3-way switch cable dilemma

TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
I might or might not have a problem with a project starting Tuesday and want to be prepared. I may have to wire up a lighting circuit using 3-way switches in a Class I, Div II area. Am being supplied with 12-3 Okanite, rubber coated armored MC cable. Because of the classification , I cannot use the armor as the ground. I must use a wire conductor for the ground. That leaves me only two conductors in the cable. Am thinking it is possible to use two cables and not violate the NEC article about have all circuit conductors within the same raceway. I do not think this is possible with a standard 3-way wiring method with the hot going to the common of one switch and the light being fed from the common of the other, with the neutral going to the light of course.

But what if a California 3-way is used? One cable would have the neutral going to the light and the traveler that is landed on the commons of both 3-way switches. The other cable would have the Hot going to one contact of each switch and the Load wire on the other contact of each switch going to the light. The power cable would come to one switch, and the load cable going to the light from the other switch.

Feel free to laugh at the sketch. The way I see it is each cable of two conductors will have current flowing in opposite directions, making it either a switch leg, with two ungrounded conductors, or a complete branch circuit of one hot and one neutral, again flowing in opposite directions.

No need to mention an inspector. This is a gas compressor station. It just isn't done.

1713747232945.png
 
TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
TwoBlocked said:
I might or might not have a problem with a project starting Tuesday and want to be prepared. I may have to wire up a lighting circuit using 3-way switches in a Class I, Div II area. Am being supplied with 12-3 Okanite, rubber coated armored MC cable. Because of the classification , I cannot use the armor as the ground. I must use a wire conductor for the ground. That leaves me only two conductors in the cable. Am thinking it is possible to use two cables and not violate the NEC article about have all circuit conductors within the same raceway. I do not think this is possible with a standard 3-way wiring method with the hot going to the common of one switch and the light being fed from the common of the other, with the neutral going to the light of course.

But what if a California 3-way is used? One cable would have the neutral going to the light and the traveler that is landed on the commons of both 3-way switches. The other cable would have the Hot going to one contact of each switch and the Load wire on the other contact of each switch going to the light. The power cable would come to one switch, and the load cable going to the light from the other switch.

Feel free to laugh at the sketch. The way I see it is each cable of two conductors will have current flowing in opposite directions, making it either a switch leg, with two ungrounded conductors, or a complete branch circuit of one hot and one neutral, again flowing in opposite directions.

No need to mention an inspector. This is a gas compressor station. It just isn't done.

View attachment 2571288
Click to expand...
Nope, I guess not. Taking another look, if the left switch is down and the right switch is up the current in the two cables will be going the same direction. CRAP! Any ideas on how to properly wire a 3-way switch circuit with only 2 conductors per cable?
 
TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
brycenesbitt said:
This article is really good:

Multiway switching - Wikipedia

en.wikipedia.org en.wikipedia.org

What exact problem are you seeing with the NEC on conductor/raceway?
Click to expand...
Thanks:

From NEC 300.20 "Induced Currents in Ferrous Metal Enclosures or Ferrous Metal Raceways. (A) Conductors Grouped Together. Where conductors carrying alternating current are installed in ferrous metal enclosures or ferrous metal raceways, they shall be arranged so as to avoid heating the surrounding ferrous metal by induction. To accomplish this, all phase conductors and, where used, the grounded conductor and all equipment grounding conductors shall be group together."

There are two exceptions that don't apply.

I thought there was a way to run a 3-way switch circuit using armored cables with only two current carrying conductors. But realized that my sketched idea would be a violation. Maybe someone else has a way to do this.

I may be lucky and this structure is a re-purposed one and the 3-way wiring is already in place. Otherwise it looks like we'll just have to get some 12-4 cable. Maybe they'll start consulting me when they are ordering stuff for a project. It also looks like they ordered stuff for one building when there are actually two. I'll have to check to see if time and materials are supposed to be charged separately, too. I think it's a bid job.
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
You could use 2-pole double throw switches instead of normal 3-ways as in the diagram below.
H1, N1 would be in one cable, and H2, N2 in the other. A double throw switch without a center-off would be most suitable.
Switching the neutral this way should be OK according to the 404.2(B) Exception because the grounded and ungrounded conductors are switched ON or OFF simultaneously.

two-pole_3-way_diagram.jpg
 
S

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
TwoBlocked said:
Thanks:

From NEC 300.20 "Induced Currents in Ferrous Metal Enclosures or Ferrous Metal Raceways. (A) Conductors Grouped Together. Where conductors carrying alternating current are installed in ferrous metal enclosures or ferrous metal raceways, they shall be arranged so as to avoid heating the surrounding ferrous metal by induction. To accomplish this, all phase conductors and, where used, the grounded conductor and all equipment grounding conductors shall be group together."

There are two exceptions that don't apply.

I thought there was a way to run a 3-way switch circuit using armored cables with only two current carrying conductors. But realized that my sketched idea would be a violation. Maybe someone else has a way to do this.

I may be lucky and this structure is a re-purposed one and the 3-way wiring is already in place. Otherwise it looks like we'll just have to get some 12-4 cable. Maybe they'll start consulting me when they are ordering stuff for a project. It also looks like they ordered stuff for one building when there are actually two. I'll have to check to see if time and materials are supposed to be charged separately, too. I think it's a bid job.
Click to expand...
unless you are using an older code, you also have to have the neutral down to one location. Hot and neutral down to the first switch. two travelers between switches. Switch leg up from the second switch, and the neutral run from the first location before the first switch in a separate MC.
 
TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
Strathead said:
unless you are using an older code, you also have to have the neutral down to one location. Hot and neutral down to the first switch. two travelers between switches. Switch leg up from the second switch, and the neutral run from the first location before the first switch in a separate MC.
Click to expand...
Sorry, I don't follow you, but it's been a looong day. Can you give the pertinent article numbers and the NEC edition?
 
TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
The potential problem has solved itself. There will be just one on/off switch, and even if there were 3-way switches, I should be able to use my sketch since the circuit will be DC.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
TwoBlocked said:
The potential problem has solved itself. There will be just one on/off switch, and even if there were 3-way switches, I should be able to use my sketch since the circuit will be DC.
Click to expand...
Now you tell us. (j/k)

You could have run two cables between switches.
 
TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
24VDC, LED lights. Haven't seen them yet.

The site will be powered primarily from a 120VAC to 24VDC power supply with a solar/battery backup. We will use a "Hillbilly" ATS: Two contactors in parallel, one NO the other NC with 120VAC coils run through a 120vac timer. After the AC has been on 5 minutes and the power supply does not have a fault, the timer energizes the contactors. This send the voltage from the power supply to the system and shuts off the voltage from the solar/battery backup. If the ac goes out or the power supply faults, the contactors immediately drop out and voltage from the solar/battery backup comes on.
 
C

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
Do you have a link to this Rubber covered MC?

With standard MC cable there is an insulated EGC in addition to the current carrying conductors. A 12/3 cable actually has 4 insulated conductors in the metal jacket/armor. There are some types of MC such as MCAP that has a bare aluminum conductor in contact with the jacket instead of an insulated conductor but I doubt the cable you are working with is designed this way..
 
TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
curt swartz said:
Do you have a link to this Rubber covered MC?

With standard MC cable there is an insulated EGC in addition to the current carrying conductors. A 12/3 cable actually has 4 insulated conductors in the metal jacket/armor. There are some types of MC such as MCAP that has a bare aluminum conductor in contact with the jacket instead of an insulated conductor but I doubt the cable you are working with is designed this way..
Click to expand...

I'll try to get you the info on this particular cable later. It has 3 conductors, all insulated: black, red, and blue. I thought we were getting #12, but looking like we are getting #10.
 
TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
wwhitney said:
That flyer says:

"Although the C-L-X sheath provides a more than adequate ground, per NEC Table 250.122, one or more grounding conductors are provided on low and medium voltage power cables."

Cheers, Wayne
Click to expand...
Correct. That is why you can only use 2 of the three wires as current carrying conductors in Class I Div II areas. The third wire must be used as a ground.
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
TwoBlocked said:
Correct. That is why you can only use 2 of the three wires as current carrying conductors in Class I Div II areas. The third wire must be used as a ground.
Click to expand...
I'm confused, does Class I Div II require redundant EGCs, or an insulated EGC, or something extra like that?

I'm not sure which C-L-X cable you are using, but per Section 4.1 of Okonite's catalog, if you are using "C-L-X Type MC-HL (XHHW-2)" then the 10/3 product, catalog number 546-31-3503, has 3 #10 conductors and 3 #14 EGCs. Which is a segmented EGC that adds up to more than 1 #10.

So I took the quoted section from the flyer you posted to mean that the cables provide one or more grounding conductors in addition to the marketed number of conductors, as is standard for building wire.

Cheers, Wayne
 
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