Can different gauge wiring be used in the same circuit?

Question

I am extending a lighting and outlet circuit in a basement. The current setup has the following characteristics

• 15 amp. circuit breaker (200 amp service)
• 4 existing lighting fixtures using CFL bulbs (~ 20 W each)
• 1 existing double duplex outlet box (no regular appliance in use)
• 14/2 and 14/3 NM cabling (lighting switched, outlets not) throughout
• less than 100 ft of cabling so far

I plan on adding three lighting fixtures (for LED spots, less than 20W each) and one duplex outlet. I will tap off one of the existing boxes to get both a switched and a constant hot line. Additional cabling will be less than 30 ft.

Is there any reason I cannot use 12/2 and 12/3 NM cabling for this project (I happen to have some on hand)?

Is there any practical benefit (e.g., less voltage drop in this small section of line)?
Is there any disadvantage in doing so, other than higher cost of cabling and the slightly harder handling of thicker wires?

If the cabling were concealed, would the presence of a 12 gauge wire in an outlet box or fixture convey something to an electrician or future DIYer that is misleading based on the existence of unseen 14 gauge wiring upstream?

Answer 1

The OP has a great reason for using 12 ga. wire. He already has some on hand, so using it is cheaper than going to get new 14 ga. wire. It's perfectly safe to use it. Don't worry about confusing future workers. If they want to add something to the circuit, they will have to go turn off the circuit breaker first, and then they will see they are working with a 15 amp circuit. Then, if they have any brains / experience at all, they will see that there is 14 gauge wire hooked up to that 15 amp circuit breaker, so they will know not to upgrade it to a 20 amp breaker. Your plan is perfectly fine.

Answer 2

It is perfectly acceptable to use 12AWG, though I would, personally, choose not to.

I wouldn't use 12 gauge wire for the project for a few reasons:

1. 12AWG isn't required for the size of the circuit

2. going from 14AWG to 12AWG confuses the use of the circuit by changing wire size -- future workers will see 12AWG and may make an assumption about the size of the circuit

3. 12AWG is more expensive than 14AWG

4. 12AWG is more difficult to work with than 14AWG

Answer 3

It is perfectly acceptable to use 12 gauge wire on a 15 amp circuit.

If the cabling were concealed, would the presence of a 12 gauge wire in an outlet box or fixture convey something to an electrician or future DIYer that is misleading based on the existence of unseen 14 gauge wiring upstream?

It shouldn't, many older homes were wired with 12 gauge throughout. Apart from that, it is actually explicitly allowed by code.

From the NEC:

Table 210.24 Summary of Branch-Circuit Requirements

Answer 4

"explicitly allowed by code"

I think the term TAP is confusing.

In Part M of Article 410, Special Provisions for Flush and Recessed Fixtures, appears Section 410-67(c), which reads as follows:

(c) Tap Conductors. Tap conductors of a type suitable for the temperature encountered shall be permitted to run from the fixture terminal connection to an outlet box placed at least 1 foot (305 mm) from the fixture. Such tap conductors shall be in suitable raceway or Type AC or MC cable of at least 18 inches (450 mm) but not more than 6 feet (1.83 m) in length.

Note that the 18-inches and 6-foot lengths are for the raceway (or cable sheath), not the conductor. (The tap lengths in Sec. 240-21 are for the conductors, not the raceways or cables.)

The other controversial matter in this Section is the use of the term “tap.” Some believe that the high-temperature wire from the junction box to the fixture must have a lower ampacity than the branch circuit overcurrent protection rating, due to the definition of “tap” in Section 240-3(c), new in the 1999 NEC:

As used in this article, a tap conductor is defined as a conductor, other than a service conductor that has overcurrent protection ahead of its point of supply, that exceeds the value permitted for similar conductors that are protected as described elsewhere in this section.

The “tap” referred to in Sec. 410-67(c) is not the “tap” as defined in Sec. 240-3(c), because that definition starts out with “as used in this article.” Therefore the definition applies only in Article 240, and the tap in Art. 410-67(c) need not have overcurrent protection greater than its ampacity. As a matter of fact, where higher temperature insulation is required, these conductors probably have an ampacity greater than the rating of the branch circuit overcurrent protection.

There, now it's not confusing.

Answer 5

I don't know why everyone is saying this is okay, and trivializing the matter of "confusing future electricians". This is a big deal. You should not mix gauges of wires in the same circuit. Regardless of what code says, I guarantee this will not pass inspection in some jurisdictions and depending on how bad a day the inspector is having. Not recommended.

14-gauge wire is cheap compared to 12-ga. You should save the 12-ga wire for when you need it. Or sell what you have and buy new 14-ga.... profit!