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I'm sure this has an answer somewhere but I am going to try my luck. I am having trouble with the following type of circuit.

240V ac going to 3 resistors in series @ 10kW (no neutral, this a simple circuit diagram). Total circuit current @ 1.44 ohms for each resistor load is 83.33 amps and 120v across any load with a voltmeter. And since this is a SERIES circuit that (83.33 amps) is also the TOTAL circuit amps (if i'm not wrong). Because in a parallel circuit it would be 249.99 TOTAL circuit amps and 240v across any load with a volt meter. I am so confused because the voltage across each load keeps adding up to 120v, and in a 240v power supply circuit that does not add up correctly, it would need to be a 360V circuit with 3 resistors @ 1.44 ohms.

I guess the question is: Should each resistive load in this circuit read 120v across itself with a voltmeter or should it read 80v across itself with a voltmeter.

If it should read the first one, why.

I apologize ahead for the bad grammar.

Max R.
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3 Answers3

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If you have three identical resistors in series across 240 volts, you will have 80 volts across each one.

The fact that the 120/240 volt "split phase" circuit can also provide 120 volts to loads connected between either "hot" wire and neutral is irrrelevant if you are not using the neutral.

If you apply 120 volts to one of your 1.44 Ohm resistors, it will draw 83.3 Amps.

With 240 volts across three 1.44 Ohm resistors in series (total resistance is 4.32 Ohms), you will get 55.5 Amps.

Peter Bennett
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  • Even though your answer had more information and I am glad it did, the other guy answered first so I had to give it him. – Max R. Aug 29 '17 at 03:32
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\$R=\dfrac{V^2}{P}=5.76=1.92*3\$ for V=240 and P=10kW

Each R will split 240/3=80Vac rms across each one.

Tony Stewart EE75
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You have resistors sized for 10kw at 120V. That means they flow 83.333 amps (W=EI).

A resistor designed for this purpose will be 1.44 ohms (E=IR).

Now you are connecting three of these resistors in series across 240V. Total resistance will be 4.32 ohms. Again with E=IR, this resistor will flow 55.55 amps.

Now, back to EI=W, 240V x 55.55A is 13,333 watts. This is barely more than one resistor/heater.

This is not particularly surprising. When you change voltage to a resistor, you square it to determine change in energy consumed. Your three resistors see 80 volts each. Changing voltage by 80/120 (2/3) results in power reducimg to 4/9. Three such resistors makes 12/9, or 4/3 or 1.33333 of the original rating of one.

Harper - Reinstate Monica
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