Go Back  DoItYourself.com Community Forums > Electrical, AC & DC. Electronic Equipment and Computers > Electrical - AC & DC
Reload this Page >

Installing ceiling light fixture in very old wiring that has never worked.

Installing ceiling light fixture in very old wiring that has never worked.

Reply

  #1  
Old 07-26-12, 02:48 PM
R
Member
Thread Starter
Join Date: Jul 2012
Posts: 9
Received 0 Votes on 0 Posts
Installing ceiling light fixture in very old wiring that has never worked.

Been in this house +20 years and bedroom ceiling light never worked from day one. Changed bulbs, no help, but wife did not like it anyway because we had a floor lamp.

Now, thinking about moving, it's time to fix that light. New wall switch, no help. Bought a new ceiling fixture. Turned breaker off, switch off, removed old fixture and cut the two old insulated wires, no color code, no ground. [110 year old house]

Turned breaker ON, but silly me, left the wall switch OFF. Using a blinking squealer, the wire on the left squealed. Nothing on the right wire.

Looking back, I suspect this was my mistake. I should have turned the wall switch on and tested. That should have shown me the hot wire.

Perhaps by mistake, I assumed the left wire was hot, and connected the two black wires from the new fixture to the left ceiling light, white wires to the right ceiling wire, and did nothing with the fixture ground wire.

Still not working. I figure I connected the wires backwards.

Questions:
1-if I hooked them backwards, shouldn't the lights still work? [although unsafely]
2-if the connections were backwards, why wouldn't it pop the CB?
3-does my theory above make sense, that I connected them wrong?
4-perhaps the biggest Q: if there is no ground wire from the metal ceiling box, how do I test to find the hot wire? If I use a VOM, it will show 115 mystery wire to mystery wire. How can I tell which is hot?
5--Even in this old house, do you think the metal ceiling box is grounded through a conduit? Everything in this old house is tube and tie. The only ground wires are those I have added when installing AC, outside flood lights, etc.

many thanks
 
Sponsored Links
  #2  
Old 07-26-12, 03:09 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
Received 1 Vote on 1 Post
1-if I hooked them backwards, shouldn't the lights still work? [although unsafely]
Yes.

2-if the connections were backwards, why wouldn't it pop the CB?
Because you did nothing to create a short to ground.

3-does my theory above make sense, that I connected them wrong?
No. It is as likely that the neutral is switched in a house this age with knob-and-tube wiring.

4-perhaps the biggest Q: if there is no ground wire from the metal ceiling box, how do I test to find the hot wire? If I use a VOM, it will show 115 mystery wire to mystery wire. How can I tell which is hot?
Plug an extension cord into a receptacle that you know is wired correctly. At the ceiling, with power and switch on, insert one probe from your analog multimeter (your VOM) into the neutral (larger) slot of the extension cord end. Touch the other probe to each wire in the ceiling. Mark them with short lengths of black and white electrical tape.

5--Even in this old house, do you think the metal ceiling box is grounded through a conduit? Everything in this old house is tube and tie. The only ground wires are those I have added when installing AC, outside flood lights, etc.
No, not conduit. K&T wiring = no conduit. But the box may still be grounded. Is it suspended from (mounted on) an abandoned gas pipe, by any chance?
 
  #3  
Old 07-26-12, 03:20 PM
R
Member
Thread Starter
Join Date: Jul 2012
Posts: 9
Received 0 Votes on 0 Posts
"Plug an extension cord into a receptacle that you know is wired correctly. At the ceiling, with power and switch on, insert one probe from your analog multimeter (your VOM) into the neutral (larger) slot of the extension cord end. Touch the other probe to each wire in the ceiling. Mark them with short lengths of black and white electrical tape."

Ah, so simple and yet very clever. Obviously, never thought of that. The fixture is at one end of the house, and almost impossible to reach or even see through the attic trapdoor. [I'm in my 70's and don't crawl around attics]

Since the wiring is so old. and the insulation rather thick, I assume I didn't scrap them clean. Making matters worse, it's a 10' ceiling over the bed. I have to lay plywood on the bed, then a step ladder on top, and a friend to keep me steady as I do this. And the fixture was a b**** to install. Not anxious to do this again, but will sure follow your steps.

many thanks

more--just thought of this. If I use a 3 wire ext cord as you describe above, I could also check for continuity from ceiling box to grd wire on ext cord. Right? If the entire house is grounded, it should indicate continuity?
 

Last edited by robertpri007; 07-26-12 at 03:30 PM. Reason: adding more
  #4  
Old 07-26-12, 03:57 PM
ray2047's Avatar
Member
Join Date: Mar 2006
Location: USA
Posts: 33,597
Received 13 Votes on 11 Posts
You can measure voltage from the box to the black wire to check for ground. Of course if you have old style BX with no bonding wire you may show ~120v but still not have a really could ground.
 
  #5  
Old 07-26-12, 04:27 PM
R
Member
Thread Starter
Join Date: Jul 2012
Posts: 9
Received 0 Votes on 0 Posts
"Of course if you have old style BX with no bonding wire..."

What is BX?
 
  #6  
Old 07-26-12, 04:43 PM
ray2047's Avatar
Member
Join Date: Mar 2006
Location: USA
Posts: 33,597
Received 13 Votes on 11 Posts
BX is a type of flexible metallic cable. Though it some cases it might actually be flexible metallic conduit, AKA Greenfield.
Name:  f0159-02.png
Views: 4067
Size:  7.6 KB
Source: BX cable: Definition from Answers.com
 
  #7  
Old 07-26-12, 05:25 PM
R
Member
Thread Starter
Join Date: Jul 2012
Posts: 9
Received 0 Votes on 0 Posts
No, nothing like that. Years ago, I managed to peek in the attic and saw just tube and knot [or whatever it's called, two wires about 12" apart from each other]
 
  #8  
Old 07-26-12, 09:08 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
Received 1 Vote on 1 Post
"Plug an extension cord into a receptacle that you know is wired correctly. At the ceiling, with power and switch on, insert one probe from your analog multimeter (your VOM) into the neutral (larger) slot of the extension cord end. Touch the other probe to each wire in the ceiling. Mark them with short lengths of black and white electrical tape."

Ah, so simple and yet very clever.
Yes, isn't it. And I can say that because I learned that trick from another mod here.

If I use a 3 wire ext cord as you describe above, I could also check for continuity from ceiling box to grd wire on ext cord. Right? If the entire house is grounded, it should indicate continuity?
In theory, yes. But the real question is whether this ceiling box is connected to ground in a way that will take care of any emergency that occurs in it. For that reason, I would trust testing for voltage from the hot wire to the box, as Ray has suggested, over the continuity test. Either way should be valid, though.
 
Reply
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


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