# three way switch wiring

#### 480sparky

##### Senior Member
And just WHY does 300.3(B)(3) NOT allow 14/2 NM to be wired as my little post-it diagram illustrates?
The 14/2 between the switches does not have the circuit neutral in it.

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#### Electromatic

##### Member
Thanks for the picture, Al!

Interesting that it doesn't really save on copper vs. the standard 3-conductor 3-way installation. But you do get an unswitched hot at the end of the run.

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
Thanks for the picture, Al!

Interesting that it doesn't really save on copper vs. the standard 3-conductor 3-way installation. But you do get an unswitched hot at the end of the run.
Two 14-2's allow the same.

#### al hildenbrand

##### Senior Member
Interesting that it doesn't really save on copper vs. the standard 3-conductor 3-way installation. But you do get an unswitched hot at the end of the run.
There is a Late-Friday-Finishing-Roughin logic that applies. When the job site 14/3 runs out, you can still finish the roughin without having to run to the supply house.

--Edit to add: It is worth repeating, this post-it note diagram runs afoul of 310.10(H) because the wire is smaller than 1/0. When both three-way switches have there toggles "down," as I drew it, the Common-To-Common conductor is wired in parallel with the Hot conductor.

#### 480sparky

##### Senior Member
Holding up 300.3(B) as if it means the neutral and hot MUST ALWAYS be in the same cable is an old trope that comes from reading the rule WITHOUT the last nine words.

When you read the complete rule of 300.3(B) you realize that 300.3(B)(3) allows conductors of the same circuit to be run in separate NM cables whether they are remote or close to each other:

Suppose you're using metal boxes?

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#### romex jockey

##### Senior Member
There is a Late-Friday-Finishing-Roughin logic that applies. When the job site 14/3 runs out, you can still finish the roughin without having to run to the supply house.
.
I'm into 14-4, wish i could have used it decades ago....

~RJ~

#### ActionDave

##### Moderator
Staff member
I'm into 14-4, wish i could have used it decades ago....

~RJ~
you could have. it's been around a looooonnnnggg time.

#### kwired

##### Electron manager
And just WHY does 300.3(B)(3) NOT allow 14/2 NM to be wired as my little post-it diagram illustrates?
I guess as long as you don't enter any metal enclosures there is no problems. My thoughts are to reduce EMF's by keeping all circuit conductors in close proximity, and I do run multiple cables at times but always route them together. Say to a bath fan/light/heater - I often run two 12-2 from switch to unit, or if you have to run three way travelers and hot/neutral between same two boxes I often use two 2 wire cables here also.

Not running them together gives you similar EMF as old K&T wiring might give you. They also ran that to metal boxes but don't recall seeing slot between entries. Probably never caused enough heating to ever be noticed either.

#### al hildenbrand

##### Senior Member
I guess as long as you don't enter any metal enclosures there is no problems. My thoughts are to reduce EMF's
300.3(B)(3) points squarely at 300.20(B) which means metal boxes are not excluded, and are not a problem per NEC.

The NEC is silent about EMF, and rightly so. If the NEC started to deal with EMF it would be addressing the EMF in almost every electrical service and grounding electrode system installed.

#### romex jockey

##### Senior Member
you could have. it's been around a looooonnnnggg time.
i never get the memo Dave....:happysad:~RJ~

#### romex jockey

##### Senior Member
. If the NEC started to deal with EMF it would be addressing the EMF in almost every electrical service and grounding electrode system installed.
at the very least, that would be great entertainment....~RJ~

#### kwired

##### Electron manager
300.3(B)(3) points squarely at 300.20(B) which means metal boxes are not excluded, and are not a problem per NEC.

The NEC is silent about EMF, and rightly so. If the NEC started to deal with EMF it would be addressing the EMF in almost every electrical service and grounding electrode system installed.
Why are metal boxes not a problem per NEC, looks to me like 300.20 requires cutting a slot between holes to me, which effectively makes it one hole just irregular shaped.

And one more reason to route all circuit conductors together is to reduce overall impedance, especially during fault conditions, which will help facilitate faster operation of overcurrent protection.

#### al hildenbrand

##### Senior Member
Why are metal boxes not a problem per NEC, looks to me like 300.20 requires cutting a slot between holes to me, which effectively makes it one hole just irregular shaped.

Seriously? According to you, the NEC says there's a problem when not all the conductors of a circuit are installed in the entries to ferrous metal boxes in a manner that satisfies the rule in 300.20(B) ? ? ? And, in this thread, that means the wiring of a three way with 14/2 NM and steel boxes is, as you are saying, "a problem per NEC."

And one more reason to route all circuit conductors together is to reduce overall impedance, especially during fault conditions, which will help facilitate faster operation of overcurrent protection.
In reading 300.3(B)(3) and 300.20(B), your post #34 reasons don't matter because they aren't part of those rules.

#### kwired

##### Electron manager
Seriously? According to you, the NEC says there's a problem when not all the conductors of a circuit are installed in the entries to ferrous metal boxes in a manner that satisfies the rule in 300.20(B) ? ? ? And, in this thread, that means the wiring of a three way with 14/2 NM and steel boxes is, as you are saying, "a problem per NEC."[/FONT][/LEFT]

How many times you seen K&T wiring enter a switch box all in same hole? That, or any other case with multiple conductors in NM wiring methods not entering the same hole is a violation. Clamps on such boxes typically are only rated to accept one cable, A separate fitting (a NM connector in a 1/2 KO) might be rated for multiple cables and would be fine.

In reading 300.3(B)(3) and 300.20(B), your post #34 reasons don't matter because they aren't part of those rules.

Isn't lowering the overall circuit impedance a pretty good reason and at least a part of why the rules are what they are even though it doesn't state why.

One thing I run into sort of on a regular basis on irrigation installations is you have a 1300-1500 foot run that was originally installed as corner grounded delta (three conductors, direct buried). POCO has (like at least 20-25 years ago) eliminated all corner ground services and when they did so they buried a fourth grounded conductor, typically about 10 feet away from the original set of conductors. Tell me that don't cause slower operation of overcurrent devices during ground fault conditions when compared to having all four conductors all in very close proximity to one another. Response time is already slow because of circuit length in these applications, adding more impedance only makes it worse.

#### al hildenbrand

##### Senior Member
I see how that works, but don't see it complying with 300.3(B) unless you route the two separate cables in close proximity to one another, and even then there is going to be some debate as to whether that complies.
You can say it. . . . c'mon, now. . . . As drawn, it complies with 300.3(B), 300.3(B)(3) and 300.20(B).

#### synchro

##### Senior Member
Isn't lowering the overall circuit impedance a pretty good reason and at least a part of why the rules are what they are even though it doesn't state why.

One thing I run into sort of on a regular basis on irrigation installations is you have a 1300-1500 foot run that was originally installed as corner grounded delta (three conductors, direct buried). POCO has (like at least 20-25 years ago) eliminated all corner ground services and when they did so they buried a fourth grounded conductor, typically about 10 feet away from the original set of conductors. Tell me that don't cause slower operation of overcurrent devices during ground fault conditions when compared to having all four conductors all in very close proximity to one another. Response time is already slow because of circuit length in these applications, adding more impedance only makes it worse.
Using the online tool below for calculating the inductance of a pair of wires with a 10 foot separation and a 1500 ft run, the inductive reactance at 60Hz is approximately equal to the combined resistance of the two wires when the conductors are #2 AWG. So with #2's the inductance would increase the total impedance by a factor of sqrt(2) ~ 1.414 times the resistance value. For smaller gauge wires the resistance would dominate, and with larger gauge wires the inductive reactance would dominate. Now the inductance decreases very slowly as the wire diameter is increased. Therefore there will be diminishing returns by increasing the wire size above #2 gauge if there's a 10 foot separation between the conductors, because the inductance of the large area loop is establishing the minimum impedance.

The inductance model at the site below ignores the inductance contribution of the conductors that connect the ends of the two wires to close the loop. But that will be negligible as long as the wire length is much larger than the wire separation.

https://www.emisoftware.com/calculator/wire-pair/

#### kwired

##### Electron manager
Using the online tool below for calculating the inductance of a pair of wires with a 10 foot separation and a 1500 ft run, the inductive reactance at 60Hz is approximately equal to the combined resistance of the two wires when the conductors are #2 AWG. So with #2's the inductance would increase the total impedance by a factor of sqrt(2) ~ 1.414 times the resistance value. For smaller gauge wires the resistance would dominate, and with larger gauge wires the inductive reactance would dominate. Now the inductance decreases very slowly as the wire diameter is increased. Therefore there will be diminishing returns by increasing the wire size above #2 gauge if there's a 10 foot separation between the conductors, because the inductance of the large area loop is establishing the minimum impedance.

The inductance model at the site below ignores the inductance contribution of the conductors that connect the ends of the two wires to close the loop. But that will be negligible as long as the wire length is much larger than the wire separation.

https://www.emisoftware.com/calculator/wire-pair/
In my described situation above most cases it was 4/0 aluminium ungrounded conductors and the grounded conductor might be as little as #2 aluminim sometimes larger but probably seldom larger than 2/0 aluminum.

#### synchro

##### Senior Member
In my described situation above most cases it was 4/0 aluminium ungrounded conductors and the grounded conductor might be as little as #2 aluminim sometimes larger but probably seldom larger than 2/0 aluminum.
In that case you're right that the 10 foot separation is creating enough inductive reactance to dominate the total impedance under a ground fault condition. Ground fault detection would solve the problem but it's not likely to be something the POCO would consider putting in.