Electrical basics

[Michael Hill]

Originally Posted by Al Skierkiewicz

They are only rated for 32 amps.

Hey, could you shed some light on current/power ratings for me? Something that's always confused me are ratings at high voltages. These say they're rated for 32 Amps @ 400 Volts. Are these the maximum for both current and voltage? That is, you should neither operate above 32 Amps OR 400 Volts? Or is it it a power rating? That is, it's rated for 32 Amps * 400 Volts = 12.8 kW. Or is it a P=I^2*R power? In that case, the current would be the max current no matter the voltage

[Al Skierkiewicz]

Micahel,
The insulation is rated to withstand 400 volts without arcing through the material. If you were to use this for line voltage it would not cause a safety hazard if handled or attached to metal. (line voltage of 120 volts is the RMS rating not the peak.) The current rating is based on temperature rise using continuous current (usually for 24 hours) If you exceed the current, the device will warm and the max voltage rating will likely fall. Once you exceed the melting point of the plastic, it will deform bringing the conducting parts closer to the surface. While CIM motors in an efficient design won't run at 32 amps continuous, under certain conditions, you may exceed that for several seconds to a minute. In that case, high temperature will result. In addition, the temperature will likely also effect the holding tension device and may just release the wire. Your mileage might vary.

[Aren Siekmeier]

Originally Posted by Michael Hill

Hey, could you shed some light on current/power ratings for me? Something that's always confused me are ratings at high voltages. These say they're rated for 32 Amps @ 400 Volts. Are these the maximum for both current and voltage? That is, you should neither operate above 32 Amps OR 400 Volts? Or is it it a power rating? That is, it's rated for 32 Amps * 400 Volts = 12.8 kW. Or is it a P=I^2*R power? In that case, the current would be the max current no matter the voltage

The current rating of a conductor is itself a power rating, basically how much power can it dissipate as heat. The power dissipated as heat is indeed P=I*V, but this V is the voltage drop across the conductor, not across the load the conductor leads to. The voltage drop across the conductor is given by V=I*R for a conductor resistance R, the power rating is P=I*V=I^2*R, and depends only on the current and the physical characteristics of the conductor (size, resistivity, length...).

The voltage rating of the device is the max voltage before the insulation between conductors or between a conductor and the outside breaks down and allows a current. This is entirely a function of the various dielectric and insulating materials being used between conductors and the electric fields they can withstand before ionizing and carrying a current. The current carried by the insulating conductors has no effect.

So these two ratings are unrelated. You should not exceed either of them, even if you are way under the other one.