Multispeed motors provide special challenges
for conductor sizing and protection
A single-speed motor has one rated speed at which it runs when supplied with the nameplate voltage and frequency. A multispeed motor will run at more than one speed depending on how you connect it to the supply. Multispeed motors typically have two speeds to choose from, but may have more.
In most industrial settings, the 3-phase, two-speed, six-lead induction motor is a common application of a multispeed motor. A typical example would be a four-pole machine (with synchronous speed of 1,800 rpm) connected to operate at 1,800 rpm (high) and 900 rpm (low).
Different requirements. The NEC requirements for sizing the overcurrent protection devices (OCPDs) for multispeed motors differ from the requirements for sizing OCPDs for single-speed motors.
To understand the multispeed motor OCPD sizing requirements, it helps to review how to size OCPDs for single-speed motors.
Sizing the OCPDs for one single-speed motor is a fairly straightforward matter of following Table 430.52, which gives the maximum ratings or settings for various OCPDs, such as fuses and inverse time breakers. And, of course, you'll find exceptions for things like the next highest device rating, increases for higher inrush currents, and so on.
Things aren't so straightforward with a multispeed motor. The complication begins with selecting the branch-circuit conductors. For single-speed motors, you select conductors that have an ampacity of not less than 125% of the full-load current (FLC) of the motor [430.22(A)] as specified in the FLC tables listed in [430.6(A)(1)]. But for multispeed motors, you base your selection on the current rating of winding(s) that the conductors energize [430.22(B)].
Notice the differences. With multispeed motors, Art. 430 doesn't refer to any tables that contain values for sizing the branch-circuit conductors. In fact, 430.6(A)(1) specifically excludes multispeed motors from the FLC tables.
Given this, how do you determine the current rating of the windings in a multispeed motor? You might conclude that the only thing you can do is use the nameplate current, and you would be right. In fact, the last sentence in 430.6(A)(1) specifically instructs you to do exactly that.
The requirements for conductor ampacity of 125% of FLC still apply. What's different? Just the method of determining the motor FLC.
You may be wondering why 430.6(1) requires you to use the nameplate current for multispeed motors. A multispeed motor won't draw the same current in the low-speed windings when running at low speed as it will when running at high speed.
It's a common misconception that low-speed conductors are de-energized when the motor runs at high speed. In actuality, the current drawn by the low-speed conductors when the motor operates in high speed will be greater than the current drawn when the motor runs in low speed. Therefore, sizing the low-speed branch-circuit conductors based on the FLC tables is incorrect. You must use the actual nameplate values.
OCPDs. The assumption behind 430.22(B) is that you'll also meet the applicable requirements of Table 430.52 — which addresses the maximum ratings of OCPDs. Each winding (i.e. speed connection) must have an overcurrent device connected in series with each phase conductor. For a two-speed motor, this would require two OCPDs — one for low and one for high. This arrangement isn't that prevalent anymore. A more common approach taken by today's MCC suppliers is to provide a single OCPD in the two-speed starter and rely on the engineer or designer to ensure proper setting of this device.
You can use a single OCPD for two or more windings of the motor, provided the rating of the OCPD doesn't exceed the applicable Table 430.52 percentage of the nameplate rating of the smallest winding protected [430.52(C)(4)]. The requirements of 430.52(C)(4) make sense when you read them in conjunction with 430.22(B). Remember, 430.22(B) requires you to size the conductors based on the current rating of a given winding. Because the low-speed winding will be the smallest winding protected, you would need to select an OCPD rated to protect this winding and its branch circuit conductors.
But this creates the potential for a problem. If you set the OCPD to the limits prescribed in Table 430.52 to protect the low-speed winding, the higher inrush current of the high-speed winding would likely trip the OCPD and consequently prevent the motor from starting in high speed.
However, the NEC recognizes this potential problem and allows an exception. If you meet three requirements, you can use a single OCPD sized for the FLC of the highest current winding [430.52(C)(4)]. To meet those requirements, you must:
Use separate overloads for each winding.
Size the branch-circuit conductors that supply each winding for the FLC of the highest FLC winding.
Ensure the controller horsepower rating isn't less than that required for the winding with the highest horsepower rating.
The first and last requirements are generally easy to meet. But the second one can easily be a stumbling block if you aren't careful. This requirement means you must use the same size branch-circuit conductors for both speed connections. The winding design of the motor is irrelevant because the single OCPD will protect both the low-speed conductors and the high-speed conductors with a setting based solely on the high-speed current.
When dealing with multispeed motors, always size all branch-circuit conductors based on the highest FLC. If you remember that and use the nameplate current rather than the current from FLC tables, you'll determine the correct conductor sizing and you'll be able to select the correct OCPD every time.
Peda is a senior electrical engineer at Black & Veatch in Overland Park, Kan.