Series vs parallel circuits: what it means for the Texas exam

Bonding Jumper editorial teamSources checked 26 Oct 20263 min read

In a series circuit, the parts are connected end to end in a single path, so the same current flows through every one and their resistances add. In a parallel circuit, each part is connected directly across the same two points, so each sees the same voltage, the currents add, and the total resistance is lower than the smallest branch.

Two circuits drawn side by side from a 120-volt source: three resistors of 10, 20 and 30 ohms in a single loop labeled series, total 60 ohms and 2 amperes; and the same three resistors on separate rungs labeled parallel, with branch currents 12, 6 and 4 amperes adding to 22 — Series: one path, one current. Parallel: one voltage, many paths.

The rules

	Series	Parallel
Current	Same through every part	Adds: total equals the sum of branch currents
Voltage	Adds: the drops sum to the source	Same across every branch
Total resistance	Sum of the resistances	Reciprocal of the sum of reciprocals

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A worked example with invented numbers

Three resistors of 10, 20 and 30 ohms on a 120-volt source.

In series: total resistance is 10 + 20 + 30 = 60 ohms. Current is 120 ÷ 60 = 2 amperes everywhere. The drops are 20, 40 and 60 volts, which add back to 120.

In parallel: each resistor has the full 120 volts. Branch currents are 120 ÷ 10 = 12, 120 ÷ 20 = 6 and 120 ÷ 30 = 4 amperes, for 22 amperes total. Total resistance is 120 ÷ 22, about 5.45 ohms, which also comes from 1 ÷ (1/10 + 1/20 + 1/30). It is smaller than the smallest branch, as it always is.

Two shortcuts for parallel: two equal resistors give half of one, and for any two, the total is their product divided by their sum.

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Where each shows up in wiring

Building circuits are parallel: every receptacle and luminaire on a branch circuit sees the same voltage, and each added load draws more total current. That is why adding loads can overload a circuit. Series connections appear in switches and overcurrent devices, which are in series with the loads they control, and in the conductors themselves, whose resistance is in series with the load and causes voltage drop. Some older string lighting and certain control circuits are wired in series.

Common mix-ups

Adding parallel resistances. That gives the series total. Parallel always comes out smaller.
Thinking current is used up along a series circuit. It is the same at every point; voltage is what divides.
Forgetting that AC circuits with inductance or capacitance need impedance, not just resistance, for the same rules.

On the Texas journeyman exam

Circuit theory sits in definitions, theory and plans, 5 scored items across both portions under the PSI bulletin, with the Calculations portion naming the area calculations and theory. Study to find a total resistance, a current or a voltage drop in a simple combination.

Practice it

The series and parallel resistance calculator shows every step for any combination. Every term in this set is in the electrical terms glossary.

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Not affiliated with or endorsed by TDLR, PSI or NFPA. NEC and National Electrical Code are registered trademarks of NFPA.

Sources

Show the list

Candidate Information Bulletin, Texas electricians (updated 9/15/2026), PSI for the Texas Department of Licensing and Regulation.
NFPA 70, National Electrical Code (2026 edition), NFPA. Free read-only access with an account.

Facts checked 26 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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