Imagine life without Motors. What would the world be like if not for Nikola Tesla, and his alternating current motor? Today, we utilize motors in various appliances, that increase our comfort of living, and within commercial and industrial facilities that produce widgets, and gadgets that afford us these comforts we would never have imagined back in the times before Mr.Tesla’s brilliant existence.

Electrician’s today are taught the nuances of special rules within the National Electrical Code® for protecting motors and the conductors that supply them. They are taught the necessary need for proper short-circuit and ground-fault protection, overload protection, and the proper sizing of associated electrical conductors that supply the motors.

This article is from a wire and cable industry perspective, seeking to examine the requirements necessary to ensure the long-term reliability of electrical conductors utilized within motor installations. Thus focusing on a single branch circuit in this article for simplicity sake, but the principals will apply broadly to all aspects regarding conductors used in motor installations.

Given : 25 HP, Continuous-Duty, 3-Phase, 208V, Design B, motor and THWN copper conductors, with 75°C terminals with the branch circuit conductors located in an ambient temperature location of 115°F.

Traditionally, the first act of the electricians is to find the full-load current, also known as the FLC. In section 430.250, and subsequent Table 430.250, the electrician shall determine the FLC as 74.8 Amps. This is the “actual load” for the motor, and will be utilized for sizing the short-circuit and ground-fault protection, and branch circuit conductor sizing.

With the aforementioned information, the electrician can focus on the branch circuit conductor sizing for the given single motor, and leave the short-circuit and ground-fault protection for another day’s lesson. The next step in sizing a motors’ branch circuit conductors would be to look at 430.22, which is titled “Single Motor. The section states the following:

430.22 Single Motor. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor full-load current rating

The savvy electrician will notice that the above quote expresses that the conductors are to have an ampacity of “not less than 125%”, but also astutely recognizes that the conductors could be larger than this minimum sizing rule where necessary due to specific conditions of use. If we didn’t take such conditions of use, such as the 115°F, into consideration then we would simply do the math as presented in 430.22 as 74.8 Amps x 125% (1.25) = 93.5 Amps and size our branch circuit via Table 310.16, and the 75°C column, which would be a 3 AWG copper THWN rated at 100 Amps.

However, we are required per 310.14(A)(3) states:

No conductor shall be used in such a manner that its operating temperature exceeds that designed for the type of insulated conductor involved

In the given example, clear change in the condition of use is evident, and places the THWN copper branch circuit conductors in a condition of use where the ambient temperature, as well as the mutual heating from other conductors, and subsequent current flow, serves to raise the overall temperature that the conductors insulation is subjected to, thus requiring additional actions to be taken to ensure the reliability and longevity of the insulated conductor. This is where we will focus on that condition of use aspect of the single motors branch circuit.

Since this article is discussing branch circuits to single motors, it must take into account the ambient temperature requirements since the condition of use is other than 30°C (86°F). To do this the electrician has to look at Table 310.15(B)(1)(1) and find the multiplier for the 115°F in the question.

Part II of Article 430 is a minimum rating for motor-branch circuit conductors. Clearly, we must make sure the single motors branch circuit conductors are at least 125% of the motors FLC.

Our stated condition of use is 115°F elevated ambient temperature, as a result the motor’s branch circuit conductors will be exposed to values other than 30°C (86°F) the normal ampacity values given in 310.16. Since our THWN is rated 75°C, as expressed in Table 310.4(A), the electrician needs to examine Table 310.15(B)(1)(1) for guidance when the ambient temperatures are other than 30°C (86°F) as depicted in section 310.16, and the subsequent Table 310.16, as previously mentioned.

Table 310.15(B)(1)(1) shows our 75°C rated THWN copper conductor at 115°F to be 0.75. This is the value to be utilized in our calculation to accommodate the condition of use expressed in the given example. Remember, we must have branch circuit conductors at least 125% of the FLC, even after the application of any ambient correction.

So we take the FLC, which was 74.8 Amps and multiply that by 125% [ 74.8 x 1.25 = 93.5 Amps]. So, the branch circuit conductors for our motor must be at least 93.5 Amps no matter what. Visiting Table 310.16, and staying within the confines of the 75°C column, the electrician would select a THWN copper conductor rated for at least 93.5 Amps. The selection would be a 3 AWG copper THWN conductor.

Now, since we know the 3 AWG has an actual ampacity of 100 Amps we can apply the ambient correction to that amapcity to see what we get after the condition of use is applied. We would take 100 x .75 = 75 Amps. As a result, the 3 AWG is now only rated for 75 Amps and that would not be acceptacle since we need a conductor that is good for 93.5 Amps because it said in 430.22 that the conductor ampacity had to be at least 125% of the FLC.

Let’s move to the next size up and do the math again. A 2 AWG CU is good for 115 Amps. Will this work now? So, 115 Amps x .75 = 86.25 Amps and still not enough. Let’s try 1 AWG CU, with an ampacity of 130 Amps. So, 130 x .75 = 97.5 Amps and would be perfectly fine for the 93.5 Amps needed to maintain the 125% of the motors FLC.

So the answer is 1 AWG CU is what would be needed to meet 430.22 and account for the elevated ambient temperature.

Conclusion : To ensure single motor branch circuit conductors are sized no less than 125% of the FLC as demanded in 430.22, while also understanding that conditions of use, such as the ambient temperature, where other than 30°C (86°F) or where more than (3) three current carrying conductors are encountered, may result in potentially larger conductor.

From a wire and cable perspective, this effort was ultimately to help educate electricians on conductors and their insulation ratings, and how the requirements of the National Electrical Code can intertwine the rules in Article 430 and Article 310 for specific conditions of use, such as ambient temperature or when you have current carrying conductors exceeding three, which are vitally important for the reliability and long-term sustainability as they pertain to single branch circuit motor installations.

Paul W Abernathy, CMECP®

Electrical Code Academy, Inc. – CEO & Founder (https://FastTraxSystem.com)

Encore Wire Corporation – Director of Technical Services (www.EncoreWire.com)

National Electrical Code® is a trademark of the National Fire Protection Association and is not associated or affiliated with Electrical Code Academy, inc.