Assume a power bank of heating resistors is rated at 40KW and is connected to a 240V supply. The resistors operate for more that three hours at a time and are located in an area with an ambient temperature of 86 Degree F. Copper conductors with THWN-2 insulation are to be used to connect the heaters. What size conductors should be used
To determine the size of conductors needed, we will consider the current-carrying capacity based on the given power and voltage, and also account for the temperature rating and ambient temperature.
First, let's calculate the current using the power and voltage:
Power (P) = Voltage (V) * Current (I)
40,000 W = 240 V * I
I = 40,000 W / 240 V
I = 166.67 A
Next, we need to determine the current-carrying capacity of the conductor considering the ambient temperature and temperature rating. The table below provides the ampacity values for copper conductors with THWN-2 insulation based on ambient temperature and conductor size:
╔═══════════╦═════════╗
║ Temp (°F) ║ Ampacity ║
╠═══════════╬═════════╣
║ 86°F ║ 90°C ║
╚═══════════╩═════════╝
Since the ambient temperature matches the insulation temperature rating, we can use the ampacity value of 90°C (194°F) for our calculations.
From the table, we select the conductor size based on the ampacity value of 166.67 A. Considering a conservative approach to avoid excessive heating, we should choose a conductor size that can carry at least 200 A.
After consulting a wire ampacity chart, we find that a 3/0 AWG copper conductor has an ampacity of 200 A, which is sufficient for our application.
Therefore, 3/0 AWG copper conductors should be used to connect the heating resistors.