UL 508A DC Wire Size in Panels

[ddawg16]

I don't have a copy of the 508A books....

But if my feeble memory serves me right, DC wires in a panel are not limited to the 14 AWG min size for AC wiring.

Translation....If I'm wiring up PLC IO cards, I can use 16 or 18 AWG wire if the ckts are DC.

For anyone who has a set of books around, it would be section 29.6 I believe.

 

[theoldwizard1]

What is the voltage and current ? Automotive "primary" wire is routinely limited to 50V worth of insulation. Wire gauge should only be "as required" for current and length,

 

[alfredeneuman]

What is the voltage and current ? Automotive "primary" wire is routinely limited to 50V worth of insulation. Wire gauge should only be "as required" for current and length,

If it's located in an enclosure it has to be rated for the maximum voltage in the enclosure

 

[ddawg16]

What is the voltage and current ? Automotive "primary" wire is routinely limited to 50V worth of insulation. Wire gauge should only be "as required" for current and length,

It would be a typical control enclosure.....120 supply going to a 24 Vdc power supply.

If it's located in an enclosure it has to be rated for the maximum voltage in the enclosure

I think we are talking two different things. Obviously, we'd use THHN rated for 600 volts.

But UL lets me use 22 AWG twisted pair for DC signals...it's rated for 600V....

I should be able to use 18 AWG for DC inputs from limit switches.

 

[mm08822]

From a UL508A book I have printed in 2010:
29.6 is for power circuits of which the minimum shall be 14 awg.

38.2 specs internal control circuits w/ocp required at:
#16 10A
#18 8A
#20* 5A
#22* 3A
#24* 2A

*Only permitted for connection to PLC I/O cards

 

[ddawg16]

From a UL508A book I have printed in 2010:
29.6 is for power circuits of which the minimum shall be 14 awg.

38.2 specs internal control circuits w/ocp required at:
#16 10A
#18 8A
#20* 5A
#22* 3A
#24* 2A

*Only permitted for connection to PLC I/O cards

That is what I'm coming up with.

I know in the case of any AC wiring in the panel....even if it's just inputs and outputs to/from the PLC, if it's 120Vac....it has to be 14 AWG min.

I'm helping a friend cut the costs on their panels by suggesting they switch all their IO to 24 VDC. Smaller wire, and the PLC cards for digital IO are a higher density....at about the same price.

Additionally, if they build anything for overseas, they won't really need to change anything since most control panels over there are 230 Vac for power but 24 VDC for all controls wiring.

 

[mm08822]

That is what I'm coming up with.

I know in the case of any AC wiring in the panel....even if it's just inputs and outputs to/from the PLC, if it's 120Vac....it has to be 14 AWG min.

I didn't find any distinction in requirements for ac vs. dc control circuits. Wire size appears to be based solely on ocp rating.

The safety factor alone w/dc control ckts is enough to move away from 120vac control circuits. Other than combination starters, I haven't touched a 120vac control ckt designed panel in the last 25 yrs.

 

[ddawg16]

I didn't find any distinction in requirements for ac vs. dc control circuits. Wire size appears to be based solely on ocp rating.

The safety factor alone w/dc control ckts is enough to move away from 120vac control circuits. Other than combination starters, I haven't touched a 120vac control ckt designed panel in the last 25 yrs.

Well, the company my friend works for has to deal with a guy who is set in his ways. Bad case of NIH (Not Invented Here). If it's not his idea, NFW.

For stuff that needs 120 Vac, I'd rather put a relay out near it.

For MCC's....DeviceNet or ArmorBlock...or FlexIO.

 

[teamextreme]

That is what I'm coming up with.

I know in the case of any AC wiring in the panel....even if it's just inputs and outputs to/from the PLC, if it's 120Vac....it has to be 14 AWG min.

That's news to me. We use #16 for 120V I/O to PLC cards all the time in our UL508A panels. Inspectors have never said anything about it. As mm08822 said, I don't think this is a rule. Table 38.1 covers conductor sizes, based on OCP, all the way down to 30AWG.

 

[ddawg16]

That's news to me. We use #16 for 120V I/O to PLC cards all the time in our UL508A panels. Inspectors have never said anything about it. As mm08822 said, I don't think this is a rule. Table 38.1 covers conductor sizes, based on OCP, all the way down to 30AWG.

I went through the UL class many years ago and I seem to recall them saying 14awg min for AC.

But as I read more, it looks like it only applies if the AC is being used to power components in the panel (PS, PLC,) or motors.

Best I can tell, 16 or 18awg for things like PLC inputs is fine.

I'll keep looking.

 

[manwithtools]

John,
You are fine with 18awg for PLC I/O wiring (AC or DC). DC is the way to go today for sure, all the control components are less expensive in DC. Only use for 120 VAC today is long runs where voltage drop could be a concern. Even them we typically use distributed I/O and move the I/O to where it's needed.

We build a couple hundred panels a month and UL is happy with every one. One thing I've discovered, the UL instructors frequently know less about their own requirements than the end users who read the requirements and apply them correctly.

 

[ddawg16]

John,
You are fine with 18awg for PLC I/O wiring (AC or DC). DC is the way to go today for sure, all the control components are less expensive in DC. Only use for 120 VAC today is long runs where voltage drop could be a concern. Even them we typically use distributed I/O and move the I/O to where it's needed.

We build a couple hundred panels a month and UL is happy with every one. One thing I've discovered, the UL instructors frequently know less about their own requirements than the end users who read the requirements and apply them correctly.

Thanks for the conformation....

And....UL is a 'for profit' organization.....

I recall most of the training being about documentation and Listed vs Recognized, etc.

 

[Dustball]

We do minimum 14 awg for any AC wiring in our panels and machines. DC wiring is sized to the circuit protection used but generally we do 18 awg for most DC panel I/O wiring. For PLC I/O to terminal blocks, we can step down to 22 awg due to space inside the modules.