help on lighting contactor circuit

[ActionDave]

The branch circuit conductors are 12AWG, yes? 20A breaker is fine.

 

[timm333]

ActionDave, yes the condutors are #12 AWG. I am not saying you are wrong, I just want to understand why 20A breaker is ok. Because if we put 20A breaker, then it will let 20A current pass through it and it will blow the photocell because the maximum current carrying capacity of the photocell is 17.5A which is less than 20A.

 

[ActionDave]

The breaker is sized to protect the conductors that are attached to it. That's all. Done. Nothing else to worry about.

I can plug in a power strip into my master bathroom receptacle, connected to a dedicated 20A breaker, plug in six hair dryers and turn them all on High. The breaker will trip(I hope). I can also turn on one 1500W hair dryer and leave it on as long as I like. The breaker will not trip.

 

[iwire]

Are you seeing something I'm not? The table above the one for breakers is for fuses. The 30 amp contactor says max 30 amp fuse.

No, you were seeing something I was not.

I did not see the chart for fuses.

 

[jim dungar]

Interesting that if I use fuses I am not limited.

That specific Square D chart is concerning the Short Circuit Current Rating of the contactor, not its full load protection.

For overloads, up to 30%, there is very little difference in the performance of fuses and circuit breakers.
Look at the Current Rating on page 12 "...are fully rated for tungsten, ballast and resistance loads. This means that a contactor can be used to control a load up to its full nameplate rating. Derating of the contactor is not necessary, as is the standard practice with circuit breakers and fuses."

Without falling further down a rabbit hole: As Jraef said, install the circuit as if there were no contactor involved, then size the contactor to the load, finally check to see if there are limitations on the OCPD size for the contactor.

 

[jim dungar]

ActionDave, yes the condutors are #12 AWG. I am not saying you are wrong, I just want to understand why 20A breaker is ok. Because if we put 20A breaker, then it will let 20A current pass through it and it will blow the photocell because the maximum current carrying capacity of the photocell is 17.5A which is less than 20A.

The 20A will flow only if the load requires it.
Limiting your load, limits your current draw, the branch circuit protection is not a factor unless the manufacturer says so.

 

[kwired]

ActionDave, yes the condutors are #12 AWG. I am not saying you are wrong, I just want to understand why 20A breaker is ok. Because if we put 20A breaker, then it will let 20A current pass through it and it will blow the photocell because the maximum current carrying capacity of the photocell is 17.5A which is less than 20A.

You have similar situation with a 15 amp switch on a 20 amp circuit, or a 600 watt dimmer on a 15 or 20 amp ciruit.

Take the dimmer situation - if it is only rated for 600 watts and you had to give it 600 watt protection - we would be seeing either supplementary protection on such devices or a lot of 5 amp branch circuits.