Article 725

[jbuthmann]

I am involved in the design of PLC control panels. After hours of paging back and forth, I think I am getting the gist of NEC Article 725. Here is how I understand it in a somewhat simplified and abbreviated way. Please feel free to critique my understanding if I got it wrong.

If you use 16 AWG or 18 AWG conductors, Article 725 applies. If the wire insulation is rated 600 Volts you can use Class 1 wiring methods. Class 1 circuits can be run with power circuits in a factory or field assembled control center. (PLC panel?)

If the insulation is rated 300 Volts, that is typical of analog cabling, you must use Class 2 or 3 wiring methods. If you use Class 2 or 3, you must use a Class 2 or 3, Listed, power supply. Class 2 and 3 circuits must be separated from Class 1 and power circuits.

What I don?t understand is, if you have 300 Volt rated cable but have a proprietary, non-listed power supply designed for the PLC, is the installation not permitted?

 

[hbiss]

Re: Article 725

I think you are putting the cart before the horse a bit.

The wiring used depends on the power supply. Devices supplying power must be listed and will have their wiring requirement indicated on the name plate. If the power supply is "proprietary and not listed" I believe it and any wiring could only be used with the approval of the AHJ. If this is part of a product which you produce you need to seek UL or other testing laboratory listing to comply with the NEC.

Art 725 can apply to any size conductor. The listing- 600V (CL1), CL2 or CL3 will be indicated on the jacket of the cable or wire as well as on the reel.

Yes, class 2 and 3 must be separated from class 1 in raceways and boxes unless the class 2 or 3 circuit also uses 600V wiring.

-Hal

 

[seattle]

Re: Article 725

Check 725.52 exception 2. The entire control circuit is reclassified as class 1 if you run class 2 or 3 with class 1 or light and power, even though you are connected to a listed class 2 or 3 power supply.

 

[jbuthmann]

Re: Article 725

In reply to hbiss,

I have often read that the NEC is not a design guide. From this point of view your comment regarding "putting the cart before the horse" is accurate. However, I am a designer so my need is to turn it around from a design point of view. The NEC looks backwards at existing installations. My job as a designer is to look forward and make decisions that will comply with the NEC.

As I understand it, the whole point of Article 725 is to permit alternative wiring methods to Article 300. I have a choice up front to pick a wire size. If I choose to use 16 AWG or 18 AWG then I must follow Article 725. If I choose to use 300 Volt cable, CL2, CL3, CM, then I must use a Listed power supply.

The power supply I am using is manufactured by a Canadian firm. It is a component part of their programmable controller. The power supply is certified under CSA 2252-01. I contend it is permitted for Class 2 under 725.41 (A) (4) that covers information technology equipment. The power supply output is rated at 24 Volts at 0.5 Amps that is well below the ratings for Class 2. It is not Listed or labeled as a Class 2 power supply.

What do you think?

 

[petersonra]

Re: Article 725

Control panels are often listed by UL these days. if so, you have to follow the UL industrial control panel rules. This usually works to your advantage as opposed to the NEC rules.

The NEC is more of a code for installation of electrical equipment rather than production of a factory assembled panel.

Keep in mind that if its a machine control panel it must also meet the requirements of NFPA79 as opposed to the NEC (aka NFPA70), although again for the most part the requirements are more lenient then NEC requirements.

You will of course have to accomodate NEC requirements, such as making sure your termination points accept the appropriate sized field wiring even if the wiring inside the panel is smaller then the field wiring.

[ October 11, 2004, 05:05 PM: Message edited by: petersonra ]

 

[hurk27]

Re: Article 725

I have seen a few power supplys that are part of a listed assembly and are only listed as a package deal. This means that in the paper work there should be some referance to what class this power supply is. when a power supply is listed like this it must be used in acordance to manufacture's instructions that are also listed with the product.