Apprentice path · Stage 3: The theory you cannot fake later · Lesson 10

Series and parallel circuits

Series means one path for the current. Parallel means more than one path. Everything else follows from that.

Why this matters on the job

It decides what happens when something fails. In series, one break stops everything. In parallel, the rest keeps working -- which is why the lights staying on does not mean a circuit is dead.

How to think about it

  1. Trace the wire from the source and count the paths back to it.
  2. One path only -- series. More than one -- parallel.
  3. In series: the current is the same everywhere, and the supply voltage is shared out between the loads.
  4. In parallel: every branch sees the same supply voltage, and the current splits between the branches.

Worked through

Two loads in series on 12 volts: the same current goes through both of them, and the 12 volts is divided between them. Put those same two loads in parallel on 12 volts and each one gets the full 12 volts, while the current coming out of the source splits between the two branches.

Check yourself: One load in a two-load circuit fails open. The other load keeps working. Were they in series or in parallel?

Answer: Parallel.

Parallel gives each load its own path back to the source, so breaking one path leaves the other one complete. In series there is only one path, so a break anywhere stops both.

Watch out for: A load still running does not mean the part you are about to touch is de-energized. Parallel branches are exactly how something stays live while something else is clearly dead.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.

Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.