Texas Electrician ExamTexas journeyman & master

One sentence per configuration and you never confuse them again

In series, current is common. In parallel, voltage is common. Everything else follows from those two facts.

On this page
  1. The two sentences
  2. Resistance, the part people memorize unnecessarily
  3. The sanity check that catches errors
  4. Where this shows up beyond theory questions
  5. What this page cites

The two sentences

Series and parallel circuits share the opposite quantity, and that is the whole trick In a series circuit there is one path, so the current is the same everywhere and the voltages across each element add up to the source. In a parallel circuit every branch sees the same voltage, and the branch currents add up to the total. Nearly every basic theory question is testing whether you know which quantity is shared. SERIES ~ one path same current everywhere the voltages add up PARALLEL ~ many paths same voltage everywhere the currents add up
Series shares current. Parallel shares voltage. Name which one you have before touching a formula. Electrical theory, not a code rule. Appears on the theory portion of most licensing exams.

In a series circuit, the same current flows through every element, and the voltages across them add up to the source voltage.

In a parallel circuit, the same voltage sits across every element, and the currents through them add up to the total current.

That is the whole topic. Every formula you have seen for combining resistances is a consequence of one of those two sentences.

Resistance, the part people memorize unnecessarily

In series, resistances add. Obvious from the first sentence: same current, voltages add, so resistances add.

In parallel, the reciprocals add. Also a consequence: same voltage, currents add, so conductances add, and resistance is the reciprocal of conductance.

Two equal resistors in parallel give half. Three equal give a third. That shortcut answers a surprising number of exam items without any arithmetic.

The sanity check that catches errors

Total parallel resistance is always smaller than the smallest branch. Always. No exception.

If your parallel answer is bigger than one of the branches, you have made an arithmetic error, and you have caught it in two seconds without rechecking your work.

The equivalent check in series: total is always larger than the largest element.

Where this shows up beyond theory questions

Parallel conductors. When conductors are run in parallel, current divides between them and the rules for doing that safely exist because it does not divide evenly unless the paths are identical.

That is why the code requires paralleled conductors to match in length, material, size and termination method. The theory tells you why the rule exists.

Voltage drop on long runs. The resistance of the conductor is in series with the load, which is precisely why the load sees less voltage than the source.

What this page cites

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Every rule here is cited to the section it comes from, and corrections are logged in public with the date. This is study material rather than electrical advice, and it does not replace your own code book or your inspector.