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Home > Home Wiring USA Archive: NEC 1999 > Definitions and Calculations > Series / Parallel Resister Circuit Calculations (NEC 1999)

# Series / Parallel Resister Circuit Calculations (NEC 1999)

By Warren Goodrich ## Toolbox

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Total Resistance in Series Circuit {Whether they are Equal or Unequal}

Total Resistance of all Resistors on a Series Circuit =
R1 + R1 + R1 + R1 = Total resistance of that Series Circuit.

Special Notes:The total resistance will increase as you add more resistors to the series circuit.

Special Notes: The total resistance of all resistors on a series circuit will always be more than the amount of any one resistor in the circuit, actually the total resistance will be the sum of all resistors on that series circuit.

Total Resistance in Series Circuit To find the total resistance of the above drawing with all resistors in a series =

3 ohms + 3 ohms + 3 ohms + 3 ohms = 12 ohms total resistance

Parallel Circuits

Total Resistance in Parallel [Equal Resisters]

Resistance of one resistor

Number of resistors in parallel = Total Resistance

Special Notes: The total resistance will decrease as you add more resistors to the parallel circuit. To find the total resistance of the above drawing with all equal resistors

resistance = 10 ohm =2 ohm total
5 resistors

Total Resistance in parallel [Maximum of two Unequal Resisters]

Resistance of one resistor R1 x R2 = Total Resistance
Number of resistors in parallel R1 + R2 To find the total resistance of the above drawing for two unequal resistors

= 3 x 12 = 36 = 2.4 Total ohms resistance
3 + 12 15

Total Resistance parallel circuit above [Minimum of Three or More Unequal Resisters]

R1 x R2 = Y Y x R3 =Resistance Total phase one of format
R1 + R2 Y + R3 3ohms x 12ohms = 36 = 2.4 Resistance
3ohms + 12ohms 15

Total phase 1 of format to find Y. You must carry “ Y ” on to phase 2 of the format to calculate the third resister into the total Resistance Calculation.

Phase 2 = “Y” = 2.4

2.4 x 4ohm {the third resistor} = 9.6 = 1.5 ohms Total resistance of circuit
2.4 + 4ohm {the third resistor} 6.4

Special Notes: The total resistance of two or more resistors on a parallel circuit will always be less than the amount of any one resistor in the circuit.

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