When it comes to wire sizes and the supposed amperage, they can tolerate there is a general rule of thumb that is applied across the board for various wire sizes. However, it’s not always as cut and dry as it seems.
So, how many amps can a 12 gauge wire handle? As far as 12 gauge wire is concerned, the consensus is that it’s good for up to 20 amps. However, there are other variables that change that consensus as well.
If you’re an Electrician Apprentice or are looking into joining the trade, knowledge about what gauge wires can handle is basic knowledge.
The rest is the kind of information you pick up on the job over time and experience.
If you’re a beginner, 20 amps are the rule of thumb for 12 gauge wires and you shouldn’t deviate from that unless under the scrutiny of a journeyman or master tradesman.
What Are The Variables With 12 Gauge Wires?
The common understanding that 12 gauge wire thickness equals twenty amps isn’t wrong, but it’s also only part of the story. For instance,
- Wires aren’t always made of the same metal
- What kind of load are you dealing with
- High ambient conditions
- What’s the allowable voltage drop across distance
- Conductor and termination points
- Insulation/temperature rating
The temperature ratings of the insulation material also affect ampacity (the number of amps the wire and the insulation can handle). Ampacity deteriorates the lower the temperature rating of the insulating material.
Also, over time, heat has a destructive effect on the insulation itself. As the insulation deteriorates, so too does the overall ampacity the wire is capable of conducting.
There are three, separate temperature categories for several different insulation types.
|According to NEC Table 310 – 316|
|TW, UF||FEPW, RH, RHW, THHW, THW, THWN, XHHW, USE, ZW||TBS, SA, SIS, FEP, FEPB, MI, RHH, RHW-2, THHN, THHW, THW-2, THWN-2, USE-2, XHH, XHHW, XHHW-2, ZW-2|
Excessive heat—produced by the electricity conducted through the wire, ambient temperatures, nearby machinery—that exceeds the specs for a given insulation/jacket, damages the insulation and reduces ampacity in the long term.
For example, a 12 gauge wire that has an ampacity of 20, is insulated by something that has a lower ampacity/temperature, then the amps cannot exceed the specs of the insulation, regardless of how much the wire is rated.
Aluminum has a lower ampacity than copper, so a 12 gauge copper wire is going to have a higher ampacity than 12 gauge aluminum wire.
A 12 gauge copper wire has an ampacity of 20 at a temperature rating of 60ºC while a 12 gauge aluminum wire has an ampacity of 15 at a temperature rating of 60ºC.
Aluminum doesn’t conduct electricity as well as copper, gold, and silver because as electricity flows through it, it forms an oxide surface that is resistant to electricity.
It also generates more heat, requiring high-voltage applications to be wrapped in layers of steel.
Circuit Breakers And Fuses
Circuit breakers and fuses are designed to interrupt the electrical flow when overload is detected by either melting and breaking the circuit (in the case of fuses), or breaking the circuit by a switch flip, in the case of circuit breakers.
So, if there is a power spike that exceeds the ampacity of a 12 gauge, copper wire, the fuse and/or circuit breaker are designed to protect important electrical equipment by breaking the circuit.
A 12 gauge, copper wire is able to handle—and is rated at 25 amps—however, the NEC’s allowable amperage is 20. A power spike that exceeded 20 amps on a 12 gauge wire, likely wouldn’t trip a circuit breaker, so long as it wasn’t over 25 amps.
If you’re running a 12 gauge wire, you’ll have to run it to a circuit breaker rated at 20 amps.
If you run it to a breaker rated at 15 amps, you would have to upgrade the circuit breaker to match the 20 amp, 12 gauge wiring. Fortunately, you wouldn’t also have to replace the wire.
If you have a 12 gauge wire running to a 30 amp circuit breaker, however, you would have to replace the wire with a thicker gauge rated for 30 amps.
When 12 gauge wire is applied to a workshop for instance, careful attention has to be applied to what tools are going to be operating on a single circuit.
Will the whole shop be on one circuit? Will the lights be tied into the same circuit as the wall outlets?
Those are important questions to ask when determining whether or not a 12 gauge conductor is appropriate.
Four, 60 watt light bulbs run on about 2 amps, whereas a circle saw is usually 15. That doesn’t mean you’re maxing out at fifteen but it will likely be close.
Fire up two circle saws at the same time, and you’ll trip the circuit breaker because you will exceed 20 amps.
Wire Length Effect On Ampacity
Conducting electricity increases heat along the length of the wire. Heat is essentially bleeding voltage and the longer the wire, the more you will lose.
Every 100’ of 12 gauge, copper wire loses 3.3% voltage. So if you’re running 12 gauge wire at 20 amps 240V, it will drop to 232.08V at the end of the conductor, a loss of 7.92V.
With an aluminum conductor, the percentage of voltage-drop at 100’ rises to 5.42%. A 12 gauge wire at 20 amps and 240V will lose 13V at the end of 100’ and your voltage will drop to 227.
At 120V, copper wire has a voltage drop of 6.6%, double the rate of loss than 240V.
There is significant drop off at anything over 100’ and it’s not generally recommended to run it that far on its own.
Can Amps Exceed 20 On 12 Gauge Wire?
That depends on the temperature rating of the conductor. Both Aluminum and copper wires have amp allowances that fall under temperature ratings.
12 Gauge Copper Wire Conductors
- 20 amps at a temperature rating of 60ºC
- 25 amps at a temperature rating of 75ºC
- 30 amps at a temperature rating of 90ºC
The higher the temperature rating, the more allowable amps there are for a particular wire thickness. 12 gauge copper wiring is typically used in kitchens (appliances such as coffee pots, toasters, microwaves, blenders, etc).
12 Gauge Aluminum Wire Conductors
- 20 amps at a temperature rating of 60ºC
- 20 amps at a temperature rating of 75ºC
- 25 amps at a temperature rating of 90ºC
The numbers are slightly lower than copper wire because aluminum isn’t as effective at conducting electricity as copper is.
Aluminum conductors can be used in nearly all of the same applications as copper wire, however, they generally have to be one gauge thicker.
12 gauge wires can handle up to 25 amps but are rated at only 20. It’s good to have a healthy knowledge of wire gauges and the amps they can conduct, especially if you’re thinking about running wire to a she-shed or a workshop.
That includes circuit breakers as well. Know what you’re tying into along with the amps required for what you’re going to use.
Otherwise, you may end up constantly tripping the circuit breaker at best, starting an electrical fire at worst.
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