In many workshops that I have instructed recently, installers have asked if the National Electrical Code (NEC) permits an appliance to be added to an existing 20-ampere (A) branch circuit without having to route an individual branch circuit. It seems that this question is asked due to the frequent impossibility of running a separate circuit because flooring, ceiling and wall materials, where finished, do not allow easy access for installing additional cable wiring and other needed accessories.

Permissive requirements
Under certain installation conditions, the answer, however, is yes, and the provisions permitting such an installation are outlined in Section 210.23(A)(1) and (2) of the NEC. Section 210.23(A) permits branch circuits rated at 15 or 20A to supply lighting fixtures and or appliances. If the overcurrent protection device (OCPD) is limited to 80 percent (1/80% = 1.25) of its rating and the added appliance fastened-in-place is the cord-and-plug type and does not exceed 80 percent of the OCPD, the installation complies with the NEC.

Sections 210.23(A)(1) and (2) additionally require appliances fastened-in-place and connected to a branch circuit supplying lighting fixtures, receptacles, and or cord-and-plug-connected appliances to be selected and sized not to exceed 50 percent or half of the OCPD rating.

Circuit classification
A branch circuit with two or more outlets is classified according to the maximum size OCPD setting permitted for protection of the circuit. The maximum size fuse or circuit breaker setting selected depends, in return, on the kind of equipment connected to the circuit and the size conductor. The NEC permits OCPDs rated at 15, 20, 30, 40 and 50A to be used to serve lights, receptacles and appliance loads. Note that it is the size of the OCPD and not the conductors that determines the rating of the branch circuit.

Sizing conductors
Branch-circuit conductors must be capable of carrying the load connected to the circuit. They also must be sized large enough to reduce the circuit’s voltage drop when it becomes necessary. The conductors also must have the capacity to handle adjustment factors and correction factors if one or both of these conditions should occur. Any or all of these conditions can occur, and it is highly advisable to recognize such problems and size the conductors accordingly. For voltage-drop problems, review sections 210.19(A)(1), Informational Note 4, 210.23, 240.23 and 250.122(B). For issues pertaining to adjustment factors and correction factors, see sections 310.15(A)(3), 310.15(B)(2) and 310.15(B)(3)(a).

Protection techniques
The NEC generally requires conductors to be protected by a properly sized circuit breaker or fuse with a rating not greater than the current-carrying capacity of the conductors of the circuit. In other words, the overcurrent protection for the circuit and equipment served must provide all necessary overcurrent protection, or a second stage of protection must be installed. For example, if the OCPD for a circuit provides only branch circuit, short circuit and ground-fault protection and does not protect from overload conditions, such as in a motor circuit, overload protection is required as the second stage of protection.

Additional appliance?
Where the load in normal operation can operate for long periods, such as three hours or more, the total connected load must not exceed 80 percent of the OCPD or 125 percent of the load connected to the circuit. This rule is verified in NEC sections 210.19(A)(1) and 210.20(A). In Article 100, the condition that defines a continuous load is that it will continue for three hours or more.

For example, is it permissible to add an 8A cord-and-plug-connected window-mounted air conditioner to an existing 20A, 120-volt, single-phase branch circuit?

To determine if the window unit can be connected to the 20A branch circuit, perform the following calculation:

Step 1: Find the ampere rating of the branch-circuit
210.23(A)(2); 440.62(C)
A = of 20A OCPD
A = 10A

Step 2: Calculate for A/C unit
210.23(A)(1); 440.32
8 125% = 10A

Step 3: Verify A
210.3; 210.23(A)(1); 440.62(B)
20A OCPD 80% = 16A

Answer: yes, the 8A A/C unit can be cord-and-plug connected to the existing 20A branch circuit without having to run an individual branch circuit.
In existing dwelling units where the cable systems are concealed and it is difficult to gain access to install new cables, an existing branch circuit is used to add a specific appliance manufactured for such use. This requirement includes provisions to add appliances to existing branch circuits for cases where more cooling or heating is needed in a particular room or area without the cost of having to add additional branch circuits.


STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.