Question 10.7: Determine if it is economically feasible to install number 1......
Determine if it is economically feasible to install number 10 (AWG) copper conductor instead of number 12 (AWG) on a 100 \ ft branch circuit that feeds a load of 16 \ A. Assume that
• The load is used 10 \ h/day and 300 \ days/year
• The cost of electricity is $0.10/kWh
• The installed costs of No. 12 and No. 10 conductors are, respectively, \$60.00 and \$90.00/1000 \ ft long cable
In addition to the electric energy used to meet the load, there is an energy loss in the form of heat generated by the flow of current, I, through the resistance of the conductor, R.
The heat loss in Watts can be calculated as follows:
Watts=R\cdot I^2
Using Table 2.2 or NEC Tables, the resistance of both conductors, No. 12 and 10, can be determined to be, respectively, 0.193 and 0.121 \ Ω/100 \ ft. Thus, heat loss for the 400 \ ft branch circuit if No. 12 conductor is used can be estimated as follows:
Watts_{12}=0.193*100/100\cdot (16)^2=49.4 \ WSimilarly the heat loss for the 400 ft branch circuit when No. 10 conductor is used is found to be
Watts_{10}=0.121*100/100\cdot (16)^2=31.0 \ WThe annual cost of copper losses for both cases can be easily calculated:
Cost_{12}=49.4 \ W*300 \ days/year*10 \ h/day*1 \ kW/1000 \ W*\$0.10/kWh=\$14.8/yearCost_{10}=31.0 \ W*300 \ days/year*10 \ h/day*1 \ kW/1000 \ W*\$0.10/kWh=\$9.3/year
Therefore, if No. 10 is used instead of No. 12, the simple payback periods, SPP, for the higher initial cost for the branch circuit conductor is
SPP=\frac{(\$90/1000 \ ft-\$60/1000 \ ft)*100 \ ft}{(\$14.8-\$9.3)}=0.54 \ year=6.5 \ monthsSavings in energy consumption through the use of larger conductors can thus be costeffective. Moreover, it should be noted that the larger size conductors reduce the voltage drop across the branch circuit that permits the connected electrical utilization equipment to operate more efficiently. However, the applicable code has to becarefully consulted to determine if larger size conduit is required when larger size conductors are used.
TABLE 2.2
Cross-Sectional Area and Resistance for Electrical Conductors
| Resistance of Aluminum Wire (Ω/1000 ft) | Resistance of Copper Wire (Ω/1000 ft) | Number of Wires | Cross-Sectional Area (cmil) | S (AWG/MCM) |
| AWG | ||||
| 10.7 | 6.51 | 1 | 1,620 | 18 |
| 6.72 | 4.10 | 1 | 2,580 | 16 |
| 4.22 | 2.57 | 1 | 4,110 | 14 |
| 2.66 | 1.62 | 1 | 6,530 | 12 |
| 1.67 | 1.018 | 1 | 10,380 | 10 |
| 1.05 | 0.6404 | 1 | 16,510 | 8 |
| 0.674 | 0.41 | 7 | 26,240 | 6 |
| 0.424 | 0.259 | 7 | 41,740 | 4 |
| 0.336 | 0.205 | 7 | 52,620 | 3 |
| 0.266 | 0.162 | 7 | 66,360 | 2 |
| 0.211 | 0.129 | 19 | 83,690 | 1 |
| 0.168 | 0.102 | 19 | 105,600 | 0 |
| 0.133 | 0.0811 | 19 | 133,100 | 00 |
| 0.105 | 0.642 | 19 | 167,800 | 000 |
| 0.0836 | 0.509 | 19 | 211,600 | 0000 |
| MCM | ||||
| 0.0708 | 0.0431 | 37 | 250,000 | 250 |
| 0.059 | 0.036 | 37 | 300,000 | 300 |
| 0.0505 | 0.0308 | 37 | 350,000 | 350 |
| 0.0442 | 0.027 | 37 | 400,000 | 400 |
| 0.0354 | 0.0216 | 61 | 500,000 | 500 |
| 0.0295 | 0.018 | 61 | 600,000 | 600 |
| 0.0253 | 0.0154 | 61 | 700,000 | 700 |
| 0.0236 | 0.0144 | 61 | 750,000 | 750 |
| 0.0221 | 0.0135 | 61 | 800,000 | 800 |
| 0.0197 | 0.012 | 61 | 900,000 | 900 |
| 0.0177 | 0.0108 | 61 | 1,000,000 | 1000 |