Sunday, March 29, 2015

The Carter 3 way wiring system beware of this dangerous wiring when doing electrical work in old homes with Knob & Tube wiring

Got a call the other day from Maintenance Director at one of the Group Homes I take care of for a Institutional customer.Seems they can not get a 3 way dimmer to work in the hall way at the home so they can dim light at night while kids are sleeping.
They installed 2 both worked then stopped and 2nd one sparked and blew up. 

First thing is I check is it the correct dimmer for the correct light.. Yes it is it is rated for both LED and Fluorescent  fixtures. OK has he  wired it correctly common wire to common wire travelers to travelers. yes he has.

So whats going on ?

Well after looking at age of the home early 1900's  and the old knob and tube wiring


 I suspect a carter wiring 3 way set up.

Unlike a conventional 3 way set up in picture below where all switching is done with just the hot conductor .

The carter system allows the switching of both the hot and neutral conductors 

Because the hot and neutral switch back and forth a 3 way dimmer can not be used  The Carter Circuit was banned in 1923 but I have found them in homes wired all the way up into the 50's and using romex wiring. Because  handymen and totally ignorant so called expert electricians refused to stop using it.

What makes this circuit deadly is the fact you can hook in a new switch improperly on a carter circuit and cause an dead short which is what happened at the group home.

How can you tell if you have a carter circuit in a home it takes a professional electrician familiar with old homes  to be able to determine this  by means of using a meter or induction probe and flipping of the switch and measuring to determine .

Are carter circuits safe to use in a home. Yes they are as long as they are not disturbed and if switches are replaced they are done so properly and due care must be taken to make sure wires are on proper terminal or you will have a short circuit .

 Due care must be taken when changing light bulbs
on a carter circuit as the shell of the light fixture socket can be hot with power even thou switch is off.

If a bulb breaks off  in socket and its an older home only safe way to replace it is to completely turn off power to the circuit.  or entire house if needed.
 and then removing the broken bulb or you risk getting shocked.


  1. Never done any wiring in homes with tube wiring before. I hear it is more difficult than normal wires. I've worked with knob wiring a few times, and think I did an ok job. If I ever work with tube I will be very sure to check and double check my work to be sure I'm doing it correctly.

  2. Nick, this was an informative post about wiring systems. My husband and I live in an older home and want to do some remodeling. W want to get an electrician to come out and help us out. It seems like the smart thing to do with an older home.
    Emily Smith |

  3. Call your professional electricians, don't try yourself. Small mistake can get fire to whole wiring.

  4. Here are some really safety tips. Thanks for explaining the routing of the energy in the 3 way wire system. We are trying to figure out a problem in our home, but we are afraid of something happening. This really gives us a clearer view as to how it might be set up a little more. electrician

  5. Thanks for the following useful blog, I agree with you that safety is first and must while dealing with electrical work.
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