Free notes
Design current Ib: single-phase and three-phaseWhat's Ib on a three-phase circuit?
Free from the notes: Sparky’s video on it, the section word for word as members see it, and a PDF to keep.
The notes
Exactly as they are on the site.
Design current Ib: single-phase
What it's for: finding how many amps a circuit will draw. That's the design current, Ib (capital I, small b). It's the first number in every design, and it depends only on the load.
P = total wattage of the load (W) · V = voltage present at the origin of the circuit (V) · Ib in amps (A). It's the power triangle with I covered.
Be careful not to say "always use 230 V". It's normally U0, the voltage to earth, 230 V. But really it's the voltage present at the point where the circuit starts.
Another example: the same load at DB1 and at DB2
3 kW heater on a final circuit From DB1 (230 V): Ib = 3000 ÷ 230 = 13.04 A From DB2 (225 V): Ib = 3000 ÷ 225 = 13.33 A Less voltage → a little more current for the same power.
8 A).
You may also see √3 and a power factor brought into current calculations. Power factor is Level 4 design work, so it's left out here. At Level 3, when you want the amps in a circuit, use the power triangle: I = P ÷ V. (If you know the volts and the resistance instead, that's Ohm's law, I = V ÷ R.)
Design current Ib: three-phase
What it's for: the design current of a three-phase circuit, where the load is spread across the three phases. The answer is the current per phase.
P = total power (W) · VL = voltage between lines, normally 400 V · √3 = 1.73 · Ib in amps per phase
Unlike single-phase, you don't use the 230 V to earth (less any volt drop). Three-phase Ib is based on the 400 V between lines. If you don't like the √ key on your calculator, just type 1.73.
Worked example: 5 kW three-phase circuit
Ib = P ÷ (√3 × VL) = 5000 ÷ (1.732 × 400) = 5000 ÷ 692.8 = 7.22 A per phase (some sources give 7.21 A, just rounding) L1, L2 and L3 each carry about 7.2 A → a 10 A circuit breaker would do.
You'll also hear it as "5000 over 400, divided by √3". That's the same sum: 5000 ÷ 400 = 12.5, then 12.5 ÷ 1.732 = 7.22 A.
Free section from Sparky Study Notes. The full Level 3 notes, 1,463 practice questions and the video lessons: sparkystudynotes.co.uk
Test yourself on it
The tests are in the members’ area. Get one wrong and it shows you why, and where to revise.
Get new notes and exam tips from Sparky
About one email a fortnight. Unsubscribe any time. Privacy noticeThat was one section. There are 583.
Sparky Study Notes is the whole Level 3 syllabus written up like this: the explanation, the worked example, and where it is in your book. With a video lesson for every part and a practice paper for every exam.
- 583sections of written notes
- 35video lessons
- 1,463practice questions
- 146diagrams, redrawn
- 18th Edition
- Building Regs
- PAT
- Fundamental Inspection & Testing
- Initial Verification
- Periodic Inspection & Testing