UL 508A vs. NEC

[cbolwerk]

We are having an interesting discussion at work about the design of electrical controls panels. Someone made the comment that UL 508A does not mention anything about using a 10 ft tap rule, yet for years everyone that I have worked with has used the tap rule from the NEC when designing power distribution within a panel. I also know that all of the panels have been inspected to UL 508A. Does anyone have any insight to this?

Another part of the discussion has to do with the differences in wire sizes. This person states that the NEC doesn't apply to internal panel wiring for UL listed panels, that UL 508A is to be used. He points to the wire size tables being different. Is that correct?

As an engineer that designs electrical control panels, which should I use, the NEC or UL 508A? Also, as this is for industrial machinery, should I use NFPA 79? What order should these be used, i.e. which has priority?

Thanks in advance for the help!!!!!

 

[cbolwerk]

Just wondering if I am out-of-line with these questions or something. With the knowledge of the people that visit this forum, I would have thought that I would have seen a reply by now.

If I am, I sincerely apologize. Please let me know.

Thanks!

 

[infinity]

You might want to check UL 508A directly look at NEC Article 409 and 409.1 FPN which references UL 508A. Over the years I have seen many so called NEC violations within electrical equipment. Multiple conductors under one lug, parallel conductor smaller than #1/0. The list goes on and on. The 10' tap rule probably falls in the same category. That being that these things are outside of the scope of the NEC.

 

[roger]

Use UL 508A for guidance, article 409 of the NEC is for direction when control panels are fabricated in the feild.

If you read 409 closely you will see that unlike some other article sections, there is no requirement for these panels to be listed.

Roger

 

[petersonra]

You might want to check UL 508A directly look at NEC Article 409 and 409.1 FPN which references UL 508A. Over the years I have seen many so called NEC violations within electrical equipment. Multiple conductors under one lug, parallel conductor smaller than #1/0. The list goes on and on. The 10' tap rule probably falls in the same category. That being that these things are outside of the scope of the NEC.

whats wrong with multiple conductors under one lug? the NEC does not even generally prohibit that.

I think the parallel conductor rule is in ul508a, would have to look Monday when I get back to work.

The important thing to remember is that these things are not within the scope of the NEC, and FPNs in the NEC are not enforcable.

 

[sandsnow]

whats wrong with multiple conductors under one lug? the NEC does not even generally prohibit that.
QUOTE]

You're kidding, right?!?!?

 

[infinity]

whats wrong with multiple conductors under one lug? the NEC does not even generally prohibit that.

I think the parallel conductor rule is in ul508a, would have to look Monday when I get back to work.

There aren't very many lugs that are listed for more than one conductor. I've seen lugs within a UPS system with about 10 conductors under it. Certainly if an EC installed such a mess it would violate the listing of the lug and therefore be an NEC violation.

The important thing to remember is that these things are not within the scope of the NEC, and FPNs in the NEC are not enforceable.

I think that someone already said that::wink:

That being that these things are outside of the scope of the NEC.

 

[petersonra]

whats wrong with multiple conductors under one lug? the NEC does not even generally prohibit that.
QUOTE]

You're kidding, right?!?!?

Where does it say that in the NEC?

Might be a UL listing issue, but not a NEC issue.

 

[sandsnow]

Where does it say that in the NEC?

Might be a UL listing issue, but not a NEC issue.

Section 110.14(A) last sentence.

Terminals for more than one conductor and terminals used to connect aluminum shall be so identified.

 

[roger]

Where does it say that in the NEC?

Might be a UL listing issue, but not a NEC issue.

Here's an illustration to accompny Larry's post.

http://mikeholt.com/reprint_request.php?url=http://www.mikeholt.com/graphics/110-14a.gif

Roger

 

[davidr43229]

UL 508A does allow multiple wires under a lug, size 10 & greater.
A disconnect is required....66.6.1 A disconnecting means shall be provided for each incoming supply source. Other than terminals,
no components shall be located on the line side of the disconnecting means.
I could not find a tap rule, within UL508A

 

[petersonra]

Section 110.14(A) last sentence.

Terminals for more than one conductor and terminals used to connect aluminum shall be so identified.

so we agree it is not a NEC issue, but rather a UL listing issue. many terminals are listed for more than one wire. I don't recall any being listed for more than 2.

 

[roger]

Where does it say that in the NEC?

Might be a UL listing issue, but not a NEC issue.

so we agree it is not a NEC issue, but rather a UL listing issue. many terminals are listed for more than one wire. I don't recall any being listed for more than 2.

No, quite the contrary, it's an NEC issue that requires the terminal to be identified for more than one conductor, UL does not come into play.

Roger

 

[petersonra]

I could not find a tap rule, within UL508A

Thats because there is not one. It does recognize tap conductors will exist in the cabinet though.

40.2.2 An overcurrent protective device, either branch circuit or supplementary type, shall be installed in each ungrounded conductor of the control circuit on the load side of the branch circuit protection in the power circuit or as specified in 40.2.1 to protect smaller tap conductors where they receive their supply and sized in accordance with 40.3.2.

 

[petersonra]

No, quite the contrary, it's an NEC issue that requires the terminal to be identified for more than one conductor, UL does not come into play.

Roger

Ok. Its an issue where someone (typically UL) has to identify a terminal is acceptable for more than one wire. The point is the NEC does not prohibit it (except for grounded conductors).

 

[cbolwerk]

Thats because there is not one. It does recognize tap conductors will exist in the cabinet though.

This is on the load side of the branch circuit, right? What I was originally wondering about was in regards to the NEC tap rule, which allows tap conductors on the line side of the branch circuit to be sized based on the size of the branch OCPD (within 10 ft of the tap).

If this does not exist in UL 508A, how are the conductors sized from the PDB to the branch circuit OCPD? For example, I have a 400 amp feed to a motor control panel. I go through a fused disconnect to a PDB. This wire is sized for 400 amp. From the PDB, I go to several motor starters, etc. According to the NEC, the wire from the PDB to the motor starter can be based on the size of the motor starter if the tap is within 10 ft. If UL 508A does not allow this, wouldn't you have to size this conductor for 400 amp?

Thanks!

 

[petersonra]

This is on the load side of the branch circuit, right? What I was originally wondering about was in regards to the NEC tap rule, which allows tap conductors on the line side of the branch circuit to be sized based on the size of the branch OCPD (within 10 ft of the tap).

If this does not exist in UL 508A, how are the conductors sized from the PDB to the branch circuit OCPD? For example, I have a 400 amp feed to a motor control panel. I go through a fused disconnect to a PDB. This wire is sized for 400 amp. From the PDB, I go to several motor starters, etc. According to the NEC, the wire from the PDB to the motor starter can be based on the size of the motor starter if the tap is within 10 ft. If UL 508A does not allow this, wouldn't you have to size this conductor for 400 amp?

Thanks!

40.2.2 An overcurrent protective device, either branch circuit or supplementary type, shall be installed in each ungrounded conductor of the control circuit on the load side of the branch circuit protection in the power circuit or as specified in 40.2.1 to protect smaller tap conductors where they receive their supply and sized in accordance with 40.3.2.

Note the part in bold.

<added> just noticed this refers to control circuits. it looks like the power circuits are handled in a similar way, although it never refers to taps at all.

31.2.1 A branch circuit protective device shall be installed in each ungrounded conductor to the load(s)
involved.

Note that it does not say where in the branch circuit it has to be. Only that it has to be before the load, and of course inside the listed cabinet.

 

[sandsnow]

Ok. Its an issue where someone (typically UL) has to identify a terminal is acceptable for more than one wire. The point is the NEC does not prohibit it (except for grounded conductors).

I guess it depends how you look at it. I see the general rule being one wire per terminal. Two or more if it is identified.

Interesting here the Code does NOT use the term listed.

Besides EGC terminal busses, anyone seen terminals rated, identified or listed for more than two conductors?

 

[infinity]

I guess it depends how you look at it. I see the general rule being one wire per terminal. Two or more if it is identified.

Interesting here the Code does NOT use the term listed.

Besides EGC terminal busses, anyone seen terminals rated, identified or listed for more than two conductors?

More than 2? Nope.

 

[Jraef]

Back to the original bigger picture;
If a control panel is UL listed (or listed by any other NRTL), the NEC applies only to the installation of the panel, not the internal components that were part of the UL listing. So when UL has different rules, such as wire sizing, it trumps the NEC because once that panel is listed, the installer can connect it as long as HIS connections meet NEC rules.

So to your 10ft tap rule, UL would apply in the construction of the panel at your shop, not the NEC. But there is the same requirement in UL 508A to address that and although worded differently, the net result is the same as the NEC. You can do it, but with rules.
For example:

UL 508A, Section 31.4
The ampacity of the tap conductors, the internal
conductors to the individual loads, shall be:
a. Not less than 1/3 the ampacity of the branch circuit conductor,
calculated as in 28.3.3; or
b. Not less than 1/10 the ampere rating of the branch circuit protection
for the group for each motor circuit provided with a
manual motor controller marked ?Suitable as tap conductor
protection in group installations? and complies with the
Standard for Industrial Control Equipment, UL 508.
The conductors on the load side of the manual motor
controller shall have an ampacity not less than calculated in
28.3.2.

Bottom line: IF you have UL listed panel, UL rules apply on the inside; NEC on the outside and coming in. If you have a-bunch-o-parts-in-a-box control panel, the NEC would apply through and through. But many states have rules that say any more than "x" parts-in-a-box will have an overall NRTL listing anyway. The NEC as a national document however stops short of the NRTL listing as a requirement, it is a suggestion.