My question is regarding how to properly size(per NEC code) circuit wires after connecting to a terminal block that is fed from a 20A circuit breaker. If I have a 20A breaker that is feeding a terminal block so I can split power out to other smaller circuits what is the proper way to size the wire for the smaller circuits connected to that terminal block. If you look at the attached drawing for example. The cable from the circuit breaker feeding the terminal block is 3 conductor 12AWG in order to comply with NEC code requiring minimum 12 AWG wire on a 20A circuit. This part is clear to me. The part that is not clear is when I connect smaller load circuits after the terminal block as shown in the drawing. I am supplying three seperate circuits to power LED lights that are rated at 1A each. The 120VAC to turn on the LED lights is supplied through the opto modules shown(the opto modules are controlled by Digital IO from a computer). Does the 120VAC wire to the opto modules have to continue at 12AWG since it is coming from the terminal block that is on a 20A circuit breaker or can it be sized smaller to meet the smaller demand of the three LED lights?
I am just trying to understand the proper way to handle this situation. I have seen it done both ways, one way someone used 12AWG and in other cases I have seen as small as 18AWG wire used for 120VAC from the terminal block to the Opto modules. If any one knows the proper way to handle this any input would be appreciated.

Sorry for the long explanation I hope my question is clear.

 

Are you hard-wiring these items in the walls of a structure?

Or you plugging them into an outlet?

Where did you find LEDs that run from AC? Usually, they run at fairly low voltage. If you try to operate a typical low-voltage LED at mains voltage, they'll rapidly get burnt to a crisp.

 

Sorry should have been more clear SgtWookie. These are actually an array of LED's They are traffic lights and they are mounted outside in a housing.

 

I'm not a licensed electrician.

However, I'll suggest that all wiring and terminal blocks or other connections/connectors, etc. need to be rated for 20A, right up to where your output modules are fused for 3A. Anything after those 3A fuses can be rated for 3A current or more.

The idea is that you want the breaker or fuses to be the weakest link; to trip or burn through before any damage to the wiring can be done. If one of the wires happened to come loose from the input of an Output Module and shorted to ground, and it were of small gauge, the wire might burn up before the 20A breaker tripped. This would be an un-good thing.

 

the rules are based on the length of the conductor in question. Based on your drawing, you could reduce it to a minimum #14.