But we’re not finished yet. Now let’s look at upsizing the wiring above code minimum to achieve further energy savings.
Most people don’t think of the wiring as an energy-consuming element of our sample motor system but, as we have already shown, it has resistance, and therefore uses energy.
From the above case, the code minimum was No. 8 AWG size wire. By upsizing to No. 6, let’s see how much more energy we will save.
| From the previous calculation: | |
| For No. 8 THHN @ 30°C: | |
| R = 0.667 Ω /1,000 ft., or 0.167 Ω / 250 ft. | |
| For Motor No. 2: | Watts Loss = (22.5)2 x (0.167) x 3 = 253.6 W Energy Loss = 253.6 ÷ 1,000 x 6,000 = 1,522 kWh/yr |
| Again, from the NEC Chapter 9, Table 9: for No. 6 THHN: | |
| R = 0.49 Ω /1,000 feet @ 75°C | |
| To correct resistance to 30°C, use NEC Table 8 footnote: | |
| R2 = R1 [1 + α( T2 - 75)] where α = 0.00323 | |
| For No. 6 THHN @ 30°C: R = 0.419 Ω /1,000 ft., or, dividing by 4, R = 0.105 Ω /250 ft. |
|
| 3-Phase Watts Loss = (22.5)2 x (0.105) x 3 = 159.5 W Energy Loss = 159.5 W ÷ 1,000 x 6,000 hr/yr = 957 kWh/yr |
|
| Savings due to upsizing the wiring, with the more efficient motor, are: | |
| 1,522 - 957 = 565 kWh/yr | |
| at $.09 per kWh | at $.07 per kWh | ||
|---|---|---|---|
| Value of Annual Savings | $50.85 | $39.55 | |
| Simple Payback = | added cost ($60) savings |
1.2 yr | 1.5 yr |
The above example assumes we have already chosen the more efficient motor. (At less than a year payback, who wouldn’t?) (With the less efficient motor, the payback time of the larger wire would appear to be slightly better, owing to the higher current drawn, but this effect is deceiving because the efficient choice uses less energy. Thus it is wiser to calculate payback following the energy savings.)
Another side benefit that we haven’t calculated here is that the larger wire will run cooler than the smaller wire. That difference will be in the vicinity of 10°C, which in turn will result in an even lower resistance in the larger wire, producing a small amount of additional savings that we haven’t taken credit for here. Although this effect is sometimes significant, at these load levels it can be ignored.