3.2 Series, Parallel, and Circuit Behavior

Circuit behavior is a reasoning tool

Journeyman exams do not test theory as an isolated classroom exercise. They use theory to see whether you understand what happens when a load opens, a connection loosens, a neutral is lost, a switch is placed in the wrong part of a circuit, or several loads are connected to one feeder. The same ideas also support conductor sizing and overcurrent protection because current depends on how loads are connected.

A circuit needs a source, an ungrounded conductor path, a load, and a return path. In normal premises wiring, branch loads are generally placed in parallel so each load receives the intended system voltage. A luminaire, receptacle load, and appliance on a 120-volt branch circuit are not intended to share voltage in series. If they did, turning one off or opening one load could affect the others.

Series vs. Parallel Quick Table

Series circuits

In a series circuit, there is only one path for current. The same current flows through every component. Total resistance is the sum of the resistances.

Rtotal = R1 + R2 + R3 + ...

If a 120-volt circuit has a 10-ohm resistor and a 20-ohm resistor in series, total resistance is 30 ohms. Current is I = E / R = 120 / 30 = 4 A. The voltage across the 10-ohm resistor is E = I x R = 4 x 10 = 40 V. The voltage across the 20-ohm resistor is 4 x 20 = 80 V. The drops add back to the source voltage: 40 + 80 = 120 V.

The field lesson is that voltage divides in proportion to resistance. A high-resistance connection in series with a load can drop voltage, heat, and leave the load operating poorly. The exam may describe dim lamps, low voltage at a motor, or heat at a splice. Theory points you toward a series resistance problem even before the code rule is considered.

Parallel circuits

In a parallel circuit, each branch connects across the same two points. Voltage is the same across each branch. Current in each branch depends on that branch's resistance, and total current is the sum of branch currents.

Itotal = I1 + I2 + I3 + ...

For resistance, the reciprocal method is:

1 / Rtotal = 1 / R1 + 1 / R2 + 1 / R3 + ...

For two resistors only, a shortcut is:

Rtotal = (R1 x R2) / (R1 + R2)

Example: a 120-volt source supplies a 20-ohm branch and a 30-ohm branch in parallel. Branch current one is 120 / 20 = 6 A. Branch current two is 120 / 30 = 4 A. Total current is 10 A. Equivalent resistance is 120 / 10 = 12 ohms. Notice that equivalent resistance is lower than either branch resistance. That is a key sanity check.

Building wiring connection

Most utilization equipment in a building is connected in parallel. A 120-volt receptacle on a multioutlet branch circuit should receive about 120 volts whether another receptacle load is plugged in or not, subject to normal voltage drop. A 240-volt water heater should not depend on a range element being energized. The parallel model is why adding loads generally increases total current without reducing nominal voltage at every load.

Controls may use series logic. A safety switch, limit switch, overload contact, thermostat, or stop button may be placed in series with a control coil so opening any one device stops the circuit. A start button or holding contact may be placed in parallel with another control path. The exam domain for control devices is smaller than wiring methods, but series and parallel reasoning helps you answer those items quickly.

Opens and shorts

An open in a series circuit stops current everywhere in that path. An open in one branch of a parallel circuit stops current only in that branch, while other branches may continue operating. A short circuit creates an unintended low-impedance path and can cause very high current until an overcurrent protective device operates. A ground fault is an unintended connection to ground or grounded parts.

Do not casually call every problem a short. A high-resistance connection, open neutral, line-to-ground fault, and line-to-line short have different symptoms and code consequences. On the exam, language matters. If the question asks for current in an open branch, the branch current is zero. If it asks for total current after one parallel branch opens, subtract only that branch current.

Worked example: branch opens

A 120-volt circuit has three parallel resistance loads: 12 ohms, 24 ohms, and 60 ohms. The 24-ohm branch opens. What is the new total current?

Before the open, currents were 10 A, 5 A, and 2 A, for 17 A total. After the 24-ohm branch opens, that branch current is zero. The remaining current is 10 A + 2 A = 12 A. The voltage across the remaining parallel branches is still 120 V in the ideal calculation.

A common trap is recalculating the opened branch as infinite current or adding the resistance values as if the loads became series. An open branch has no complete path, so it has no branch current.

Calculator setup

For series resistance, addition is direct. For parallel resistance, use branch current when voltage is known; it is often cleaner than reciprocal resistance. If the question gives voltage and branch resistances, calculate each branch current and add. If the question gives only resistances and asks for equivalent resistance, use the reciprocal formula and check that the answer is less than the smallest branch resistance.

Exam traps

Watch for the word total. Total current in a parallel circuit is not the current through each load. Watch for equal resistances. Two equal resistors in parallel have half the resistance of one resistor, and three equal resistors have one-third. Watch for voltage in series. The full source voltage is not automatically across each series component. It divides according to resistance.

Finally, keep code and theory in their proper places. Series and parallel math can tell you the current. The NEC then tells you how conductors, overcurrent devices, boxes, wiring methods, and equipment must be selected and installed.