I have an existing 15 Amp circuit running to 3-4 outlet boxes and a single overhead light (120 Watt bulb). I want to wire off one of the boxes to move it to a different location and power a new single outlet and an in-floor radiant heating system.

- I pulled the exiting wire (circa 1975 or 1985) from the old outlet box. It is white Romex with individually sheathed white and black solid copper wires, plus a bare copper ground. The only labeling I can read is "600 Volts."

- I have wired from the old box (which I plan to retire) through a new wall to a new box about 10 feet away and want to hook up a new outlet, as well as the in-floor radiant heat (Thermosoft). The floor heat thermostat is rated at 16 Amps for 212 square feet, but the floor I am installing is only 75 square feet (so my working assumption is that it should only draw 1/3 of 16 Amps or a little over 5 Amps). The thermostat I am using has a built-in GFI.

- I will want the new outlet box primarily for a clothes iron (the one we use now is labeled 1400 Watts at 120V, yielding about 11.4 Amps).

So my questions is: What should I do to wire this properly? Or Specifically:

1. What is the capacity of the old wire in the wall that I am planning to connect to? (the circa 75-85 Romex). I am planning to upgrade the breaker to a 20 Amp. Will the old wiring support this and how can I test the wire to make sure?

2. What is the recommended max on the new 20 Amp breaker. I've been told I should have a breaker that supports 20% more than the load. Is this right? Given the 11 amps for the iron, and the assumption that the radiant floor will draw a max of 5, I am already at 16 Amps or 80%. Add the other outlets (most of which I could retire), plus overhead lights, and I fear that 20 Amps is not enough. I would just set up a dedicated breaker, but that is a big deal since the electrical box is very far from where I need power and will be very difficult to pull wire there.

:confused:

Help! And thanks so much for the advice!

My answers to your questions are in bold.


1. What is the capacity of the old wire in the wall that I am planning to connect to? (the circa 75-85 Romex). I am planning to upgrade the breaker to a 20 Amp. Will the old wiring support this and how can I test the wire to make sure?

To measure the wire you can get the wire strippers that have a notch for each wire size and match up the wire to the correct notch. For a 20 amp circuit, the wiring all needs to be #12, or larger. You most likely have #14 wire, and that needs to remain on a 15 amp breaker.

You should be able to read the wire size on the cable sheathing somplace, as it is stamped every so often.

2. What is the recommended max on the new 20 Amp breaker. I've been told I should have a breaker that supports 20% more than the load. Is this right? Given the 11 amps for the iron, and the assumption that the radiant floor will draw a max of 5, I am already at 16 Amps or 80%. Add the other outlets (most of which I could retire), plus overhead lights, and I fear that 20 Amps is not enough. I would just set up a dedicated breaker, but that is a big deal since the electrical box is very far from where I need power and will be very difficult to pull wire there.

A 20 amp circuit can handle 2400 watts for a non-continuous load, considered as 3 hours or less. If the load runs more than 3 hours, the max load is derated 80%, or 1920 watts.

A 15 amp circuit can handle 1800 watts for less than 3 hours, or 1440 watts for more than 3 hours.

To calculate the load of the heat mat, find out exactly what the watts per square foot truly is, do not guess or estimate.

OK, so I check the wire with my cutter. The old wire (on the 15 Amp breaker) is only #16!

Does this mean my existing wiring is not safe?

I find it hard to believe its 16 gauge. I have never seen that used. If it was built in the 70's or 80's it is more likely 14/2.

As Tom said, look along the white jacket for the wire size. It has to be there somewhere.

Maybe your wire strippers are like mine? My 14 gauge stripper is bigger than it should be: I grounded the tool once while stripping a live line . A new one has been on my Christmas list for years, but I keep getting socks and ties. :-)

While all we can do is trust your observation, RTW is correct, #16 was not used for Romex cable, generally.

However, I seem to recall seeing very old Romex cable that would use 2 #14 wires for the black and white, and a bare #16 for the equipment ground.

Which wire did you measure?

Is you wire stripper defective?

Re-check the cable sheathing for any markings.

If you still have problems sizing this wire, cut off a piece and bring it to an electrical supply distributor or contractor to check the size.

If this is all #16 wire, then... aww hell your in deep crap!