Mixing wire size on same circuit

hvac809

Member
Location
Rhode Island
Why can't different gauge wires be used on a single circuit breaker if the current is dependent on the load? For example, a single pole 20amp breaker feeds outlets with 12 gauge wire and splices off with 14 gauge to run a few led lights. If an amp clamp is used while the lights are on and a power tool is running, the lights read 0.5amps(14 gauge) and the tool is 15amps(12 gauge). At the panel (12 gauge) the meter reads the combined value of 15.5amp. The led lights will never see the higher current (wired parallel) so why can't wire gauges be mixed on a circuit? Thank you
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Why can't different gauge wires be used on a single circuit breaker if the current is dependent on the load? For example, a single pole 20amp breaker feeds outlets with 12 gauge wire and splices off with 14 gauge to run a few led lights. If an amp clamp is used while the lights are on and a power tool is running, the lights read 0.5amps(14 gauge) and the tool is 15amps(12 gauge). At the panel (12 gauge) the meter reads the combined value of 15.5amp. The led lights will never see the higher current (wired parallel) so why can't wire gauges be mixed on a circuit? Thank you

Part of that has to do with the ability of a 14 gauge equipment grounding conductor to carry enough ground fault current to trip the breaker. Years ago #12/2 nm had a 14 or16 gauge ground wire and that was an issue so they obviously did some testing and decided that a 20 amp breaker needs a #12 wire to interrupt the circuit.

I am sure there are better explanations.
 

retirede

Senior Member
Location
Illinois
The short answer is yes, different gauge wiring can be used on a single circuit.

It’s done all the time when a long portion of a run is upsized to minimize voltage drop, then reduced again to accommodate termination limitations.

But as others have noted, the smallest wire must be protected properly by the OCPD.
 

FionaZuppa

Senior Member
Location
AZ
All depends monkey wrench. The AHJ has final say so.

Wire sizing and ampacity mean different things depending on the purview. In NEC it means one thing, chassis wiring std's mean something else. Ever look inside a chassis, fed by #12 only to find #14 or #16 inside. Hmm, the thingy is UL listed and is specified to have it on a 20A ckt. Hmmmmm.

I have downsized from #12 to #14 on a 20A gfi OCPD before (ocpd --> #12 for ~100ft --> jbox --> then some #14). The AHJ approved, approved my specific scenario, which was an outdoor wiring scenario. The #14 was in EMT w/ insulated EGC wire. Duly noted, the specifications were put into the permit application, they reviewed it w/o issue, permit issued, work done & inspected, final approved and closed.

In my case, there's was plenty of wire and metal to carry all fault current to trip the ocpd.

Most times probably be denied, sometimes it may get approved. AHJ has final say-so.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I have downsized from #12 to #14 on a 20A gfi OCPD before (ocpd --> #12 for ~100ft --> jbox --> then some #14). The AHJ approved, approved my specific scenario, which was an outdoor wiring scenario. The #14 was in EMT w/ insulated EGC wire. Duly noted, the specifications were put into the permit application, they reviewed it w/o issue, permit issued, work done & inspected, final approved and closed.
Sounds like your AHJ gets to make up their own code.
 

480sparky

Senior Member
Location
Iowegia
No. They didn't make up code, they provided some wiggle room. Bottom line is, the AHJ accepted it.
But like I mentioned, 12 to 14 on 20A probably denied 99.9% of the time.
You're right....... they didn't make up a Code. They made up an exception.

14 on a 20a OCD is perfectly legal in more cases than you think. Take a look at T240.4(G) for a comprehensive list.
 
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