Three lamps with 40 Ω, 30 Ω, and 15 Ω resistance are connected in parallel to a 90 volt circuit. What is the total current used by the three lamps?
A) 0.94 A

B) 15.25 A

C) 11.25 A

D) 1.06 A

Option C) 11.25 A

Explanation:

Given that:

Voltage V = 90 volts

total current I = ?

Three resistors each with 40 Ω, 30 Ω, and 15 Ω in parallel, the equivalent resistance of the combination (Rtotal) of the circuit is as follows:

i.e 1/Rtotal = (1/R1 + 1/R2 + 1/R3)

1/Rtotal = (1/40Ω + 1/30Ω + 1/15Ω)

1/Rtotal = (3 + 4 + 8)/120Ω

1/Rtotal = 15/120Ω

1/Rtotal = 1/8Ω

To get the value of Rtotal, cross multiply

Rtotal x 1 = 8Ω x 1

Rtotal = 8Ω

Then, apply the formula V = I x Rtotal

90 volts = I x 8Ω

I = 90 volts / 8Ω

I = 11.25 Amperes

Thus, the total current used by the three lamps is 11.25 Amperes

most voltage drop calculators want the distance between the source and destination. then internally, they double it so both the source and return wires are calculated. but you need to know which distance the calculator uses so you're not half of what you should be or double what you should be.

most circuit breakers will not take a #6 directly, so the electrician will have to splice on a smaller pigtail. you also have to upsize the grounding wire when you upsized the ungrounded (hot) wire (see note in 250.122). this will preclude using most cable assemblies, as a cable with #6 copper will most likely only have a #10 grounding conductor. you need a #6 grounding conductor if the circuit is 20a. a #8 cable is no better, as it will have a #10 grounding conductor too. but most 20a breakers will take a #8 directly.

in your related post, you didn't say what type of load this is. if it is a light or heater, voltage drop won't be a big issue. if this is a motor, the current needs are huge at startup and the voltage drop can pose a problem. if this is a motor, knowing if the motor is under much load during startup will answer the voltage drop effect (a saw is easier to start than a compressor).