Induction in switch loop
- Thread starter BassFace
- Start date
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
I don't think this is allowed anymore.
Bob, I think you are thinking about 404.2(C), the exceptions actually allow you to omit the neutral to the switch in many installations.
Roger
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
I don't think we are talking about running to the switch, but rather from the switch to the switched device.
I agree, I was just pointing out what I think Bob was referring to.
It is perfectly fine to just run a feed and a switch leg in a raceway or cable, they will cancel each others EMF effects.
Roger
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Got it.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Don't worry about it, we've been wiring 2 wire switch loops for over a hundred years without any problems.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
180425-2030 EDT
BassFace:
Often times experiments may be a useful way to prove to someone a concept.
But first about your first post. Induction heating is not of the wire, but of the conduit. However, heating the conduit will transfer some heat to the wire. The wire itself is not where the induction heating is occurring. Rather the AC current thru a single wire in the conduit is inducing a current in the conduit, and it is the induced current in the conduit that is heating the conduit.
On experiments. You can take a 500 or 1000 ft roll of wire and use it to sense some 60 Hz stray magnetic fields. Small diameter wire is desirable from a size and weight point of view, but you can somewhat usefully use #16 or #14. I ran a test with #16. Both ends of the coil must come out of the spool so you can connect to the coil. Besides a mV meter you probably want about 2 ufd of capacitance (paper, Mylar, or Polypropylene) across the coil (across the coil is the same as meter input) to reduce high frequency noise. This way I was around 0.1 mV of background. Other wise without the shunt capacitor of 2 ufd it was about 10 mV.
With a 250 W lamp load on a normal rip cord and the cord close to the sensing coil the meter reading is not much different than background. About 0.3 to 0.4 mV. In this case we have exactly the same current in each wire of the rip cord, but one current is flowing in the opposite direction of the other, and their magnetic fields at only a short distance from the wire pair mostly cancel each other.
Next, if I rip the cord apart to create two separate wires, actually I used two separate wires, put one wire close to the sensing coil, and the other wire about 1 foot away, then I saw about 5 mV of induced voltage. Probably not the maximum I can produce. This demonstrates that when you sufficiently separate the two current paths, to and from, that a substantial induced voltage can be produced.
I have better sensing coils than this, but I wanted to see what I could do with something anyone on this forum might be able to find.
.
BassFace:
Often times experiments may be a useful way to prove to someone a concept.
But first about your first post. Induction heating is not of the wire, but of the conduit. However, heating the conduit will transfer some heat to the wire. The wire itself is not where the induction heating is occurring. Rather the AC current thru a single wire in the conduit is inducing a current in the conduit, and it is the induced current in the conduit that is heating the conduit.
On experiments. You can take a 500 or 1000 ft roll of wire and use it to sense some 60 Hz stray magnetic fields. Small diameter wire is desirable from a size and weight point of view, but you can somewhat usefully use #16 or #14. I ran a test with #16. Both ends of the coil must come out of the spool so you can connect to the coil. Besides a mV meter you probably want about 2 ufd of capacitance (paper, Mylar, or Polypropylene) across the coil (across the coil is the same as meter input) to reduce high frequency noise. This way I was around 0.1 mV of background. Other wise without the shunt capacitor of 2 ufd it was about 10 mV.
With a 250 W lamp load on a normal rip cord and the cord close to the sensing coil the meter reading is not much different than background. About 0.3 to 0.4 mV. In this case we have exactly the same current in each wire of the rip cord, but one current is flowing in the opposite direction of the other, and their magnetic fields at only a short distance from the wire pair mostly cancel each other.
Next, if I rip the cord apart to create two separate wires, actually I used two separate wires, put one wire close to the sensing coil, and the other wire about 1 foot away, then I saw about 5 mV of induced voltage. Probably not the maximum I can produce. This demonstrates that when you sufficiently separate the two current paths, to and from, that a substantial induced voltage can be produced.
I have better sensing coils than this, but I wanted to see what I could do with something anyone on this forum might be able to find.
.
kwired
Electron manager
- Location
- NE Nebraska
If all you have is two conductors in the raceway and they are carrying equal current but in opposite directions - which is what you have with an "end of run switch" as well as at the "load at end of run", the EMF effects of those two conductors cancel one another out.
If you had a neutral run in that raceway but it doesn't connect to any load - it essentially isn't there when it comes to it contributing to EMF's.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
This is also why, when relocating a circuit from one panel to an adjacent one, the neutral for that circuit should also be relocated to the other panel through the same nipple.
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