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Monday, July 25, 2016

Question:

Thank you for starting us off every morning with some NEC learning.

My question today relates to adding a 120V single-gang duplex receptacle in a garage.  The design is to tap off of an existing single-gang duplex receptacle (on a 20-amp circuit) and run the three conductors (hot, neutral and ground) in EMT along the garage wall to the new receptacle (less than 50 ft.).  The new receptacle is for powering an irrigation system controller (120VAC to 24 VDC power supply plugged into the receptacle for this purpose).  The question – what is the conductor size required by NEC and what is the NEC reference?  In an attempt to answer the question I reviewed NEC Article 210 and found 210.18(4) Exception No. 1 states the minimum conductor rating for a tap on a 20-amp circuit shall not be less than 15 amps (or 14 AWG).  Also, Table 210.24 indicates the same, 14 AWG for taps on 20-amp circuits.  However, connecting a 19-amp load to the new receptacle could potentially overheat the wire without tripping the 20-amp breaker if we use 14 AWG.  Hence, the question.

Best regards, Tom Stoll, PE, LC

A

Answer:

Hey Tom thanks for your question and you are welcome. Branch circuit conductors must be sized to have sufficient ampacity for the load as stated in 210.19(A)(1). Conductors must also have overcurrent protection based on their ampacities as stated in 240.4. Because you have multiple receptacles and 14 AWG is limited to 15 amps for a maximum overcurrent protective device the conductor must have a minimum ampacity of 20 as stated in 240.4(B) and 240.4(D)(3).

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