You could do the math and figure out the correct size of wire for the wire distance or using their rule of thumb and half the current. Too much current for the insulation, which would go through irreversible deterioration and possible fire, death and destruction. 30A with a load 60 ft from panel violates the 3% maximum voltage drop. The table below shows various data including both the resistance of the various wire gauges and the allowable current (ampacity) based on a copper conductor. At 30A, this equates to a feeder resistance of \$0.12\Omega\$ or feeder length of 120ft or 60ft from panel. If you figure a maximum of 3% voltage drop to a feeder, this means a specific maximum distance for #10.Īt 120V, 3% means 3.6V. Contact your local electrician to find out what is legal! NOTE: For installations that need to conform to the National Electrical Code, you must use their guidelines. The Maximum Amps for Chassis Wiring is also a conservative rating, but is meant for wiring in air, and not in a bundle. The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very very conservative. In reality, this maximum must also be derated for safety (normal load should be 80% of rated ampacity), ambient temperature, armoured sheaths, raceways or adjacent power conductors.Īs you might guess, the rated ampacities are just a rule of thumb. The amperage rating of a cord is a function of the wire gauge, number of current carrying conductors and length of the cord. The type of insulation used determines the maximum ampacity of a wire. While the required gauge to usefully send a certain amount of current with a certain fraction of loss will depend upon the voltage and transmission distance, there are many situations where it will exceed the gauge that would required to avoid overheating. If one were trying to send 2400W of power over a cable, one wouldn't want to lose hundreds of watts in the cable even if it could dissipate that much heat safely. The 15A rating takes into account another factor: any power that gets converted into heat won't be usefully delivered to its destination. The 55A and 30A figures assume the limiting factor is heat dissipation. The difference between the 55A rating in one table and the 10A rating in the other is likely because of this. Passing a certain amount of current through a piece of bare copper wire which is surrounded on all sides by air will not cause its temperature to rise nearly as much as as passing that same current through a like-gauge wire which is buried under 4 inches of fiberglass insulation. The temperature a wire will reach when carrying a certain amount of current depends upon the gauge, the ambient air temperature, and the amount of thermal insulation between the wire and the air.
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