There are circumstances when a plug and receptacle configuration can be used above the normal limit of 80% of their rating.
We received some manufacturer's literature on commercial cooking equipment from an engineer. He was concerned about two items in particular, namely, some microwave ovens with nameplate ratings of 19A, and a commercial coffee maker with a nameplate rating of 29A. The units were factory-designed for cord- and plug-connection. The microwave ovens had 20A 125V cord caps, and the coffee maker had a 30A 125/250V cord cap. The engineer wanted to know if the units could be used with the cord configurations as described.
We asked him to check and see if the testing laboratory (UL in this case) had actually seen the cord connections. We have long experience with commercial cooking equipment being produced and listed with only a knockout, and then improper cords and plugs being added in some warehouse by unqualified personnel. To the field installer, it looks like the equipment was listed with improper configurations.
In this instance, however, UL had seen the equipment with the cords and plugs. In addition, UL supplied some relevant pages from applicable standard, Commercial Electric Cooking Appliances, UL 197. The relevant material is in Paragraph 10.5.5, as follows:
10.5.5 The current rating of the attachment plug of a n appliance rated more than 15 amperes, shall not be less than 125 percent of the maximum current input of the appliance when tested in accordance with the Power Input Test, Section 41.
Exception: The attachment plug may be rated not less than the current and voltage rating of the appliance it, when operated continuously for at least 3 hours with no food load or as described for the normal temperature test, the average current input to the appliance is 80 percent or less of the ampacity of a branch circuit equal to or higher than the nameplate rating. See marking required in 61.10.
We have no reason to doubt that the appliances were properly tested and presumably qualified under the exception. We also assume the cords are properly rated and sized. The question that follows then is, does the allowance in UL 197 violate the NEC?
The EC&M Panel's response
We think that the allowance in the standard is proper, but an installer providing branch circuits and receptacles for these loads must be very careful as to the choice of devices. The basic rules are in Sec. 210-21(b)(1) and(b)(2):
(1) A single receptacle installed on an individual branch circuit shall have an ampere rating of not less than that of the branch circuit.
Exception: Where installed in accordance with Section 430-81 (c).
(FPN): See definition of "Receptacle" in Article 100.
(2) 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).
The fine print note refers to the definition of a receptacle in Art. 100, and that definition makes dear that a duplex receptacle is two receptacles, even though it would be installed at a single outlet. Therefore, a single 20A receptacle on an individual branch circuit installed for each microwave oven would comply with Sec. 210-21 (b)(1), as would the 30A receptacle for the coffee maker. If the receptacle were a 20A duplex, the applicable rule would be Sec. 210-21 (b)(2) and the limits in Table 210-21(b)(2) would apply, limiting the permitted receptacle loading to 16A.
Although there are no specific receptacle placement rules for commercial occupancies, Sec. 210-50(b) does apply:
(b) Cord Connections. A receptacle outlet shall be installed wherever flexible cords with attachment plugs are used. Where flexible cords are permitted to be permanently connected, it shall be permitted to omit receptacles for such cords.
A single receptacle must be provided for each of these cooking appliances, at a location where the correct receptacle will, in fact, be the one used.
What about equipment restrictions?
There are other restrictions in the NEC that may appear to disallow this, however, principally Sec. 210-23. For example, Sec. 210-23(a) reads as follows (pay particular attention to the second sentence):
(a) 15- and 20-Ampere Branch Circuits. A 15- or 20-ampere branch circuit shall be permitted to supply lighting units, other utilization equipment, or a combination of both. The rating of any one cord- and plug-connected utilization equipment shall not exceed 80 percent of the branch-circuit ampere rating. The total rating of utilization equipment fastened in place shall not exceed 50 percent of the branch-circuit ampere rating where lighting units, cord-and plug-connected utilization equipment not fastened in place, or both, are also supplied.
Be careful with the second sentence. It refers to the rating of any one utilization equipment. It applies to applications where there are multiple receptacles or multiple outlets. The idea is to reserve some branch-circuit capacity for the vacant half of a duplex receptacle, for example, or for other receptacles on the circuit. Then, if equipment is permanently installed, the rules reserve even more capacity (50%) for the other outlets or receptacles. The result is a logical progression from a 100% loading allowance for an individual branch circuit, to an 80% limit on any one cord- and plug-connected appliance, to not over 50% for permanent equipment.
The parent language in Sec. 210-23, which governs all subsections that follow, supports this argument:
210-23. Permissible Loads. In no case shall the load exceed the branch-circuit ampere rating. An individual branch circuit shall be permitted to supply any load for which it is rated. A branch circuit supplying two or more outlets shall supply only the loads specified according to its size in (a) through (d) below and summarized in Section 21024 and Table 210-24.
The second and third sentences of this section must be read in concert. The second sentence clearly allows, as in this case, a 20A individual branch circuit to supply a 19A microwave oven, and a 30A branch circuit to supply a 29A coffee maker. The third sentence has the effect of removing the receptacle restrictions from the individual branch circuits described here. The subsections that follow only apply where there are "two or more outlets."
This will be further clarified in the 1996 NEC. The words "or receptacles" will be added after the phrase "supplying two or more outlets." This will directly correlate this wording with Sec. 210-21(b)(2), which refers to "two or more receptacles." The requirements in Sec. 210-23(a) through 23(d) will only apply to single receptacles on individual branch circuits.
This analysis assumes noncontinuous loading. The branch-circuit overcurrent protection must properly protect the plugs and receptacles as installed, and Sec. 210-21(b)(1) requires the branch-circuit protection to not exceed the receptacle configuration ratings in these cases. If the load were effectively continuous, the branch-circuit overcurrent protection would need to be increased to cover the additional 25 % load allowance required by Sec. 220-3(a), and that would, in turn, increase the required receptacle (and therefore, the required plug) ratings. This in turn is why the UL standard (see Par. 10.5.5 above) normally requires a 125% allowance and only permits the 100% sizing with actual continuous loading not over 80% of the branch circuit rating.
These answers are given by our panel of experts. I am chairing this panel, and the other panel members include Bill Summers, James Stallcup, and Dan Leaf. The opinion expressed is that of the panel. If a panelist disagrees with the majority opinion, his explanation is printed following the answer. Although authoritative, the answers printed here are not, and cannot be relied on as formal interpretations of the National Electrical Code.