The electrician.wiki guides
Learn electrical concepts
Guides that explain the ideas behind the calculators: how a transformer works, why Ohm’s law holds and when it doesn’t, what power factor costs, how earthing clears a fault and what each code edition changed. Every number in them is computed by our tested engine.
Featured guide
Featured · Electrical theory
What is a transformer and how it works
Working principle, parts, the EMF equation and turns ratio, losses, efficiency, types and who invented it.
By The electrician.wiki editors · · 22 min read
Read the guide →
Electrical theory
The ideas every calculation rests on, explained with worked numbers from our engine and original diagrams.
Electrical theory
Ohm’s law
The statement, V = IR and the triangle, the class 10 experiment, solved numericals and where the law breaks down.
· 18 min read
Electrical theory
Power factor
The formula PF = kW ÷ kVA, lagging vs leading, what a low power factor costs and how to correct it.
· 17 min read
Electrical theory
The 555 timer
Pinout, the inside of the chip, astable and monostable circuits and the formulas behind them.
· 17 min read
Wiring and safety
How installations behave in the field, and what the codes ask of them.
Wiring and safety
Voltage drop
What causes it, the 3 % and 5 % figures under the NEC, IEC 60364 and BS 7671, and how to reduce it.
· 16 min read
Wiring and safety
Grounding and bonding (NEC Article 250)
Grounding electrodes and rods, the main bonding jumper, EGC and GEC sizing, and subpanels.
· 15 min read
Wiring and safety
Earthing systems: TN-S, TN-C-S, TT and IT
How each system clears a fault, the broken-PEN risk, RCDs and disconnection times.
· 14 min read
Codes explained
What the codes are, how editions differ and which one applies where. Facts and section numbers, never code text.
Codes explained
NEC: the National Electrical Code explained
What NFPA 70 is, how it is organised, the 2017 to 2026 editions and the articles used most.
· 11 min read
Codes explained
2026 NEC changes
Article 120, the 2 VA/ft² dwelling load, EV chargers and every number that moved.
· 16 min read
Codes explained
GFCI requirements by NEC edition
Every 210.8 location and limit from 2017 to 2026, side by side.
· 10 min read
Codes explained
BS 7671 explained
The IET Wiring Regulations, 18th edition with Amendment 4: what changed and what applies.
· 13 min read
Codes explained
IEC 60364 explained
The international wiring standard, part by part, and how national codes build on it.
· 14 min read
Resistor colour codes by value
The values people look up most, each with its 4-, 5- and 6-band colours, SMD codes, tolerance range, wattage and uses.
- 1 ohm resistorbrown, black, gold, gold
- 100 ohm resistorbrown, black, brown, gold
- 220 ohm resistorred, red, brown, gold
- 330 ohm resistororange, orange, brown, gold
- 470 ohm resistoryellow, violet, brown, gold
- 1k resistorbrown, black, red, gold
- 4.7k resistoryellow, violet, red, gold
- 10k resistorbrown, black, orange, gold
- 100k resistorbrown, black, yellow, gold
- 1M resistorbrown, black, green, gold
How these guides are written
Each guide answers its question in the first screen, then proves the answer. The numbers in the worked examples and tables are computed by the same engine as the calculators, and the engine is tested against published worked examples before every release, so a figure in an article never drifts from the figure a calculator gives. The diagrams and cover drawings are our own, made for the page.
Where a guide touches a code, it names the edition and cites section numbers in place of reproducing code text, and it tells you when an answer depends on where you work: the NEC in the US, BS 7671 in the UK, IEC 60364 and its national versions elsewhere. Each guide links to the calculator that does its arithmetic, and the calculators link back. If you find an error, the form at the foot of every page reaches the editors, and fixes are logged on the corrections page; how we check is set out in the methodology.