Dan Akers wrote:
You wrote:
"An inspection on our home (7years old) recently revealed that both our
air conditioning units have 40 amp circuit breakers instead of 30 amp
(the unit asks for 30 amps). How big of a deal is this? And if so, is it
an expensive thing to change?"
______________________________________
Re;
Assuming you're talking about the compressor power supply breaker, you
should replace them and it's not expensive if you do it yourself. The
breaker size selection is indeed selected to protect the compressor
motor. That's why a maximum breaker ampacity is given on the unit name
plate. Even though the compressor has an internal motor protective (IMP)
winding temperature limiting device. These IMP devices are last resort
current/temperature limiters and tend to operate (open the circuit) at
temperatures well above (100F+) normal operating temperature of the
windings AND they do reclose only to reopen again if the compressor does
not start. They will cycle like this indefinitely if the wrong supply
breaker is installed, intermittently "cooking" the windings and greatly
reducing the life of the insulation on the windings, until either power
is removed or the reason for the stall, such as high head pressure,
resolves itself.

Back to the breaker. You should select the breaker amperage rating to
be less than the nameplate maximum over-current protection and about
twice the compressor running current. My two 2-ton compressors have a
running current (RLA) of 11 amps and have 20A breakers for protection.
Locked rotor current (LRA) is stated to be 61A. The nameplate calls for
max over-current protection to be NO MORE THAN 30 A. You want the
breaker to trip as soon as possible to prevent "cooking" the compressor
motor windings (even though there is an IMP device) to prevent
degradation of the winding insulation that occurs in a long term
fail-to-start situation as in the case of a blown starting capacitor.

If the breaker is sized right, it will activate (open) before the IMP
does on a stall or start failure.

I hope that helps...
Dan Akers



The motors internal protection is not backup protection, but its primary
protection as far as the motor engineers are concerned. Motors are
damaged by heat, thats why they have thermal breakers. External
breakers are not sensitive to the thermal conditions at any point in
time, within a motor. The motor itself is adequately protected by its
internal breaker is does not get "cooked." The external breaker is
necessary to protect the external wiring normally.

The assembly internal wiring should be able to handle motor stall
current. But if they are trying to cut costs, they may downsize the
wiring then put the onous on the installer to protect the internal
wiring by using a smaller external breaker.

So if the faceplate specifically calls for a breaker size he better
stick with it. But if it doesen't, he is fine.


--
Thank you,



"Then said I, Wisdom [is] better than strength: nevertheless the poor
man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16