Inductive heating

AHJ · 03-07-2012, 08:06 PM

I have always understood that the way Inductive heating is limited by two ways, One, 120 volt single phase circuit, induction is reduced by the grounded conductor, by bringing the unbalanced load back.
Two, 120/208 Volt three phase or single phase, by having two phases or more in the same conduit this cancels out the induction?
So with that said, I have a two tub panel, 120/208 volt three phase system. I nippled into a contactor control cabinet for refrigeration that the panel feeds. I used two, 2" nipples, in the top nipple I ran my load conductors (switch legs)
and in the second conduit I ran all of my Line conductors. My load consists of Door heat, anti sweat heaters, fans for dual temp cases and lights.
Question, My inspector hit my on the way that I grouped my conductors, his argument is, Need to run my Hots, and the switch leg for that hot in the same raceway to prevent Inductive heating?
Only place I have seen this in is 300.3 (B) but I don't think this is the code that the inspector is talking about, curious to know where this information is.

Superrick · 03-08-2012, 10:09 AM

300.5 (i)

Yoyizit · 03-08-2012, 03:12 PM

It's further limited by using a twisted pair. The mag field from each wire tends to cancel.

AHJ · 03-17-2012, 11:15 AM

Ok, what is the difference between my installation, or setting a junction box above ceiling for a pull box, conductors in, and out? Besides i used chase nipples, so technically its not conduit. I personally think the inspector is full of it, but I did change it only because he is Authority Having Jurisdiction.

ddawg16 · 03-17-2012, 12:33 PM

mmmmm......I think you guys are using the wrong term.....

At 60 Hz for straight runs....the amount of 'inductive heating' from the wires due to magnetic field produced is so small that I don't think you can measure it....sorry...but your getting a lot of misinformation.

About the only heating in the wire your going to get is from the temp rise due to high currents. This is the reason for fill specs for conduit...

Allow me to give you a little science lesson....

When current is passing through a wire, a magnetic field is generated around the wire. Think left hand rule...wrap your hand around the wire with your thumb pointing in the direction of the current flow....your fingers show the direction of the magnetic field. When that wire is next to another wire, you do get some mutual inductance into the other wire....on long runs you will get some induced voltage into the other wire...but no current....in other words...if you measure it with a high impeadance meter, you might measure 50 Vac..but grab it with your hand and the voltage would drop to almost nothing.

In order to get any heating from inductance you need a magnetic load....like iron....wrap the wire around an iron rod...pass current through it...now you can get some heating....wrap the same wire around something non-magnetic....no heating....if you do get some heating...it's only because of the resistance of the copper....hence, resistive heating. A good example is the inductive heating stove tops....you have to use the special pots on them...

If inductive heating was such as issue then there would be a lot of comercial installations with issues....for example....some of those 1000 HP motors that we run power to where each leg is in it's own conduit....no neut cond in the conduit to cancel the 'inductive heating'.

Regarding the 'twisting' of the wires....that is used to reduce noise coupling. The twist actually puts the wires at a 90 deg angle to each other which helps to eliminate noise....does nothing for 60 Hz....more applicable to high freq....like your Ethernet cable.