UL508A ground conductor sizing 6

[PaulKraemer]

Hi,

I have been trying to design an Industrial Control Panel following the guidelines in UL508A. I have asked several questions related to this project, and members in this forum have been incredibly helpful. I am now at the point where I have ordered all my components, and now I am trying to come to grips with the wire and cable I will need.

I have read UL508A twice, but I'm still a bit unclear about how to choose the conductor size for some of my ground wires. In particular, the component in my panel with the highest current draw will be a Solid-State Relay I'll be using to control power to a resistance heater. I will have this SSR fused at 35 amps. According to UL508A table 28.1 (Ampacities of insulated conductors), I believe I should use 8 AWG copper wire for the current carrying conductors going into and out of this SSR (please correct me if I am wrong). This SSR has a terminal that according to the manufacturer's instructions must be connected to earth ground. It is not clear to me whether I should use 8 AWG for this ground wire, or if it is acceptable to use a smaller ground wire for this purpose.

Any advice will be greatly appreciated.

Thanks in advance,
Paul

 

[mparenteau]

Perhaps this:

https://us.rs-online.com/expert/industrial-control-panel-grounding-and-bonding/

Mike

 

[che12345]

Mr PaulKraemer (Electrical)(OP)4 May 23 18:12
".....I have read UL508A twice, ...... I will be using to control power to a resistance heater. ...... fused at 35 amps. According to UL508A table 28.1 (Ampacities of insulated conductors), I believe I should use 8 AWG copper wire for the current carrying conductors going into and out of this SSR (please correct me if I am wrong)..... This SSR has a terminal that .... must be connected to earth ground. It is not clear to me whether I should use 8 AWG for this ground wire, or if it is acceptable to use a smaller ground wire for this purpose" .

Per the NEC Table 250-122 Minimum size Equipment grounding conductor .....
Rating or setting of automatic OC device in circuit ahead of equipment ... not exceeding
(a) 40A, the minimum equipment grounding conductor Copper AWG 10,
(b) 60A, the minimum equipment grounding conductor Copper AWG 10.
BTW: The minimum equipment grounding conductor size is dependent on the OCPD ( fuse 35A).
Che Kuan Yau (Singapore)

 

[PaulKraemer]

Thank you Che and Mike -

Che - Can I ask what version of the NEC (NFPA 70) you are referring to? I am looking at the 2020 version, but table 250-122 in my copy only shows OCPD ampere ratings of 15, 20, 60, 100 (and higher) amps. There is no row in my copy for 40 amps.

Regardless, this table make it seem like it is acceptable to use 10 AWG copper wire for the grounding conductor if your OCPD does not exceed 60 amps. The article that Mike shared also states that "Grounding conductors must be sized per the NEC based on the size of the overcurrent device protecting the circuit."

UL508A table 28.1 (ampacities of insulated conductors) makes me believe I have to use 8 AWG copper wire for the current carrying conductors in the circuit I will have fused at 35 amps. NFPA 70 (NEC) 2020 table 310.16 seems to agree with this. For a moment, I thought 10 AWG might be permitted by the NEC, but there is an asterisk referring to section 240.4(D) that makes it clear that 10 AWG would only be acceptable if the over-current protection did not exceed 30 amps.

Anyway, I am pretty confident that 8 AWG copper wire is the correct choice for my current carrying conductors. Given the apparent consensus that the grounding conductor must be sized based on the size of the overcurrent device protecting the circuit, it seems to me that table 250.122 justifies my use of 10 AWG for my grounding conductor according to the NEC.

I just am not sure if there is anything more restrictive in UL508A that says that the grounding conductor has to be at least the same size as the current carrying conductors. I re-read the UL508A sections that pertain to grounding pretty thoroughly, but I didn't find anywhere that this is stated.

With this being the case, with my goal being to satisfy the requirements of both NEC and UL508A, I *think* that using 8 AWG copper wire for my current carrying conductors and 10 AWG copper wire for my ground conductor would be considered acceptable. If either of you can tell if my analysis is correct or flawed, I would greatly appreciate it.

Thanks again and best regards,
Paul

 

[resqcapt19]

Paul, There is no row for 40 amps in Table 250.122, but since 40 exceeds 20 and does not exceed 60, the required copper equipment grounding conductor size is 10 AWG.
As far as the conductor size per the NEC (assuming overcurrent protective device is rated at 35 amps), it would be 8 AWG if all of the terminations are rated 60°C, but the NEC would permit 10 AWG if the equipment terminations were rated 75°C. I don't know if UL508A works the same as the NEC or not.

 

[che12345]

Mr PaulKraemer (Electrical)(OP)6 May 23 16:30
".....with my goal being to satisfy the requirements of both NEC and UL508A, I *think* that using 8 AWG copper wire for my current carrying conductors and 10 AWG copper wire for my ground conductor would be considered acceptable..."
1. My sincere apology for posting the outdated data, as NEC and UL508A standards are not practised in my location (Singapore).The Table is taken from my personal collection; publication 1999 NEC Handbook by Mark W. Earley , Joseph V. Sheehan and John M. Caloggero
2. I strongly suggest that you should refer to the latest NEC and UL standards, as both standards are revised regularly.
3. Yes , I am of the opinion that " using 8 AWG copper wire for my current carrying conductors and 10 AWG copper wire for my ground conductor would be considered acceptable " with the OCPD fused at 35A.
Che Kuan Yau (Singapore)

 

[waross]

In the 2013 Edition of UL508A There is a row for 40 Amps.
However for overcurrent protection of 30A, 40A, and 60A #10 AWG copper is allowable for bonding.
Try this download:

https://www.starsplas.com/wp-content/uploads/2019/01/UL508A-2014.pdf

Table 15.1
PDF Page 30
Document Page 26

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[PaulKraemer]

Hi Che, resqcapt19, and Waross -

Thank you very much for your replies. I am now feeling comfortable that I can use 10 AWG copper for my grounding conductor for this circuit, and 8 AWG for the current carrying conductors.

As shared by Waross, I do remember seeing table 15.1 when I was reading the current version UL508A. What threw me off was that the way this chapter is written, it made me think this table might only apply for the main circuit that supplies power to the whole Industrial Control Panel. It was not clear to me that I can also refer to this table for the grounding of individual components inside the panel.

It makes sense to me that the requirements would be the same, but I just wanted to make sure I wasn't missing something. I really appreciate all of your help!

Best regards,
Paul

 

[waross]

As side note, some codes size the bonding conductor based on the ampacity of the supply conductors rather than the size of the overcurrent protection.
Rational:
On a long circuit, fault current may not be high enough to trip the instantaneous element of the breaker.
Likewise, with dual element fuses, the instantenouls element may not clear.
That leaves the inverse time element to clear the fault.
However:
1) In the event that all the fault current returns through the bonding conductor, (There are no parallel grounding paths, such as through a metal building frame.) The supply conductors and the bonding conductor will form a voltage divider. The bonding conductor typically has a higher impedance than the supply conductor and the voltage drop across the bonding conductor may be more than 1/2 of the normal system to ground voltage.
On a 480 Volt circuit, the voltage to ground will be 277 Volts and the touch voltage on a the surface of a faulted motor or junction box may be greater than 138 Volts until the protection clears the circuit.
On a long circuit, the supply conductors may be increased in size to mitigate voltage drop and so the possible touch voltage may be even higher.
When the bonding conductor size is based on conductor ampacity, the bonding conductor size will be increased along with the supply conductors and the touch voltage will not be increased when the supply conductors are increased in size.

2) Motor circuits.
While the motor overload protection and motor supply conductors are sized based on the motor current, the overcurrent protection may be several times greater than the conductor ampacity. Sizing the bonding conductor based on overcurrent protection may result in grossly oversized bonding conductors, or at least bonding conductors of the same ampacity as the supply conductors.
This leads to point 3).

3) Cables. In some industries, cables are replacing conduit and individual conductors to a very large degree.
Typical cable assemblies have an included bonding conductor that is two AWG sizes less (smaller) than the supply conductors.
This is reflected in the code tables for bonding conductor ampacities.

4) Special cases. In some areas, engineering standards and/or special industry codes for bonding conductors recognize the possibility of high touch potentials and the possibility of arcing to adjacent grounded metal.
In the areas of the petro-chemical industry that I am familiar with, local bonding conductors of several times the ampacity of the supply conductors are required for motors.
These conductors do not run back to the supply but connect to the steel skid or structure on which the motor is mounted.
These local bonding conductors do not replace the code required bonding conductors but are suplementary to code grounds.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

Table 15.1 is used to pick the terminal required for the service ground conductor. But, if your supply only needs a 10awg wire then internally it can be 10awg too. There is a note about door bonding following that table as well, indicating it can be used for internal conductors. NEC is the same service ground conductor sizing.

 

[PaulKraemer]

Thank you Waross and LionelHutz for your most recent posts, and thank you again to everyone else who participated in this thread. I have learned a great deal by asking questions related to my current project. All of your help is very much appreciated.

Thanks again!
Paul