Splitting up wiring to smaller guages

[hrc]

Feeding into a control box with a 208Y feed, using 6/5 SO cable, full load on the system is 43.846A/phase

[derived from having a power supply with a draw of 14.42A, so using two legs and three supplies split across, each phase will see a draw of 28.8A, and then a 15A single phase 120V load for each phase]

So, the main feed from the load side of the control box breaker will come to a terminal block using #10 wire (free air), and the other side feeds two power supplies AND the 115V outlet. So the PS see a draw of 28.8A and that requires #14. Tapping off this phase and the neutral, the 15A feed only needs #18. So is it 'legal' to take two different wire sizes off the terminal block, or the studs of the circuit breaker?

 

[dpc]

Not in general. The wire must be protected by the upstream overcurrent protective device. #18 would not be protected by a 15A breaker.

There are "tap rule" exceptions in the NEC that might allow this under certain situation, but I don't think #18 could ever be used.

 

[hrc]

Per my charts, in free air, copper wiring #18 is good to 16A

 

[debodine]

I just happened to have a copy lying around of the now cancelled MIL-C-5809/1 spec which used to qualify circuit breakers.

The first important condition to be noted is that these standard circuit breakers have an ultimate trip limit of 138%, which means that for some short period of time a 15A circuit breaker could allow up to 20.7A (15A X 1.38 = 20.7A) to flow through your wire.

The second condition (and most important to your application) is that these standard breakers have a lower trip limit of 115%, which means that the circuit breaker could allow 17.25A (15A X 1.15 = 17.25A) INDEFINITELY.

The circuit breakers you are contemplating may be qualified to a different spec, but the same principle applies when sizing a circuit breaker to protect wire. You have to know as a minimum (there are other factors) what trip limits your circuit breaker will have so that you can characterize accurately what your wire will experience.

Best regards,

Debodine

 

[hrc]

Thanks for that insight, should have been obvious to me on that one.

 

[dpc]

If this is an installation covered by the National Electrical Code, the smallest conductor allowed for 15A branch circuits is #14. Also, whatever termination devices or lugs you are using would also have to be UL listed for #18 wire.

Molded case circuit breakers rely on the connected conductors as a heat sink to keep the breaker from overheating and tripping below its rating. UL testing for 15A (and smaller) breakers is done with #14 AWG. So, that is smallest size conductor that can be (legally) used for NEC compliance.

 

[hrc]

I was planning on using #14, thanks for the confirmation. This is a power chassis, and will be run the NRTL certifcation so yes, it will be a NEC covered product.