UL508A and Wire Sizing

4 posts in this topic

Hi All,

I am pretty new at electrical design for PLC control panels. We are a UL508A shop, so this is the code I refer to most. Something struck me as odd when I was trying to size wire for this control panel.

UL508A 28.3.1 says this: The required size of the field wiring terminal shall not be less than 14 AWG (2.1 mm2) and shall be determined by:

a) Calculating the required ampacity per 28.3.2 – 28.3.6; and

b) Determining the minimum field wiring conductor size from Table 28.1 having a corresponding ampacity that is equal to or greater than the required ampacity from (a).

How can the minimum wire size be #14 AWG? We use #16 AWG for wire to the PLC modules? Are we breaking code or is there something I am missing?

Also, understanding code books can be difficult. Is there a website or place I can go to get a plain English translation? I feel like I will read a sentence over and over again to try to understand what they are saying, and feel like there's is a much easier way to say it than the way it is written. For example, "28.3.1 b" ……...what is it really saying?


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no you are not missing something

the rules are different for wiring in the control cabinet.

When i did machine wiring the minimum wire size was 14 THHN for all control wiring (120 VAC & 24 VDC control).

The analog was a different story.  

Yes the code book is confusing.

they sell a NEC handbook that explains some part with easier to understand text

also this is handy for the common stuff Uglys Electrical Reference


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The key that you are missing here is that you are in section 28, Power Circuits. The smallest size is 14 AWG. If you look in section 37, Control Circuits you will find that smaller sizes of wire are legal. 

The terminal connections for all field wiring will still need to be capable of taking a 14 AWG wire regardless of the circuit type (37.2.1). The exception is with Class 2 circuits, they may have a smaller terminal block, see section 37.4

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