Guides
Guides
Short, practical explanations of the rules behind the calculators — and the mistakes that cause most failed inspections and re-worked jobs.
- What Size Wire for a 100 Amp Sub Panel?A 100 A feeder is a standard size, but the conductor that carries it depends on three things people usually forget: the termination temperature rating, the ambient temperature, and how many current-carrying conductors share the raceway.
- What Size Wire for a 60 Amp Hot Water Heater?Water heaters are usually treated as a continuous load, which means the branch circuit is sized at 125% of the nameplate current. That single factor moves both the conductor and the breaker up a step.
- The 3% Voltage Drop Rule, ExplainedThe 3% figure is a recommendation in an informational note, not a hard rule — but it is the number inspectors and equipment manufacturers expect you to design to.
- Conduit Fill Percentages: 53%, 31% and 40%The allowable fill depends only on how many conductors are in the raceway: 53% for one, 31% for two, and 40% for three or more.
- THHN vs THWN: What the Letters MeanThe letters describe the insulation: heat resistance, moisture resistance and where the conductor may be installed. The designation decides which temperature column you are allowed to use.
- The 80% Rule for Circuit LoadingThe 80% rule is shorthand for the continuous-load requirement: a load expected to run for three hours or more must be limited to 80% of the circuit rating, which is the same as sizing the circuit at 125% of the load.
- Copper vs Aluminum ConductorsAluminum carries roughly 78% of the current of copper at the same size, so it takes a larger conductor to do the same job — a trade that often still pays off on large feeders.
- How to Read an Ampacity ChartAn ampacity chart gives you the starting number, not the answer. The answer comes from applying the ambient, conductor-count and termination adjustments to it.
- Ampacity Correction Factors in PracticeCorrection factors are multiplicative. Two moderate factors can cost you a third of the ampacity, which is usually one or two conductor sizes.
- How to Calculate Box FillEvery box has a volume in cubic inches. Each current-carrying or grounded conductor takes an allowance based on its own size; equipment grounds, devices, clamps and support fittings use the largest conductor that applies to them. The total must not exceed the box volume.
- Recessed Lighting Spacing and LayoutLayout is driven by illuminance, not by a fixed grid: work out the total lumens the room needs, divide by what each fixture produces, then arrange that many fixtures evenly.
- NEC Editions and Wire Sizing: What ChangesConductor sizing is one of the more stable parts of the code, but table reorganisations and revised correction factors do happen — and adoption is local, so the edition in force is whatever your jurisdiction has adopted.