300.3(b) does EMF = induced voltage?

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Jps1006

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Northern IL
I found (3) sets of 3-ways on warehouse lighting where the feed originates in the ceiling, comes down one pipe on one end of the building to the 1st switch box to feed the 3-ways hot. Goes up a separate emt conduit with only travelers to the switch-leg side, and then goes back up to the ceiling to switch the lights.

Clearly a violation of 300.3(b). Would this arrangement induce voltage and/or current onto the conduit?
 

Jps1006

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Location
Northern IL
Well I rewired it all dragging a neutral around to properly route it with the its corresponding switch leg. It was one of the least satisfying jobs I've done.

It originally started when I was called because a 3-way switch burned up. I figured it was a bad switch and noticed I had 2 pipes in the switch box, one with only travelers and one with only switch legs, and right away it looked wrong. "wheres the neutral?" I told the customer it was probably unrelated to the switch burning up, but the high-priced, big shop guys that wired it all up were in violation of 300.3(b). I told him I'm not an engineer and cant tell him what its doing to his system, all I can tell him is that it is a code violation and he should pursue the original guys before his 5 year warranty is up. I didn't have a 20-amp brown 3-way with me, so let them replace that too when they come out to honor the warranty.

I was told I guy came out and poked around for 5 hours and said it was fixed. Then when the switch burned up again a year later (same switch, was never replaced, overheated connection) they called me out. I couldn't believe when I opened the box to find everything still the same. Customer said he didn't want to deal with the other guy, just fix it. So scissor lift and 10-hours later I bat clean-up for the shop that got the build-out contract with the opportunity to up sell and overbuild the place (duly taken), and cant get it right, and probably didn't even understand what was wrong about it. Then I get the satisfaction to struggle with boxes buried 25' above the floor in a drop ceiling behind the demising wall (extension ladder through the grid) only to flip the switch on a system that was already working with little satisfaction that I've done anything other than reduce the emf and clear up a technicality.

I must admit, the travelers seemed awfully warm for a 10-amp load, and I suspect that had something to do with the lack of neutral in the pipe. I will check them next time I go back for something else.

Is there other satisfaction I get out of this other than correcting a technicality?
 

GoldDigger

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I must admit, the travelers seemed awfully warm for a 10-amp load, and I suspect that had something to do with the lack of neutral in the pipe. I will check them next time I go back for something else.
I could see the unbalanced current heating the pipe itself inductively, but for just the wires getting warm, I would look for more than 10 amps current flowing. That might also explain the switch burning up. Did you actually meter the circuit or just add up what you were told about the lamp loads?
Next time you are back, throw a clamp-on ammeter on the wires.
 

Jps1006

Member
Location
Northern IL
That was 10-amps measured with a clamp on meter.

If you say you could see the imbalance heating up the pipe, I would imagine a hot pipe could heat the conductors too then. That gets back to the root of my first question. Is it current induced in the pipe that generates the heat? How does the heat get generated?
 

GoldDigger

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That was 10-amps measured with a clamp on meter.

If you say you could see the imbalance heating up the pipe, I would imagine a hot pipe could heat the conductors too then. That gets back to the root of my first question. Is it current induced in the pipe that generates the heat? How does the heat get generated?

That was 10 amps per conductor, measured individually?

If both ends of the pipe are grounded, there will be a current induced in the pipe potentially equal to the sum of the traveller currents. Think of it as an enormous transformer with only one turn. That current will generate heat. But even without a closed loop, if the pipe is a magnetic material, a reversing magnetic field will be induced in the pipe, resulting in both magnetic hysteresis losses which cause heat and circulating currents within the material of the pipe which also cause heat. The thicker the pipe, the worse both of these may be.
 

don_resqcapt19

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Illinois
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retired electrician
There will be very little heating of the conduit at that current level. The Canadian Electrical Code equivalent of 300.20(B) does not even apply unless the current level exceeds 200 amps.
 
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