Improperly substantiated horsepower designations on pool motors and other appliances may resize many circuit elements without increasing safety.

We received a question from an inspector about a listed swimming pool filter assembly. The 115V motor was marked 1 1/2 hp, but the full-load current on the motor nameplate was much lower than one would expect based on the usual ratings of those motors. Then, three days later, this editor inspected a different brand of 115V filter assembly personally in his official capacity as a part-time electrical inspector. It was also marked 1 1/2 hp, but with a nameplate full-load current rating of 12.4A. The horsepower label was paper, printed with a dot-matrix printer and glued on, whereas the current rating was stamped on the metal nameplate riveted to the motor. That nameplate had no horsepower notation, but did show that the motor was thermally protected. We will focus on this motor since we have direct information about it.

It came from the factory with a 3-ft 123 SJT W-A cord assembly, including a 20A 125V locking cord cap molded on to the end of the cord. The installer matched the receptacle configuration to the cord cap (125V, 20A), which in turn was installed on a 20A branch circuit using No. 12 THHN supplied through a 20A GFCI circuit breaker marked for 75 [degrees] C terminations. The cord assembly was included in the listing evaluation for this equipment. The question is, are the elements of this branch-circuit in accordance with the Code?

The EC&M Panel's response

We think the horsepower marking on the motor creates a violation of the literal text of the NEC, but we question whether there is an actual safety problem because we doubt the validity of the marking itself. Assuming, for the sake of argument, that the marking is valid, then the rule in Sec. 430-6(a) will apply to this motor:

(a) General Motor Applications. Other than as specified for torque motors in (b) below and for ac adjustable voltage motors in (c) below, where the current rating of a motor is used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, etc., the values given in Tables 430-147, 430-148, 430-149, and 430-150, including notes, shall be used instead of the actual current rating marked on the motor nameplate. Separate motor overload protection shall be based on the motor nameplate current rating.

This provision overrides nameplate information for determining all elements of a motor circuit other than running overload protection. In this case, the Table 430-148 value for this motor is 20A, which is over 60% higher than the nameplate rating of 12.4A! To see where all this leads, let's size the circuit elements based on a 20A motor load.

* The branch-circuit conductors. The conductors must have an ampacity of 125% of the motor full-load current, in accordance with Sec. 430-22(a):

(a) General. Branch-circuit conductors supplying a single motor shall have an ampacity not less than 125 percent of the motor full-load current rating.

The No. 12 THHN has an ampacity, as determined from Table 310-16, of 30A. The obelisk note at the bottom of the table doesn't apply to motor circuits. The note doesn't apply if the Code specifically allows connections based on full ampacities, and Sec. 240-3(f) leaves all responsibility for sizing motor circuit protection to Art. 430. In this case, 125% of the motor full-load current (20A) is 25A, and in motor circuits, this is the value that you must check for excessive terminal heating, per Sec. 110-14(c). A No. 12 conductor running at 25A will reach, but not exceed, an operating temperature of 75 [degrees] C.

You determine this by looking in the second ampacity column (75[degrees] C) of Table 310-16. We simply assume that a No. 12 conductor carrying less current (below 25A) must reach some temperature below 75 [degrees] C, and vice-versa. Therefore you could use No. 12 conductors for this motor. For design reasons you might want to increase the conductor size to counteract voltage drop, but since that is design criteria, you would be free to use the nameplate value of 12.4A in making that decision.

* The cord. The 12-3 cord will only have two conductors carrying current, and therefore column B of Table 400-5A applies, resulting in a cord ampacity of 25A. Therefore the cord is large enough for the motor. The cord is also marked "W-A." When you have an application where flexible cord will be left outdoors, as is typical for swimming-pool equipment, the listing restrictions on cord require that it be marked "Outdoor" or "W-A" to indicate suitability for this use. This cord was so marked, and its other markings indicated suitable mechanical strength for the environment.

* The plug and receptacle. The plug is a disconnecting means covered in Sec. 430109 Ex. 5 which provides in part:

For a cord- and plug-connected motor, a horsepower-rated attachment plug and receptacle having ratings no less than the motor ratings shall he permitted to serve as the disconnecting means.

In order to find the horsepower ratings for plugs and receptacles, you need to look in the UL Electrical Construction Materials Directory. This is the same place we found the information about how the cord needed to be marked. It bears repeating, that you cannot design and properly install electrical systems without the general information in either this publication (the "Green Book") or the shorter "White Book," which covers the same material but omits the specific listee data.

In this case, a 30A receptacle and mating cord cap would be required. We think the motor should not have been listed with the 20A cord cap, unless the horsepower marking was evaluated and then changed so as not to exceed 1 hp, which is the maximum permitted for a 20A 125V configuration. Note that the configuration rules in Sec. 210-21 don't apply here, because pure motor circuits are beyond the scope of Art. 210. The listing restriction is the only governing factor on the receptacle configuration.

* The overcurrent protection. In this, as in almost all motor circuits, the overcurrent protection is split. The running overload protection for the motor is, in this case, integral with the motor. You determine this by looking for the "thermally protected" notation on the nameplate. Short-circuit and ground-fault protection uses a separate device, which must be provided in the field, again based on the table values and not the nameplate.

An inverse-time circuit breaker such as the one in question must be able to start and run the motor, and could be as large as 250% of 20A, or 50A, per Table 430152. Indeed, if the motor wouldn't start, the protection could go all the way up to 400% (or 80A for this motor) without needing to increase the wire size or receptacle configuration. As a practical matter, however, we suspect this motor would run very nicely on a 20A circuit, and it certainly would run on a 30A circuit.

Believe the horsepower label?

Unless the inspector is willing to entertain an allowance under Sec. 90-4, we think the horsepower label must be observed. We would, however, encourage the inspector to entertain a variance in cases where the horsepower label doesn't appear to remotely correspond to the nameplate current. At the present time, UL and other testing laboratories aren't policing horsepower claims on appliances. The nameplate current rating is carefully checked, but the horsepower label can be freely used to advertise the product, with only the faintest basis in reality. In this case I would say that this filter motor is probably a 1 hp or even smaller motor based on conventional standards.

This is a recurring problem. For example, we are aware of some 120V garage door openers marked 3/4 hp, but with a 6A nameplate full-load current. According to Table 430-148, this is a 13.8A motor. Since this appliance is fastened in place, Sec. 210-23(a) won't allow it to be more than 50% of a multioutlet branch circuit supplying other receptacles and lighting. If you believe the horsepower label, you need an individual 20A circuit; if you believe the current rating, you can put it on the customary lighting circuit in the garage.

In my capacity as an inspector, since UL saw both the cord cap and the horsepower label at the same time, I have asked UL to review the listing on the pool motor, and they are doing that as of this writing. The larger question, however, is whether installers and designers should be at the mercy of appliance manufacturers' advertising departments in working out circuit designs. Remember that the specific appliance data is usually unknown at the design stage.

We understand UL's reluctance to intervene in advertising campaigns. However, as long as Sec. 430-6(a) applies, something needs to be done. Either the product standards need to prohibit ridiculous horsepower designations, or the Code needs an escape clause for these appliances. I was fortunate. The Massachusetts Electrical Code includes an additional exception to Sec. 430-6(a) that does exactly that, and I used it here.