Sizing a cable in Singapore follows SS 638, the Code of Practice for Electrical Installations (Appendix 4). The steps run in order: find the design current, choose the protective device, size the cable for current, then check the voltage drop. If the voltage drop is too high, go up a size.
The terms
| Ib | Design current: what the circuit actually carries |
| In | Rated current of the circuit breaker |
| It | Tabulated current-carrying capacity of the cable, from the SS 638 tables |
| Iz | Current-carrying capacity after corrections for the real installation |
| I2 | Current at which the protective device reliably trips on overload |
| Ca | Correction for ambient temperature other than 30°C |
| Ci | Correction for thermal insulation around the cable. In Singapore installations cables are rarely run through thermal insulation, so Ci is normally taken as 1 |
| Cg | Correction for grouping with other circuits |
Step 1: Find the design current
The design current Ib is the current the circuit will actually carry in normal use: the load's full-load current, or the sum of the loads it feeds. Everything else is sized from it.
| Single-phase | Ib = P / (V × cos φ) |
| Three-phase | Ib = P / (√3 × VL × cos φ) |
P is the load in watts, V the supply voltage (230 V single-phase, 400 V line-to-line three-phase) and cos φ the power factor, 1 for a resistive load such as a heater.
Step 2: Choose the circuit breaker
Pick the next standard rating at or above the design current, so that Ib ≤ In.
Motor circuits: a motor started direct on line draws about 6 to 7.5 times its full-load current for a few seconds. The device must ride through that inrush without tripping, so In is often 1.5 to 2.5 times the motor's full-load current. The cable is then sized to that larger In, so check how long the inrush lasts against the device's tripping curve.
Step 3: Size the cable for current
The cable and its protective device must satisfy three conditions:
| Ib ≤ In | The device is rated for the load |
| In ≤ Iz | The cable can carry the device's rated current |
| I2 ≤ 1.45 × Iz | The device trips on overload before the cable overheats |
For MCBs to IEC 60898-1, the third condition is met automatically once the second one is, because their tripping current I2 is no more than 1.45 × In.
The cable's effective capacity is Iz = It × Ca × Ci × Cg, and it must be at least In. Rather than working out Iz for every candidate size, divide In by the correction factors and look the result up in the tables. Where the device is a miniature circuit breaker to IEC 60898-1:
| Single circuit | It ≥ In / (Ca × Ci) |
| Grouped circuits | It ≥ In / (Ca × Cg) |
Pick the smallest cable whose tabulated It meets the figure. For groups whose circuits cannot all be overloaded at the same time, SS 638 allows a less onerous alternative based on Ib instead of In; see Appendix 4, section 6.1.2.
Runs through different conditions
A circuit often passes through more than one set of conditions on its way to the load, for example bunched with other circuits in trunking for part of the way, then running alone through a hotter room.
- Work out the correction factors and the size needed for each section separately.
- Use the largest of those sizes for the whole run. The breaker rating must not exceed the rating of the weakest section.
- Check the voltage drop over the total length of the run with the chosen size.
Cables in enclosed trenches (installation methods 18, 19 and 20) are the exception. Their grouping factors in Table 4B3 depend on the cable size, so the calculation works the other way round: start from a likely size, for example the one voltage drop calls for, apply that size's factors to its tabulated rating to get Iz, and confirm Iz is at least In. If it falls short, check the next size up.
Ambient temperature factors
The tabulated ratings assume 30°C air around the cable. Singapore plant rooms, risers and ceiling voids often run hotter, so Ca from SS 638 Table 4C1 is applied. It depends on the insulation's maximum operating temperature:
| Insulation | 30°C | 35°C | 40°C | 45°C | 50°C |
|---|---|---|---|---|---|
| General purpose PVC, 70°C H05VV-F, H07V-K |
1.0 | 0.94 | 0.87 | 0.79 | 0.71 |
| Rubber, 60°C, flexible cables H07RN-F |
1.0 | 0.91 | 0.82 | 0.71 | 0.58 |
| Thermosetting (XLPE), 90°C | 1.0 | 0.96 | 0.91 | 0.87 | 0.82 |
These factors apply where the circuit is protected by a circuit breaker.
Grouping factors
Cables bunched together cannot shed heat as well as a cable on its own, so each one must be derated. SS 638 Table 4B1 gives Cg by installation method and number of circuits. For cables enclosed in conduit or trunking, or bunched and clipped to a surface (method 1):
| Circuits | 2 | 3 | 4 | 6 | 9 |
|---|---|---|---|---|---|
| Cg | 0.80 | 0.70 | 0.65 | 0.57 | 0.50 |
Cables laid in a single layer derate far less: spaced one cable diameter apart on a surface, Cg stays at 0.90 or above however many circuits there are, and beyond two diameters no factor is needed at all. Three points from the notes to the table:
- The factors assume cables of one size. Mixed groups need more care.
- A cable that will never carry more than 30% of its grouped rating can be left out when counting the group.
- If cables with different maximum operating temperatures share a group, rate them all at the lowest temperature in the group.
Step 4: Check the voltage drop
In a consumer's installation in Singapore, the voltage drop from the origin of the installation (usually the supply terminals) to the last point must not exceed 4%: 9.2 V on a 230 V circuit, or 16 V on 400 V three-phase.
The tables give voltage drop in mV per amp per metre (mV/A/m), covering all the circuit's conductors. For three-phase circuits the values are for the line voltage with a balanced load.
Voltage drop (V) = mV/A/m × Ib × length (m) ÷ 1000
If the result is over the limit, choose the next size up and check again. On long runs voltage drop often sets the size, so some designers work out the largest mV/A/m allowed first: the permitted drop in mV divided by the design current and the run length.
A more accurate voltage drop
The tabulated values assume the conductor is at its maximum operating temperature. If the cable runs well below its capacity, it is cooler, its resistance is lower and the real voltage drop is smaller. SS 638 allows two refinements:
- Operating temperature: for conductors up to 16 mm², multiply the tabulated mV/A/m by a factor Ct that reflects how hot the cable really runs. Above 16 mm², apply it to the resistive part only.
- Load power factor: for conductors up to 16 mm², multiply by the load's power factor, cos φ, which gives an approximate figure. Above 16 mm², use cos φ × (mV/A/m)r + sin φ × (mV/A/m)x, from the resistive and reactive columns of the table.
Both corrections can be combined. They matter most on long runs where voltage drop, rather than heating, sets the cable size.
The temperature correction applies only where the ambient temperature is 30°C or higher. For very large cables, where the reactance is at least three times the resistance (x/r ≥ 3), it can be ignored.
This is a summary of the method in SS 638:2018, Appendix 4. Always use the tables and factors in the standard itself, and have the design checked by a Licensed Electrical Worker. Need help choosing a cable size? Ask our team.
