300V Cabling

Status
Not open for further replies.
N

nhee

Member
I often see instrumentation circuits (24 VDC, 4-20 mA transmitters) installed using 300V shielded pair cable (#18AWG and smaller, Type CM or similar). These circuits are often fed from power supplies that are not listed/labelled as class 2.

My understanding is that 725.27 would require these cables to be Class 1 circuits, with 600V insulation. Otherwise, they would need to be installed per Chapter 3, and would need to be minimum #14 AWG. Am I missing something somewhere else in the code which permits this type of installation? Because we come across it often.

Thanks
 
Re: 300V Cabling

I agree that the power supply should be marked to indicate how it should be wired. Just because it isn't marked doesn't mean it's Class 1. Unfortunately the NEC is lacking in the area of low voltage and some manufacturers couldn't care less about the NEC. That's why we have this situation.

-Hal
 
Re: 300V Cabling

Hal,
I think that under the current code, you can't have a Class 2 or 3 system unless you use a listed Class 2 or 3 transformer, power supply or other listed equipment that is marked to identify it as a Class 2 or 3 power source. See 725.41(A). A few code cycles ago there was some voltage and current information that could be used to identify Class 2 and 3 power supplies, but that information is no longer in the code. If it isn't marked or identified as a Class 2 or 3 power source, then it is by default a Class 1 system.
Don
 
Re: 300V Cabling

I dunno. Read 725.21. Sounds like he is using a non-listed power supply. I don't agree that no marking by default makes it a Class 1 though that is probably the best thing to assume.

-Hal
 
Re: 300V Cabling

Wouldn't an unmarked PS be Class 1 by default if met the maximum volts/VA requirements of 725-21?

I guess my original questions was is there somewhere else in the code that allows installation of 300V cable fed from a 24 VDC non-Class 2 PS?
 
Re: 300V Cabling

If the 300V Cable is listed type ITC, then does 727 allow its use on these 24 VDC circuits, provided they are fused at less than 5A?
 
Re: 300V Cabling

Class 2 circuits, using 300v insulated #18, #20, etc. cable are used with transformers not to exceed 100 VA for circuits 30V or less, and .05 VA for circuits over 30V up to 150V. (Chapter 9, Table 11)
 
Re: 300V Cabling

I concur with the idea that if the power supply is not listed as Class 2 or Class 3, then it falls to Class 2 in the world of UL Listing.

NEC Article 727.5 states that Type ITC cable shall not be used with Class 1 circuits.
 
Re: 300V Cabling

Originally posted by cha:
I concur with the idea that if the power supply is not listed as Class 2 or Class 3, then it falls to Class 2 in the world of UL Listing.
Click to expand...
You concur with who? I don't think anybody has said that.

I agree with Don. This sounds like a class 1 circuit, with chapter three wiring methods being required :(
 
Status
Not open for further replies.
Top