Need help sorting out an Odd voltage issue on 3wire cable


  #1  
Old 10-09-14, 07:51 AM
W
Member
Thread Starter
Join Date: Oct 2014
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
Need help sorting out an Odd voltage issue on 3wire cable

Just bought a new home & notice that the light in the ceiling fan in the master bedroom would not turn on. after taking the fan down we discovered an interesting issue; I'll try to describe as best as I can so bear with me if this gets long winded..

The 3 wire cable (Red, Black & white +gnd) that runs from the ceiling to the switch is connected to the ground & black to a hot black in the box in the ceiling. The red & white are disconnect. However, when we test for voltage all 3 cables had voltage at the switch. The black has 120v, the red & white both have 27v . So we started questioning if we had the right cable, a continuity test (all wires disconnected) proved that we did; still it didn't make sense as to why we would have voltage on the red & white when they were not connected to anything on either end. We assumed there must be a nail or staple in the cable causing the voltage of the Black wire to cross over to the other wires. I went to the attic & pull the cable out & thoroughly inspected it. We could not find any shorts or damage to the cable at all. So out of frustration & curiosity I cut the cable in half & connected the black wire back to the hot wire in the ceiling & like wise for the ground. we then tested for voltage & found that we had 120v on black, & now 24.5v on both red & white. Still confused we connected the shorter half & tested it... 120v on black but only 11v on the Red and White.

At this point I assumed that it was the ground that was carrying the small voltage (27,24,11) but if that was the case how would my meter read voltage on the red & white if they aren't connected to anything on either end. None the less we decided to do some testing. first, since we only read voltage when the black is connected to a hot wire we did some continuity testing between the black & the ground... no continuity there. We did the same for all other wires & had no continuity across any of the wires in the cable.

At this point nothing made since to me so I completely removed the cable jacket & visibly inspected each individual wire. I did not find any defects in any of the wires.

I'm buying some new cable tonight & will wire it up - fully expecting to NOT have voltage on my red & white wires; if I do I may go crazy... Still, I'm curious why this was happening & what I should of been looking for.

Anyone have any idea what was going on?




Oh, and I should also mention that the incoming hot (load) wire is in the box in the ceiling not in the switch. I plan to redo the wiring to make it so that the incoming load is in the switch - which I find to be preferable.
 
  #2  
Old 10-09-14, 08:00 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 65,497
Received 4,059 Upvotes on 3,642 Posts
Welcome to the forums.

If you are using a digital meter it is very common to see lower voltages that are induced into the cable just from the dead cable being run parallel to a live cable or in your case you put 120v on the black wire and you measure a lower voltage on the red and white since they are run next to each other. A analog meter would have shown 0 volts. Digital meters are very sensitive to voltage.

You have a three wire from fan to the switches. You are carrying power down on one wire.... in this case black and returning it on red and white. White should be taped another color so that it's not confused as a neutral.
 
  #3  
Old 10-09-14, 08:17 AM
W
Member
Thread Starter
Join Date: Oct 2014
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
I actually considered that... and when all was said & done I asked my helper..... Did we just pull this cable out & do all this work for something that is normal & expected?
 
  #4  
Old 10-09-14, 08:31 AM
Handyone's Avatar
Member
Join Date: Aug 2000
Location: U.S.
Posts: 4,807
Upvotes: 0
Received 0 Upvotes on 0 Posts
The 3 wire cable (Red, Black & white +gnd) that runs from the ceiling to the switch is connected to the ground & black to a hot black in the box in the ceiling. The red & white are disconnect.
The above statement is confusing.
In order to troubleshoot properly, we need to know how many cables are in each box.

Please confirm:
Switch box has one 3 conductor cable; red - black - white

Ceiling box has 2 cables:
One is incoming power, black and white.
One goes to switch box, black - white - red.
 
  #5  
Old 10-09-14, 08:37 AM
W
Member
Thread Starter
Join Date: Oct 2014
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
Yea, I screwed that up. Switch has (HAD) one 3 wire cable; ceiling has (HAD) 2 cables - one 2 wire (black & white) and one 3 wire cable.

Tonight, I'm moving the incoming power cable to the switch box & running a new 3 wire cable from the switch box to the ceiling. I would assume it would be preferred to have the incoming hot wire at the switch - Correct?
 
  #6  
Old 10-09-14, 09:49 AM
Handyone's Avatar
Member
Join Date: Aug 2000
Location: U.S.
Posts: 4,807
Upvotes: 0
Received 0 Upvotes on 0 Posts
It's the simplest way. Hot and Neutral come in via 2 conductor cable.
Neutral goes directly to fan by connecting whites of 2 and 3 conductor cables.
When it comes to hot, you have two choices:
Have one switch that controls both fan and light.
Have two switches that control fan and light separately.

Which do you want?
 
  #7  
Old 10-09-14, 05:49 PM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: United States
Posts: 9,881
Received 188 Upvotes on 168 Posts
when all was said & done I asked my helper..... Did we just pull this cable out & do all this work for something that is normal & expected?
Yes, you did. You should have come here before tearing anything out.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: