Tuesday, February 25, 2020

Upgrading a 15A Breaker Safely

Does this require replacing the existing wires?  Or can an electrician look at the existing wires and decide if 20A is safe?  All my pitiful three outlets in the garage are on a 15A breaker.  The chop saw draws 15A; the mill and PC draw something greater than 0, unfortunately.


  1. You should have #12AWG wire to use a 20A breaker.

  2. 15 amp uses 14ga per code. So unless it is wired with 12 or 10ga you have to go larger wire for 20amp.....sorry.

    Do you have a 200 amp service panel? If yes, are there empty slots for breakers? Hopefully there are and you can add new 20 amp circuits and outlets....of course they require new wire runs. Surface conduit attached to Sheetrock in garage will be easier then pulling wire.

  3. don't worry you only operate one at a time. 14awg is rated for 15 amp breakers 12awg is rated for 20amps


  4. I've heard that some builders will wire a whole house with 12-AWG cable just so that they don't need to stock two types, but I think it is generally unlikely that it would be 12AWG and OK to breaker at 20A rather than 14AWG and restricted to 15A.

    I forget when you said the house was built, but around about 2001 it become common for 12AWG Romex cable to have a yellow sheath instead of white. So if you can view either end of the cable sheath either in the circuit panel or behind the outlet, and it is yellow not white, that would be one way to know if you got 20A capable cable .as a bonus.

    Check to see if there is a different circuit for the door-opener outlet on the ceiling.

  5. NEC rates breakers to match the ampacity of the cable.

    15A breakers can feed 14 AWG cable, maximum.
    20A breakers can feed 12 AWG cable, maximum.

    If your circuit has 14 AWG cable and you need more than 15A for your equipment, then you must upgrade the cable AND the breaker.

  6. Yes, an electrician can readily look at the wires to determine whether the circuit can handle 20 amp loads. Quick and easy to figure out.

  7. And good on you for being willing to ask the question! It prevents guys like me from having to show up in a fire engine. We like putting out fires, but don't really want anyone to have to go through the experience.

  8. A 15 amp circuit runs on 14 gauge wire, a 20 amp circuit requires 12 gauge wire. Opening the distribution panel (remove the cover) and inspect the wire leading to the circuit breaker in question; 14 GA is white, 12 GA is yellow, but it will also be imprinted on the non-conductive sheath.
    If the wires to the receptacles in question is 14 gauge the circuit must be controlled by a 15 amp breaker.

    NEC (National Electrical Code) stipulates a circuit should not be operated at greater than 80% of its rated capacity. NEC also spcifies lighting circuits operate at 15 amps; it is possible your garage utility outlets are also on the same circuit as the garage lighting.

    The best solution is add a couple 12 amp utility circuits, using 12 gauge wire, and the cheapest way is to run surface-mounted 3/4" EMT (Electrical Metallic Tubing) around the perimeter of the garage, placing quad receptacles approx every 6 feet (EMT comes in 10 ft lengths). NEC does not specify the maximum number of receptacles on a circuit, but in high-draw situations four quads per circuit seems about right. GFCI protection will be required for all 120 volt circuits in a garage; individual GFCI receptacles are more convenient but more costly, both receptacles in a quad can be protected with one receptacle. Cheapest is a GFCI breaker. Avoid using one GFCI receptacle to protect all "downstream" receptacles; it works fine and meets code, but is a PITA when the GFCI trips.

    TIP: While you're hiring the electrician, have a 240 volt circuit installed, with a receptacle on each of the 3 walls in the garage. You'll find a use for it (air compressor, welder, etc.) someday. And, here's an opportunity to install, or plan for future, more lighting in the garage, and consider a transfer switch if you're ever considering a generator.

  9. In modern electrical codes, a 20A breaker is for outlets, and requires 12 gauge wire. Depending on where the outlet(s) is and what it is used for depends on how many outlets should be on one circuit. A dedicated outlet, like for a refrigerator or washing machine is one. A kitchen counter area might have 2 or 3 for small appliances. A workshop with high amperage power tools may be limited to a quad or duplex outlet per 20A circuit.

    Older 15A circuits most likely have 14 gauge wire, which is smaller and handles less power. Just changing the breaker can cause the wire to heat up in the walls if you draw too much before the breaker trips.

  10. As a rule, the breaker is sized to keep the wire from getting too hot before the breaker pops. So the breaker is sized for the circuit amp capacity, as is the wire size.

    Look at the wire: If it is 14 gauge, then it will safely handle 15 amps at 120V. If it is 12 Gauge, then it will handle 20 at 120. 10 is 30 amps.

    Likely, you will have to replace the wire. Seldom is the breaker undersized for the wire size.

    Often, if you have room in the breaker box, it is just as easy to add a breaker and another circuit. It isn't hard to do.

    If unsure, or this is confusing, consult a professional. **Don't** just replace a breaker with a larger one. This is how fires start.

  11. This probably involves rewiring. The circuit is probably on 14 gauge wire, rated for 15 amps. A simple breaker swap is --not-- recommended. Would it possibly work? yes, but the wire is not rated to carry more than that load safely.

    you need a 12 gauge wire / 20a circuit.

  12. 15A requires 14ga wire. 20A requires 12ga wire. 12ga is bigger than 14ga. The size is written on outside of cable sheath or you can measure the conductor diameter. With a little experience to trai your eye or a sample of both sizes you can tell by eye.

    I doubt those outlets are wired for 20A. If they ran 12ga they would have put in 20A outlets and breakers.

  13. Yes. 12 ga to 10 ga.

    Pull some new circuits: You can do it all yourself, or if you wish do the work and hire an electrician to wire into the box.

  14. I am NOT an electrician though my Dad is and I learned a lot but check your local electrical codes.

    My understanding is: 20Amp circuits require at least 12 gauge wiring, most existing 15A circuits are wired with 14 gauge wire.

    If you run 20A over 14gauge wire it will be warm and is not recommended, i.e. Code violation.

    I'd recommend leaving 15A alone and used for low draw tools. Have an electrician install a 20A circuit and outlet for the high draw saw. Remember you should also not put more than 80% load on an circuit. So that's 16amps on a 20amp circuit and 12amps on a 15amp circuit.

    Just because you can doesn't mean you should. :)

  15. An electrician would check the wires at the breaker box and every junction box/outlet/fixture to make sure they are all 12ga copper or better. If there is anything less he would have to pull new wires. He would also check all the outlets and fixtures to make sure they were marked as 20A rated.
    This applies to everything downstream of the breaker, not just the outlet with the heavy draw.

  16. You need to have an electrician check what type of wire is in the wall. Putting a 20A breaker on wire rated for 15A is a fire risk. My understanding is that you need at least 12 AWG copper to support 20A, and even larger if it's aluminum. Also, if your outlets are rated for 15A, you need to be sure that you won't be pulling more than that - or replace the outlet.
    Safety first - get an electrician.

  17. Do your circuits use 12 gauge wires? I don't know what the rules were in 1958 when my house was built, but currently 14 gauge is considered too light for 29 amps.


  18. If you have access to a micrometer, and you have room to measure the bare copper, this chart says that 12 gauge is about 80 thousands in diameter, and 14 gauge is about 64 thou.
    I checked a piece of recently purchased 12/2 and it was about 80 thou.

    Lots of good intel from the commenters.