You are employed in a large industrial plant. A 480-V, 5000-W heater is used to

Question

You are employed in a large industrial plant. A 480-V, 5000-W heater is used to melt lead in a large tank. It has been decided that the heater is not sufficient to raise the temperature of the lead to the desired level. A second 5000-W heater is to be installed on the same circuit. What will be the circuit current after installation of the second heater, and what is the minimum size circuit breaker that can be used if this is a continuous-duty circuit? You are an electrician. You have been asked by a homeowner to install a lighted mirror in a bathroom. The mirror contains eight 40-watt lamps. Upon checking the service panel you discover that the bathroom circuit is connected to a single 120-volt, 20-ampere circuit breaker. At the present time, the circuit supplies power to an electric wall heater rated at 1000 watts, a ceiling fan with a light kit, and a light fixture over the mirror. The fan motor has a foil-load current draw of 3.2 amperes and the light kit contains three 60-watt lamps. The light fixture presently installed over the mirror contains four 60-watt lamps. The homeowner asked whether the present light fixture over the mirror can be replaced by the lighted mirror. Assuming all loads are continuous, can the present circuit supply the power needed to operate all the loads without overloading the circuit? A car lot uses incandescent lamps to supply outside lighting during the night. There are three strings of lamps connected to a single 20-ampere circuit. Each string contains eight lamps. What is the largest standard lamp that can be used without overloading the circuit? Standard size lamps are 25 watt, 40 watt, 60 watt, 75 watt, and 100 watt.

Explanation / Answer

1)

Rated Continuous Current drawn by heater is Ir = 5000/480 = 10.42 A

Total rated current IT  = 2*10.42 = 20.8 A

Since Load is continuous Breaker rated current should by 125% of Load rated Current

ICB = 1.25*20.8 = 26 A

According to NEC 240.6 table stand Circuit breaker current rating required is 30 A

2)

Given Circuit breaker rating 120 V , 20 A.

Load Connected is Continuous and total current drawn by the load is

IT = 1000/120 + 3.2 + 180/120 + 240/120 = 15.03 A

Breaker current rating should be ICB = 15.03*1.25 = 18.8 A

According to NEC stand table of circuit breaker rating 20 A rating breaker is required, so it is present now.

Now given that removing light fixture over mirror and replace with lighted mirror which contains 8*40W lamps

New Total current the load is ITN = 1000/120 + 3.2 + 180/120 + 320/120 = 15.7 A

Circuit breaker current ICBN = 19.625 A

There it is under rated value of installed circuit breaker i.e of 20 A. Hence we can replace with lighted mirror.

3)

Observing the ratings of each light 75 W rating will be the highest rating bulb can be connected without over loading the 20A circuit.

8*75 W *3 = 1800 W

I = 1800/120 = 15 A

ICB  = 15*1.25 = 18.75 A

Thus these incandescents bulbs can be used.

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