Control Panel Wiring Practice

[DM2]

I'm looking for information relative to wiring guidelines for industrial control panels. I've had a look through UL 508A and NFPA 79, but it couldn't find the information I was looking for.

I'm looking for information on wire connections to terminal blocks, definition of the different type terminal blocks, etc. One issue I'm struggling with is terminating to barrel type (I think that's the technical term), terminal block that use either solid wire or a Fork terminal.

For the barrel type terminal block, it sometimes has a small plate that the screw compresses against the terminated wires. If you're not using a fork terminal, because you may be using solid wire, I was always told that there is a maximum wire size difference between the wires terminated on both sides of the terminal. If, for example I have a 14 AWG solid wire on one side of the screw terminal I can't connect a resistor to the other side because the plate mentioned above, wouldn't properly compress on the two different wire sizes. Also is there information published on the different colors used for fork terminal, and ferrules?

I thinking there is a standard out there, military maybe, that provides these guidelines, along with others that I can reference. Can someone point me in the right direction?

Regards,
Dan Marr

"Real world Knowledge isn't dropped from a parachute in the sky but rather acquired in tiny increments from a variety of sources including panic and curiosity."

 

[JLSeagull]

Differentiate high voltage power from signal terminals. Many require ring terminals for 480 Vac.

Most companies in the process industry use the DIN rail mounted clamp type terminals with no lugs on the conductors. These are availabe in many arrangements that accommodate jumper bars etc. Examples include Weidmuller, Phoenix Contact, etc. Some companies prefer the screw-less clamps such as Wago. I don't know any client that permits the Bucannon style screws on a bakelight strip that accommodate the forked terminals.

I cannot think of a domestic US industry standard. I know of many operating company general specifications from most major operating companies across several industries. My work is mostly oil, gas, refinery, petrochem etc. and this generalization applies to my food and pharmaceutical experience too.

 

[dpc]

I agree that there is no standard in the US. If given a choice, most panel builders use DIN-rail mounted type blocks because they are easy to build up and require less wire prep. These seem to be gaining market share all the time.

Many utilities still specify the old 30 A strap-screw barrier type blocks and these will require lugs. Most require ring lugs. Some allow locking fork lugs. For current transformer circuits, ring lugs are almost always used. There is definitely a quality difference in the compression lugs. The cheap ones sold at hardware stores will not last long, especially if exposed to temperature extremes. Insist on a quality, name brand lug. They are worth the extra penny.

Regarding termination of solid wire - use of solid wire is rare in control circuit applications. We never allow it.

Cheers,

Dave



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)