480 and 120 In Same Conduit

Is it ever okay to run 120 vac control wires in the same conduit as 480 vac motor leads? I know it is not good practice but we are about to buy some equipment that seems t have this in several places.

I could not find anything stating that it was against code?

I would think as long as the wires carrying the 120 were rated for 600V, it shouldn't be a problem. I'd make sure the 120V wires were a different colour then the motor leads.

All wires within a conduit must have insulation rated for the maximum voltage within that conduit. Thus the 120 Volt control wires can be run with the 480 Volt wires if everything is rated for 600 V. Be careful of quick connect cables, there are plenty that are only rated 300 V.

In a previous job (at a power facilty) we always got a multiconductor cable (12C) that had the control wires and motor leads in the same cable. Made connecting at the local station easier. This was definately pre-PLC.

The Plc Kid said:

...we are about to buy...

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Then ask for what YOU want.

Its very bad practice to do this. Risk of catastrophic failure rises as you induce such mix. Why then accept it. If only the law is what drives the permissible then we are $crewed.

Want to pressure them?

...ask the supplier how his system is rated for Arc flash.

My only concern would be from plc input signals, especially from things like proximity sensors. You could induce enough current in the signal lines that inputs may turn on on their own.

If this is a reasonably standard machine I wouldn't worry too much. If it is a one-off I would be a bit more concerned.

Keith

mvaughan said:

I was a controls electrician for 15 years and now have moved more into an engineering position. So, with 20-25 years experience, I have never seen a catastrophic failure from mixing 120/480 wiring.

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You have lived a sheltered life. Count your blessings.

As Kamenges said, it isn't the voltage you should be concerned with, it is the current.

If the insulation works as designed, the voltage should be a non-factor.

But the larger the current, the stronger shield you will need on your wires.

I have seen 110v unshielded power cables going to a computer cause encoder pulse count errors on a servo motor when the shield was not properly connected at the end of the encoder cable.

Imagine the magnetic field around a wire carrying 30 Amps to a motor!

This is quite common practice, here in Canada Tech, the cable manufacturer make a composite cable 3 phases for motor, 3 wires for Start Stop. One cable from MCC to Motor.
I would hesitate to use this cable for a PLC based system however.
In parts of the world where they insist of running water pipe to the motor as long as the wires have the higher voltage insulation should be OK also.
Roy

Pierre said:

Its very bad practice to do this.

Want to pressure them?

...ask the supplier how his system is rated for Arc flash.

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Not bad practice at all. This is perfectly acceptable.

BTW - you should demand a arc flash rating with the main panel whenver you purchase a machine. This should not be pressure, it should be standard. You must supply the manufacturer with your system fault current so the machine can be properly designed with respect to arc flash and breaker sizing for AIC.

mvaughan said:

I have never seen a catastrophic failure from mixing 120/480 wiring. We did it as common practice, especially with disconnect interlocks. I would have no problem designing a machine with mixed voltages(120/220/480) in the conduits as long as the wires were all rated and derated correctly.

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Never seen it either. For this to happen, the system must be designed improperly for interrupting rating, both cables must wear through only with respect to each other and not ground and then a short must happen.

TheStarr said:

But the larger the current, the stronger shield you will need on your wires.

I have seen 110v unshielded power cables going to a computer cause encoder pulse count errors on a servo motor when the shield was not properly connected at the end of the encoder cable.

Imagine the magnetic field around a wire carrying 30 Amps to a motor!

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No shielding in multiconductor cable. Shouldn't be used for servos, just hard wired controls.

This isn't poor engineering, it is acceptable and done all the time.

Originally posted bu brucechase:

You must supply the manufacturer with your system fault current so the machine can be properly designed with respect to arc flash and breaker sizing for AIC.

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This may not be the place for this but I tend to disagree with this statement.

Any given enclosure, component, etc. is capable of producing produces a certain amount of arc flash energy. This energy must be determined so appropriate protection can be used. I have personally never heard of designing a machine working backward from a specified arc flash energy.

Likewise an enclosure, component, etc can be safely exposed to a specific peak fault current. It is up to the installer/user to make sure this fault current level is not exceeded at the supply terminals of the device. It is the device manufacturers responsibility to determine this peak fault current value (SCCR) and publish it to the user.

Keith

kamenges said:

This may not be the place for this but I tend to disagree with this statement.

Any given enclosure, component, etc. is capable of producing produces a certain amount of arc flash energy. This energy must be determined so appropriate protection can be used. I have personally never heard of designing a machine working backward from a specified arc flash energy.

Likewise an enclosure, component, etc can be safely exposed to a specific peak fault current. It is up to the installer/user to make sure this fault current level is not exceeded at the supply terminals of the device. It is the device manufacturers responsibility to determine this peak fault current value (SCCR) and publish it to the user.

Keith

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Why isn't this the place?

I have designed many systems backwards from an arc flash energy standpoint. The incident energy is dependant on many things including the available fault current at the main breaker. This can be changed with the selection of proper circuit protection. When I have a cabinet that has any rating above a level "0", then I go back to the main breaker and see if I can find one (or use current limiting fuses) that will reduce the energy. I have been sucessful on all my machines on the floor (over 150) in reducing the level to "0".

Yes, an enclosure, cabinet, system can be safely exposed to a spedific peak current (usually a 3 phase bolted fault) and each component must be designed to handle that. Too many times the machine manufacturer puts in a standard breaker that can handle 22,000 amps. It is up to the user (plant engineer/technician) to specify what the available fault current is at the main terminals and the manufacture needs to design the cabinet to handle that fault current. Since I have 42,000 amps at my bus, all my machines need to be able to handle close to that current. That makes the machine more expensive (getting a 60,000 amp breaker) but that is the price for safety.

I don't see any way for the installer to make sure the machine sees less than the rating of the main breaker. I believe that is the function of the engineer designing the machine and the machine manufacturer.

Originally posted by brucechase:

Why isn't this the place?

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This isn't specifically related to the original post. But when has that ever stopped any of us before!!

Originally posted by brucechase:

The incident energy is dependant on many things including the available fault current at the main breaker.

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On this we agree. But there really is nothing a machinery manufacturer can do to guarantee what is seen at the input terminals of their cabinet. That is determined by the facility it is installed in and the installer.

Originally posted by brucechase:

It is up to the user (plant engineer/technician) to specify what the available fault current is at the main terminals and the manufacture needs to design the cabinet to handle that fault current.

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This is where we conditionally disagree. If this value becomes part of the sales contract it becomes a commercial issue to be dealt with. But fundamentally all a manufacturer needs to do is tell the user what the system can safely withstand. It is up to the user to make sure that value is not exceeded. If that means different supply protection with a lower let-through to the machine or additional components to limit peak fault current so be it. While I agree that is is pretty unfriendly for a manufacturer to supply a machine with a 200 amp service requirement with an SCCR of 10000 amps, the way the regulations are specifified that is OK as long as the manufacturer clearly states what the SCCR is.

Keith

Kid
getting back to the original question 120/480 in same conduit.
Art 90 of NEC nebulously lays out its purpose which is mainly fire prevention and personnel safety. It also says it is not intended to address reliabilty, installation quality, etc etc.

You are not supposed to have low voltage (TV, phone, communication etc etc) mixed in with high voltage.

ASSUMING the 120 VAC in same pipe as 480 is used to drive electromechanical relays I guess you could do it with no major problems caused by EMI or capacitive coupling between conductors.

If it is only 3 to 5 feet between cabinets I do not see a big deal.
If it is 100 feet or more I would not do it from the maintenance standpoint ie if you have a problem in either circuit or you need to change the control wiring now you have to pull both out and repull both.
In my mind it comes down to
penny wise pound foolish
OR
Pay me little now (minimize capital up front cost)
vs pay me MORE later (total life cycle cost including maintenance and repair plus lost production).

Dan Bentler

mvaughan said:

I was a controls electrician for 15 years and now have moved more into an engineering position. So, with 20-25 years experience, I have never seen a catastrophic failure from mixing 120/480 wiring. We did it as common practice, especially with disconnect interlocks. I would have no problem designing a machine with mixed voltages(120/220/480) in the conduits as long as the wires were all rated and derated correctly.

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I agree. I have seen this for years and years and done countless installs using this method, never ever seen a problem. If you install it right, this should not be a problem.