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Fuses, circuit breakers, transistors, ect... are all rated in amps. Why are resistors rated in watts instead of amps? Does their breakdown/failure include more than just heat?

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  • \$\begingroup\$ Pragmatically, the manufacturers do not want to provide individualized Ampere ratings for each resistor in a series. So they use power ratings. This allows them to say that the power rating is 0.25W (or whatever) and then just list all the available values in one table. \$\endgroup\$
    – mkeith
    Sep 7 '15 at 20:06
  • \$\begingroup\$ Transistors, zener diodes, are also rated in watts \$\endgroup\$ Sep 8 '15 at 2:59
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Resistors are rated by power because all of a given series of resistor (same length, diameter, material) will have the same power rating (same maximum temperature and thermal resistance to the surrounding air). So, a given series of resistors will have dozens of different resistance values, but all will have the same power rating.

If resistors were rated by current, then every single resistor part number would have a different rating, and the current ratings would be all over the map. For instance, this resistor kit has 20 different resistance values, all with the same power rating (1/4 watt):

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If resistors were rated by current then each resistance value would have its own current rating. Much more confusing.

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Resistors commonly fail when asked to dissipate too much power - the permissible power is given in Watts. The power dissipated in a resistor is determined by its resistance, and the currrent through it, or the voltage across it.

Resistors do also have a maximum voltage rating, which is independent of the resistor value. All resistors of a given style (size and construction) will have the same power and voltage ratings.

I don't think I've seen a maximum current rating for a resistor.

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  • \$\begingroup\$ My train of thought was on high power transmission lines that use very high voltage and lower current to deliver the same amount of wattage long distances with less joule heating. If a resistors failure mode is due to heat then shouldn't the same principle apply? So would a 10Kohm 1W resistor fail just as soon with 100V and 10mA as a 1ohm 1W resistor would with 1V and 1A even though the later should heat up more? \$\endgroup\$ Sep 7 '15 at 20:10
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    \$\begingroup\$ Actually I think I just reasoned through my own confusion. In my previous comment I used 2 different resistors and I realized that was incorrect that I should have posed the question using a single resistor. But with a fixed resistance value you cant arbitrarily select both voltage and current. \$\endgroup\$ Sep 7 '15 at 20:35
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A power rating on a resistor already specifies the current and voltage ratings on it as well. We know that P = I*V = I^2*R = V^2/R. From a power rating, we can derive the voltage and current limits.

The failure mode of resistors is primarily due to heat. The resistivity of materials is dependent on temperature and the thermal stress due to excessive temperatures and deform or destroy the structure or packaging of the resistor.

Something like a fuse is rated in amps because the purpose is for it to fail open at a specified current.

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