An electric annealing oven is located in an area with an ambient temperature of 125°F. The oven contains a 50-kW electric heating element and is connected to 480 volts. The conductors are to be copper with type THHN insulation. The termination temperature is not known. The furnace is expected to

Question

An electric annealing oven is located in an area with an ambient temperature of 125°F. The oven contains a 50-kW electric heating element and is connected to 480 volts. The conductors are to be copper with type THHN insulation. The termination temperature is not known. The furnace is expected to operate more than three hours continuously. What size conductor should be employed to make this connection?

Accepted Answer

Determine the amount of current needed to operate the furnace.

[latex]I = \frac{P}{E}[/latex]

[latex]I = \frac{50,000}{480}[/latex]

[latex]I = 104.167 amperes[/latex]

Because the load is continuous, it must be increased by 125%:

104.167 × 1.25 = 130.2 amperes

The next step is to apply the correction factor for temperature. Type THHN insulation is located in the 90°C column. The correction factor for a temperature of 125°F is 0.76, as shown in Figure 11-10. To determine the current rating of the conductor at 125°F, divide the current by the correction factor:

[latex]I = \frac{130.2}{0.76}[/latex]

[latex]I = 171.3 amperes[/latex]

Because the termination temperature is not known and the current is over 100 amperes, the conductor size will be selected from the 75°C column of NEC Table 310.15(B)(16). A 2/0 AWG conductor will be used.

Question 11.8: An electric annealing oven is located in an area with an amb...

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