Inductance in Panduit and Conduit

[Fred Raud]

Another thing i noticed in the panel building thread,,is some people talked about running wire so tight together to cause problems,,i assume through inductance,,anybody have any 'nuts and bolts' comments on that,,like why it can cause problems and what problems it could cause,,

i can imagine problems with wires that carry signals,,and inductance messing up data being sent,,but when it comes to figuring load,,how many wires pulling how much current in X amount of space is where i start getting lost,,I went through rsdorans tutorials and i didnt see it touch on that

Fred Raud

 

[BobB]

Here is a good example of what inductance can do. I work with diesel generators a lot - it is a major part of my business. A competitor insisted on running power cables for fuel pumps and PLC input wiring in the same multicore orange circular cable. Whenever a fuel pump turned on - these are only little pumps of about 0.55 kW - enough voltage was induced into the PLC input wiring that the PLC input for low oil pressure turned on and shut down the set.

I always insist on separate cables for power and signals - the signal cables are always 0.5 sq mm twisted and screened. It is not usually even necessary to ground the screen would you believe and these cables run on the same tray as the power cables. Have never had a problem. If I ever do, I will ground the screen at one end.

Further, if a twisted pair cable is run separately but alongside power cables, the theory is that the twist in the pair will have voltage induced into it but due to the twist the induced voltage will "cancel" itself out as the cable twists throughout its length.

 

[kamenges]

Fred-

You touched on the thing that most people immediately think about when the discussion of wires in a conduit comes up. That is noise from a power conductor being coupled into a signal conductor.

But the second part of your post brings up the other equally important issue. That is conductor heating. You may not typically think about it but all industrial conductors generate heat, since all conductors have resistance. The tighter you pack these conductors the more heat per unit volume you produce. In addition, thightly packed conductors tend to thermally insulate each other as you decrease the amount of air available for circulation and heat transfer.

If I remember correctly, the NEC recommends a 60% maximum conduit fill percentage if you use the NEC listed ampacity of for wires. If you fill past this point you need to start derating the conductor ampacities to account for heating. The NEC also has some equations listed that allow an individual to calculate more exactly how much heating takes place and how much current a specific conductor can carry. I can't remember where exactly this is listed, though.

As a side note, the old single wire and standoff wiring in old houses really tends to maximize the ampacity of any given conductor. These are single conductors strung through free air. There is no mutual heating like you get in the cables of today and very little thermal insulation value. Granted, the electrical insulation wasn't the best. But you have to give to get, I suppose.

Keith

 

[darrenj]

Fred-

You touched on the thing that most people immediately think about when the discussion of wires in a conduit comes up. That is noise from a power conductor being coupled into a signal conductor.

But the second part of your post brings up the other equally important issue. That is conductor heating. You may not typically think about it but all industrial conductors generate heat, since all conductors have resistance. The tighter you pack these conductors the more heat per unit volume you produce. In addition, thightly packed conductors tend to thermally insulate each other as you decrease the amount of air available for circulation and heat transfer.

If I remember correctly, the NEC recommends a 60% maximum conduit fill percentage if you use the NEC listed ampacity of for wires. If you fill past this point you need to start derating the conductor ampacities to account for heating. The NEC also has some equations listed that allow an individual to calculate more exactly how much heating takes place and how much current a specific conductor can carry. I can't remember where exactly this is listed, though.

As a side note, the old single wire and standoff wiring in old houses really tends to maximize the ampacity of any given conductor. These are single conductors strung through free air. There is no mutual heating like you get in the cables of today and very little thermal insulation value. Granted, the electrical insulation wasn't the best. But you have to give to get, I suppose.

Keith

Not sure about the NEC but the Canadian code says 40% conduit fill..

 

[Tark]

Adjustment factors is not limited to conductors in conduit, it also comes into effect with conductors that are bundled together in lengths longer than 24 inches. So if you have a group of conductors in wire duct and you cable tie these together and make a bundle that is longer than 24 inches you need to apply adjustment factors to these conductors as well. NEC 310.15(B)(2)

 

[rdrast]

Why limit your scope to inductance?

If you are talking about signal cabling, you do indeed have inductance, as well as resistance, but more importantly, you have capacitive coupling, and tranformer coupling. For communications cabling, you have those, as well as cross-talk (which is a function of the above two).

Then, there are always ground-loops, which are just plain evil.

For power conductors, heating is a major concern, as has been mentioned above, and you must absolutely derate the conductors if they are grouped together.

'Best Practices' dictate that you should follow your local national codes, for power conductors in wireway on a panel just as you would for the same conductors running through a conduit. They also dictate that you should seperate out runs of signal/communications wireing from anything switching a load, or at the very least cross load-bearing conductors at a right angle to signal conductors.