You might think this is a good idea, but it isn't. The code requires a single cable of the required current carrying capacity. What would happen if one of the cables was damaged and the hot wire severed. Then the other would be carrying 40 amps and you wouldn't know it until your house burned to the ground.
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No, the electrical code states that no conductors of similar conductivity in sizes smaller that #1/0 copper shall be permitted in parallel.
#6 wire is good for 65 Amps as long as it is type THHN.
140 AMPS
3/0 gauge
Yes but there are two stipulations, one that the join or splice must be made in a junction box that is accessible and has a removable cover and the second stipulation is that the breaker that is supplying the power can not be larger that the smallest conductor. In this case no breaker larger that a 20 amp breaker as #12 wire is only rated at 20 amps..
Typically 30 amps.
15 amps
12 guage wire israted for 20 amps and 10 guage is rated for thirty amps! 12 gauge is what is required, as it is rated for 20 amps. 10 gauge is rated for 30 and will be more then enough. Oversizing wire in your case isn't necessary. The thicker 10 gauge wire is harder to work with and will more then likely cost more.
The amps that a four gauge wire will handle will depend with the thickness of the wire. If the wire is thin, the four gauge will handle 95 amps.
Yes, 12 AWG is good for 25 Amps if copper wire.If aluminum wire, then good for 20 Amps.
If the wire used on the circuit is sized for 15 amps (usually 14 gauge) you run a very serious risk of fire.If the wire used on the circuit is sized for 25 amps (usually 10 gauge, good for 30 amps) there is no increased danger.Additional InformationIf you don't know if the wire is sized for 25 amps, DON'T DO IT!A 25 amp circuit would require 10 AWG wire. The fire hazard mentioned above is real and you shouldn't ignore that advice.
A #6 copper wire with an insulation factor of 75 and 90 degrees C is rated at 65 amps.
If you have 100 amp wire, you can use it for a 60 amp circuit, or for any circuit of 100 amps or less. But if you have a 60 amp circuit, 60 amp wire is thinner and cheaper than 100 amp wire.
#6 wire is good for 65 Amps as long as it is type THHN.
140 AMPS
You should never try to use more than 15 amps through a 14 gauge wire.
Wire is not sized by voltage. It is sized by current measured in amps. Some common copper wire sizes and their current capacities are: 15 amps -- 14 gauge wire 20 amps -- 12 gauge wire 30 amps -- 10 gauge wire 40 amps -- 8 gauge wire
70 amps.