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 also look at what Article 210, entitled Branch Circuits, states concerning branch circuit conductor sizing. In Part II of Article 210, we find the "Branch-Circuit Ratings", and in 210.19(A) it states what appears to be an informational note, while not enforceable, is important nonetheless.
It reminds the electrician that Part II of Article 430 is a minimum rating for motor-branch circuit conductors. Clearly, they are still branch circuits, and 210.19(A) is going to apply in the overall scheme of conductor sizing. As a result, let's move to 210.19(A)(1) to give conductor sizing it's additional consideration.
Section 210.19(A)(1) is broken down into two first level subdivision, one being (a) where the loads are considered continuous (125%) and may or maynot also contain non-continuous loads (100%) as well and the other being (b), where a condition of use is present, such as in our given example. Since our given motor example is considered continuous by virtue of 430.22, which already imposed the minimum branch circuit sizing via the FLC at 125%, there is no relevant need to consider (a) since the 125% is already taken into account, we will move on to first level subdivision (b).
Now, keep in mind that our actual load is 74.8 amps as verified earlier in this article.
First Level Subdivision (B) of 210.19(A)(1) states:
(b) The minimum branch-circuit conductor size shall have an ampacity not less than the maximum load to be served after the application of any adjustments or correction factors in accordance with 110.14(C).
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) 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) 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, 210.19(A)(1)(b) seeks to ensure that the branch circuit conductor has enough current carrying ability to handle the actual load, in our case 74.8 amps, after the application of any adjustments or corrections.
Since we know the actual load we can utilize division to work up to the desired conductor sizing and then multiplication to validate the effort. So, 74.8 ÷ .75 = 99.73 Amps. 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 99.73 Amps. The selection would be a 3 AWG copper THWN conductor. Interestingly, in this given example, the conductor size is the same as selected when using the 125% rule in 430.22.
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 necessary to handle the actual motors load.
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 210 for branch circuit conductor reliability and long-term sustainability as they pertain to single branch circuit motor installations.
Check out this video on the topic related to this article.
Paul Abernathy, CMECP® | CEO & President
Electrical Code Academy, Inc. | www.FastTraxSystem.com 3913 Edward Drive., McKinney, TX 75071 Office: 214-945-0653
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I appreciate that you often include a temperature derating as part of the calculation. You get used to operating under normal ambient conditions and then forget temp deratings when they are most important