UL508A Back Panel as Ground Connection

bwat

EE
Location
NC
Occupation
EE
Shot in the dark...

Anybody know conclusively whether the back panel of a UL508A control panel can be used as EGC bus? Meaning, let's say you have a 3-terminal mechanical lug for incoming connections, and then need to also have 3-terminal mechanical lug for outgoing. Do you absolutely have to run a jumper between the two different lug sets, or is it good without the jumper because both lug sets are connected to the solid, single-piece back panel.

The language in 14.3 says "...by means of metal-to-metal contact or by means of an internal bonding conductor that complies with 15.2".

"metal to metal" seems to imply that this is ok.... but "metal to metal" seems way too broad and easy to attain (number of threads, etc), so it seems like it would be harder than that.

We usually just end up just playing it safe and running a jumper, but I don't like not knowing whether it is actually required or not.
 

DrSparks

Senior Member
Location
Madison, WI, USA
Shot in the dark...

Anybody know conclusively whether the back panel of a UL508A control panel can be used as EGC bus? Meaning, let's say you have a 3-terminal mechanical lug for incoming connections, and then need to also have 3-terminal mechanical lug for outgoing. Do you absolutely have to run a jumper between the two different lug sets, or is it good without the jumper because both lug sets are connected to the solid, single-piece back panel.

The language in 14.3 says "...by means of metal-to-metal contact or by means of an internal bonding conductor that complies with 15.2".

"metal to metal" seems to imply that this is ok.... but "metal to metal" seems way too broad and easy to attain (number of threads, etc), so it seems like it would be harder than that.

We usually just end up just playing it safe and running a jumper, but I don't like not knowing whether it is actually required or not.
I don't know conclusively but from my experience, yes, you can. As long at your tapping and threading the screws to the panel with no less than 32 tpi.

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DrSparks

Senior Member
Location
Madison, WI, USA
For example, if you were to bolt on a lug using a bolt and nut through the panel, that would not be considered sufficiently bonded unless you scraped the paint off behind the lug (and that's just ugly). By tapping and threading you are bonded via the threaded connection.

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bwat

EE
Location
NC
Occupation
EE
I don't know conclusively but from my experience, yes, you can. As long at your tapping and threading the screws to the panel with no less than 32 tpi.

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Thanks. Any reference to the 32 tpi? That makes it sound like there's a specific code rule that's being met, and I haven't seen one.
 

DrSparks

Senior Member
Location
Madison, WI, USA
Thanks. Any reference to the 32 tpi? That makes it sound like there's a specific code rule that's being met, and I haven't seen one.
250.8(a)(6) requires 2 threads to engage the metal. 32 tpi would be required to tap 1/8 inch thick metal panel or enclosure. Usually a 10-32 screw.

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bwat

EE
Location
NC
Occupation
EE
250.8(a)(6) requires 2 threads to engage the metal. 32 tpi would be required to tap 1/8 inch thick metal panel or enclosure. Usually a 10-32 screw.

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I hear you, and that's a solid basis for an answer, but that's NEC and UL508A, and they are not equivalent in all aspects. This might be one of those areas where UL508A lacks clarity and the best we can do is fall back to something like 250.8(A). Then there's a least a method to the madness.
 

DrSparks

Senior Member
Location
Madison, WI, USA
I hear you, and that's a solid basis for an answer, but that's NEC and UL508A, and they are not equivalent in all aspects. This might be one of those areas where UL508A lacks clarity and the best we can do is fall back to something like 250.8(A). Then there's a least a method to the madness.
Well we as electricians must adhere to the NEC. If you're building a control panel, we'll now you're an engineer. You can totally build a control panel with individual UL listed components and have it field approved. I don't see why UL and the NEC would be at odds.

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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Shot in the dark...

Anybody know conclusively whether the back panel of a UL508A control panel can be used as EGC bus? Meaning, let's say you have a 3-terminal mechanical lug for incoming connections, and then need to also have 3-terminal mechanical lug for outgoing. Do you absolutely have to run a jumper between the two different lug sets, or is it good without the jumper because both lug sets are connected to the solid, single-piece back panel.

The language in 14.3 says "...by means of metal-to-metal contact or by means of an internal bonding conductor that complies with 15.2".

"metal to metal" seems to imply that this is ok.... but "metal to metal" seems way too broad and easy to attain (number of threads, etc), so it seems like it would be harder than that.

We usually just end up just playing it safe and running a jumper, but I don't like not knowing whether it is actually required or not.
This is just one of the things in UL508a that is not explicitly stated, although strongly implied.

If it bothers you, maybe you should just put your lugs on the some bolt.
 

DrSparks

Senior Member
Location
Madison, WI, USA
This is just one of the things in UL508a that is not explicitly stated, although strongly implied.

If it bothers you, maybe you should just put your lugs on the some bolt.
Right, but there are provisions for it in the NEC, so stands to reason UL would be OK with it too.

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Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I did it for years and had many panels looked at by UL during inspections, they never said a word about it.


I hear you, and that's a solid basis for an answer, but that's NEC and UL508A, and they are not equivalent in all aspects. This might be one of those areas where UL508A lacks clarity and the best we can do is fall back to something like 250.8(A).
UL508A says exactly the same thing, although it adds an exception;
Exception: Two full threads are not required if fewer threads result in a secure connection in
which the threads will not strip upon application of a 20 pound-inch (2.3 N·m) tightening torque.
 

bwat

EE
Location
NC
Occupation
EE
I did it for years and had many panels looked at by UL during inspections, they never said a word about it.
That's good to know that at least some UL inspectors are ok with it. Thank you.


UL508A says exactly the same thing, although it adds an exception;
Isn't that piece that you quoted ("Exception: Two full threads are not required..") from 28.2.3 which has to do with wire binding screws? I'm not seeing the correlation to connections for bonding or continuation of EGC purposes.
 

bwat

EE
Location
NC
Occupation
EE
Well we as electricians must adhere to the NEC. If you're building a control panel, we'll now you're an engineer. You can totally build a control panel with individual UL listed components and have it field approved. I don't see why UL and the NEC would be at odds.

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We have UL508A shops and yes I'm an engineer. It is uncertain whether we will be listing these panels yet or not, but we try to build all of our panels to UL508A of out good practice and other reasons. For the most part, you are right; UL and NEC are extremely similar. But they are not identical, so when we're designing UL508A panels we only want to leverage NEC where the UL508A standard falls short on clarity.
 

bwat

EE
Location
NC
Occupation
EE
This is just one of the things in UL508a that is not explicitly stated, although strongly implied.

If it bothers you, maybe you should just put your lugs on the some bolt.
To be clear, which way are you saying is implied? Jumper or backpanel as connection? I know you have UL shops also and are knowledgable on these topics. I'd be interested to hear what you guys typically do as well.
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
250.8(a)(6) requires 2 threads to engage the metal. 32 tpi would be required to tap 1/8 inch thick metal panel or enclosure. Usually a 10-32 screw.
It's just picking at nits, something at which I excel, but 1/16" panel is common and needs a 32 tpi or finer screw. A 1/8" panel could use a 16 tpi screw.
 

DrSparks

Senior Member
Location
Madison, WI, USA
It's just picking at nits, something at which I excel, but 1/16" panel is common and needs a 32 tpi or finer screw. A 1/8" panel could use a 16 tpi screw.
Yeah you're probably right. I always use 10-32 anyhow. But yeah code only wants to see 2 threads engaged no matter how you achieve that.

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