Industrial Control Panels

[david.mullins]

I am new here, thanks to the Admins for approving me!

I have a couple questions about equipment and code inside an industrial control panel but I'm not sure if this will be within the scope of the forum, i.e. NFPA 79 vs NEC.

Since I have worked mostly with the NFPA 79 and only some experience with the NEC, as a whole, I hope to ask some questions as to which standard will apply inside the panels.

If that's cool, I'll follow up with a couple of basic questions, here, first. Thanks!

David Mullins

 

[Smart $]

Welcome... :thumbsup:

There's a little crossover, but 79 dominates inside an ICP. The NEC dominates on field wiring, i.e. wiring that enters and exits the panel, and not furnished by the manufacturer as part of an unassembled multi-unit machine.

 

[david.mullins]

Control Taps

Control Taps

I know that wires need to be protected by an overcurrent device to the rated current of the wire size and type. How does this apply to a small guage wire, ~14AWG, that connects to the input side of the input breaker of a higher current (say 440VAC, 1000A three phase) line by means of a screw terminal with a ring lug to supply a step down transformer for the control electronics which need to be always on regardless of the state of the breaker. And, does this change if the tap is added (physically drilled and tapped into the connecting block) by the equipment integrator?

 

[petersonra]

I know that wires need to be protected by an overcurrent device to the rated current of the wire size and type. How does this apply to a small guage wire, ~14AWG, that connects to the input side of the input breaker of a higher current (say 440VAC, 1000A three phase) line by means of a screw terminal with a ring lug to supply a step down transformer for the control electronics which need to be always on regardless of the state of the breaker. And, does this change if the tap is added (physically drilled and tapped into the connecting block) by the equipment integrator?

I would take a very dim view of someone modifying a piece of equipment by drilling and tapping a hole in a lug, even though it seems to be a common practice.

The rules require one to protect all conductors from overcurrent in some way. the size of the wire does not matter any.

What you are describing is called a tap conductor. There are NEC rules about how tap conductor overcurrent protection is handled. they are the same for tap conductors that originate inside a control panel as they would be for tap conductors that originate elsewhere.

Is this a UL508a listed control panel you are asking about?

Is this for general information or specific to something you have in house? Or something you are yourself designing/building?

 

[david.mullins]

...

The rules require one to protect all conductors from overcurrent in some way. the size of the wire does not matter any.

...

Is this a UL508a listed control panel you are asking about?

Is this for general information or specific to something you have in house? Or something you are yourself designing/building?

It was my understanding that wires cannot be connected to a breaker larger than the size for which the wire is rated, as in 14 AWG to a 20A breaker? :? Otherwise, the switchgear for the incoming power would be protection for the 14AWG wires.

The panel is not UL. Unfortunately, I'm not at liberty to say what it is but we are just a contract manufacturer, build to print, only. I need to make sure I know what I'm talking about before I present this to the two senior engineers.

Thanks for the info! Now I have to buy the NECs to back up my arguments!

 

[GoldDigger]

Read the NEC rules on "tap conductors".
Normally any OCPD protecting conductors must be at the source end. But under specific conditions the OCPD can be at the other end instead. That is a tap.

 

[petersonra]

It was my understanding that wires cannot be connected to a breaker larger than the size for which the wire is rated, as in 14 AWG to a 20A breaker? :? Otherwise, the switchgear for the incoming power would be protection for the 14AWG wires.

Some things are impractical to do so tap conductors are allowed under limited circumstances. the length of a tap conductor is limited and it has to be well protected physically (such as in a box) or outside where it is unlikely to harm anyone. the short length of tap conductor is protected from overloads by the downstream OCPD. it does not have the normal short circuit protection though. but the level of physical protection is considered to be adequate to prevent short circuits. tap conductors that leave an enclosure have restrictions on what size OCPD they can terminate on, probably so a short circuit will trip the upstream OCPD.

The panel is not UL. Unfortunately, I'm not at liberty to say what it is but we are just a contract manufacturer, build to print, only. I need to make sure I know what I'm talking about before I present this to the two senior engineers.

if it is build to print and not UL why do you care? It is the problem of the guy that designed it.

 

[david.mullins]

Some things are impractical to do so tap conductors are allowed under limited circumstances. the length of a tap conductor is limited and it has to be well protected physically (such as in a box) or outside where it is unlikely to harm anyone. the short length of tap conductor is protected from overloads by the downstream OCPD. it does not have the normal short circuit protection though. but the level of physical protection is considered to be adequate to prevent short circuits. tap conductors that leave an enclosure have restrictions on what size OCPD they can terminate on, probably so a short circuit will trip the upstream OCPD.

if it is build to print and not UL why do you care? It is the problem of the guy that designed it.

The two tap wires (one on each of 2 of 3 phases) travel 4 to 5 inches from the taps on the main breaker terminals to a plastic raceway, then about 18 inches to the two pole breaker that feeds the 2KVA control transformer.

Mostly, I care because I care. The company we build them for is a sister company of ours, both owned by a larger holdings company. Their end customer is also a very large customer of ours, as well. It's just the right thing to do, all the way around.

 

[petersonra]

Well it's not a bad thing to ask when you see something you don't understand.