figuring gauge of old wire


Old 08-01-05, 08:12 AM
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figuring gauge of old wire

My house was built in 1931. How can you tell the gauge of the original wire? I want to know if I can safely use a 20 amp ciruit breaker. The wire appears as thick as #10 AWG. In my service panel, there are some 15 and some 20 amp breakers. I doubt that the electrician 75 years ago used different sized wires for different circuits. Thanks.
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Old 08-01-05, 08:16 AM
Join Date: Sep 2000
Location: United States
Posts: 18,497
I recommend you go to your home center and buy one foot of each of several sizes of wire. When you compare the sizes, make sure you are only comparing the size of the bare copper, not the size of the insulated wire.

I certainly would not make the assumption that the wire size in the panel is carried through the whole circuit. So figure out what's on the circuit, and open up every box on the circuit and check all the wires in every box.
Old 08-01-05, 08:39 AM
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
Is this an existing circuit?--- if "Yes", what load(s) are connected?

Good Luck & Enjoy the Experience!!!!!!!!!!!
Old 08-01-05, 09:37 AM
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John, so it is as simple as comparing wire size? I've got some #14 and #12 laying around. The copper in the stuff running through my house looks larger than the copper in #12. This circuit goes to a finished attic. It services lights and some electronics (tv, computer, etc.) I sometimes use a space heater up there in the winter. The space heater has a built in thermastat. Sometimes, when it kicks on it trips the 15 amp breaker.

Most of the wiring in my house is original wiring. The exception is the basement, which was wired more recently with grounded outlets, plastic coated wire, etc. Also, the fuse box was replaced with a service panel. The old fuse box currently serves as a giant junction box, connecting the original wire with new wire running to the service panel. Thanks.
Old 08-01-05, 08:02 PM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
You need to compare wire _size_, _construction_, _material_ and _insulation_.

_size_ will tell you the wire gage if the wire is solid. However if the _construction_ is stranded, then the wire will look larger for the same conductor cross section. #12 stranded wire is somewhat larger than #12 solid.

_insulation_, and more specifically, the temperature rating of the insulation, helps to determine a conductor's ampacity; higher temperature insulation systems permit the wire to run hotter, which means that in can safely carry more current. For home circuits with small conductors (10ga, 12ga, 14ga), rules other than ampacity set the breaker size, so the insulation won't change the breaker that you can use, unless other factors such as ambient temperature cause tremendous reduction in ampacity.

_material_ is critically important. Sometimes _Aluminium_ has been used for wires rather than Copper. Aluminium is cheaper, but it doesn't conduct electricity as well. A #10 Al wire can only be used on a 20A breaker for example. Given the age of your house, you probably don't have aluminium wire, but may have tinned copper wire or some variation on that theme.

If everything on a particular circuit is 12ga, then you can _probably_ change to a 20A breaker. But you need to check _everywhere_, and that quickly becomes more effort than pulling a new circuit.


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