Can a 20A receptacle be placed on a 15A circuit?
A reader asks, “On a multioutlet circuit at 120V, I know that I can put a 15A receptacle on a 20A circuit, so why can I not put a 20A receptacle on a 15A circuit?”
First, let’s look at 210.21(B)(3), which permits 15A receptacles on a 15A circuit and 15 or 20A receptacles on a 20A circuit. This dates back to the 1947 NEC. In the 1937 and earlier versions of the Code, the 15A receptacle was permitted on a 15 or 20A branch circuit; the 20A receptacle required a 25A circuit. A 15A plug will fit into either a 15 or a 20A receptacle, but a 20A plug will fit only into a 20A receptacle.
A general rule to follow is that for a UL-listed appliance, the cord and plug furnished with the appliance will tell you what size circuit is required. If the appliance is furnished with a 30A plug, then a 30A branch circuit is required; the same thing goes for 15 and 20A plugs. But don’t count on it.
NEC 210.23 says: “Permissible Loads, (A)(1) Cord-and-plug-Connected Equipment. The rating of any one cord and plug-connected utilization equipment shall not exceed 80 percent of the branch circuit ampere rating.” In my office is a portable electric heater, UL listed, factory-equipped with a 15A plug, with a nameplate rating of 1,500W, 120V. A 15A circuit at 120V has a maximum rating of 1,800W (120 x 15). Eighty percent of 1,800 is 1,440W. Therefore my heater cannot be supplied by a 15A circuit but must be on a 20A circuit. But there is no marking on it to so indicate. How is Joe Homeowner to know this, or for that matter how am I to know?
The 1,500W heater will work quite well on a 15A circuit. The heater is thermostatically controlled, so it is not a continuous load. If the rest of the circuit is not loaded beyond the 300W “cushion” left when the heater is on, there is no problem. If the circuit is overloaded, the overcurrent device will open the circuit and no harm done.
If the 15A circuit is a single outlet circuit, then it can feed any load within its rating, according the 210.23, Permissible Loads. But because it is a multi-outlet circuit, any one receptacle can supply only 1,440W. The same rule, stated in a different way is in 210.21(B)(2): “(2) Total Cord and Plug-Connected Load. Where connected to a branch circuit supplying two or more receptacles or outlets, a receptacle shall not supply a total cord-and-plug-connected load in excess of the maximum specified in Table 210.21(B)(2).”
How is a homeowner expected to know that his appliance with a factory-attached 15A plug must be on a 20A circuit?
Equipment fastened in place may be installed on a circuit also serving luminaires and cord-and-plug-connected utilization equipment not fastened in place, or both, but the fastened-in-place equipment is not to exceed 50 percent of the circuit rating. Although it is common practice to install kitchen waste disposers on a separate circuit, a 1/3 hp unit at 115V with a Table 430.148 ampacity of 7.2A could be on a 15A circuit also supplying lighting outlets and/or general-use receptacles in other than the kitchen, dining room, pantry, breakfast room or similar areas.
Now to the question asked at the very beginning: the existence of a 20A receptacle suggests to the user that a load of 20A could be supplied. If it is a multi-outlet circuit, any one cord-and-plug-connected load is limited by the Code to 16A, but the user does not know that. In any case, if a 20A receptacle were installed on a 15A circuit the user would be misled and could easily overload the circuit. On the other hand, a 15A receptacle on a 20A circuit does not present a hazard except for the slightly longer response time of a 20A circuit breaker over a 15A circuit breaker, which is minimal. EC
SCHWAN is an electrical Code consultant in Hayward, Calif. He can be reached at firstname.lastname@example.org.