210407-1537 EDT

ThatShouldHaveWorked:

You are list as an engineer, and I assume electrical. Thus, I would assume you studied thermal and electrical problems and considerations.

First, uninsulated copper wire in free air will have some nominal current rating based on material, somewhat by diameter, ambient temperature, and desired maximum operating current. This is a steady state test.This same wire will have a much higher fusing current value, and is dependent upon ambient temperature.

When you add insulation to the wire you do at least two major things. Change allowed maximum steady-state temperature, and the insulating effect of the insulation.

To some extent the trip-time characteristics of a breaker or fuse are related to wire tolerance to peak current. Generally there are tables that relate wire size for a particular breaker. Thus. it is probably safe to assume wire sized to a particular breaker will be adequately protected.

If you look at what happens to wire vs current there are some items of importance.

Temperature rise above ambient is about proportional to power dissipated in the wire per unit length. If 1 W produces a rise of 1 C, then 10 W produces about 10 C rise.

For most wire as temperature rises so does resistance, and therefore at constant current the resistance and power dissipation both go up.

Since wire has mass it also has a thermal time constant. So some fuses are rated based on I^2*t, and so you can apply the same concept to wire.

QO breakers also have an instantaneous trip characteristic. I believe this is about 6 times rating, This is not really instantaneous, but probably means about 1/2 to 1 cycle at 60 Hz.

Another factor to consider is the repetition rate of the load. A 50% duty cycle would likely be a big problem.

If your steady-state current is 25 A, and starting current is 110 A, then then the energy ratio for the 15 S is (110/25)^2 = 19.4 . Depending upon the thermal time constant of the wire this can produce lot of excess temperature rise.

Another factor to consider is the type of load and the actual voltage at the load. A motor at the end of a long line can be a big problem, just a resistor and it may not matter.

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