gander:)
 
Awesome replies gentlemen! But to make sure I am understanding you guys correctly, it is your interpretation that the primary side (120VAC) of the ignition transformer can be considered a control circuit? Thus I do not have to use 14 gauge wire as dictated by UL508A for power circuits.

So, since you guys are on a roll, I'm going to pitch one more question at you. I have a control panel fed with 480VAC/1Ph to a fused disconnect switch which then feeds a control power transformer. Everything on the secondary side of the control power transformer is control circuits. Disconnect switch and control power transformer are both in the control panel with the control circuits. Does this panel have an SCCR value or not? I know that a panel with control circuits only does not have a SCCR value, but it would seem to me that the panel would have an SCCR value since the 480VAC disconnect and transformer are in the panel.
 
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