What determines the required cable size?

Design current, installation method, ambient temperature, grouping, permitted voltage drop and protection against fault current all influence the selected conductor size.

How does this calculator choose a size?

It evaluates every conductor in the selected size system in ascending area order and recommends the smallest one whose calculated voltage drop is at or below the design limit you enter.

Does it check current carrying capacity?

Not automatically. OHMICLY does not hold a verified ampacity table, so no allowable current is assumed. If you enter a base allowable current from your own table, the calculator applies your correction factors and compares it with the load current.

Where do the derating factors come from?

They are values you enter from the standard applicable to your project. The calculator never invents temperature correction, grouping or installation method factors.

Can I use it for copper and aluminium?

Yes. Both copper and aluminium are modelled, in metric mm2 sizes as well as AWG and kcmil sizes.

Cables & Circuits

Cable Size Calculator

Compare every listed conductor size against your voltage drop limit and select the smallest one that passes, with your own correction factors applied to the design current.

Last updated: 11 September 2026Calculation basis: Transparent formulas & worked examples

Cable size selection

OHMICLY does not hold a verified ampacity or derating table, so this version selects the smallest listed conductor that satisfies your voltage drop limit. Correction factors and any allowable current are values you enter from the standard applicable to your project — nothing is assumed on your behalf.

Applicable references

BNBC 2020
Bangladesh National Building Code
National building code covering electrical installations, safety and building services requirements in Bangladesh.
NFPA 70 / NEC
National Electrical Code
Widely adopted code for safe electrical design, installation and inspection practice.
Source

Method, data sources and current limitations

How a size is selected

Selection criterion: Every conductor in the chosen size system is evaluated in ascending area order, and the smallest one whose calculated voltage drop is at or below your design limit is recommended. The same resistance and reactance model as the OHMICLY Voltage Drop Calculator is used, so both tools always agree.
Design current: I_design = I_load ÷ (k_temp × k_group), where both factors are the values you enter from your applicable standard. It is reported for your ampacity comparison and does not alter the voltage drop calculation, which uses the actual load current.
Conductor data: Resistivity is 0.0214 Ω·mm²/m for copper and 0.0344 Ω·mm²/m for aluminium at an assumed 75 °C operating temperature; AC reactance uses 0.000164 Ω per metre per conductor. Distances entered in feet are converted at 0.3048 m per foot. These are generic engineering assumptions, not clauses from a published code.

Engineering data OHMICLY does not yet hold

Conductor ampacity tables: No verified allowable-current table (per material, size, insulation rating and installation method) has been loaded into OHMICLY.
Ambient temperature correction factors: No verified temperature correction table is available; the factor must be supplied by the user.
Grouping / bundling adjustment factors: No verified adjustment table for the number of current-carrying conductors is available; the factor must be supplied by the user.
Installation method factors: No verified installation-method (reference method) factor table is available.

Because of this, version 1 does not perform an automatic thermal (current carrying capacity) check. It never guesses a table value: allowable current and correction factors are only used when you supply them. A conductor selected here must still be verified against the ampacity, short-circuit withstand and protection requirements of the code applicable to your project.

About this cable size calculation

Sizing a cable is a sequence of independent checks, and the conductor you install has to pass all of them: it must carry the load current continuously without exceeding its insulation temperature, keep voltage drop within the limit your design or specification sets, withstand the prospective short-circuit energy for as long as the protective device takes to clear, and coordinate with that device so a fault at the far end is detected. Version 1 of this tool performs the voltage drop check across every listed conductor size and returns the smallest one that passes.

Enter the load current, system voltage, phase arrangement, conductor material, run length and the design drop limit, and the calculator compares the whole size table for you rather than making you re-run a voltage drop check by hand for each candidate. The number of current-carrying conductors and ambient temperature are recorded with the correction factors you supply, so the design current used for your own ampacity comparison is shown explicitly alongside the result.

On long runs the voltage drop check usually governs the final size, while short runs at high current are almost always governed by ampacity. If the recommended size looks larger than expected, the practical levers are the same as for voltage drop: add conductors per phase, shorten the route, move the distribution board closer to the load, or raise the distribution voltage.

Formula notes

Version 1 selects the smallest listed conductor whose calculated voltage drop is within your design limit, using the same resistance and reactance model as the OHMICLY Voltage Drop Calculator.
Design current is the load current divided by the temperature correction and grouping factors you enter; those factors are taken from the standard applicable to your project, not generated by this tool.
OHMICLY holds no verified ampacity, temperature correction, grouping or installation method table, so the thermal current carrying capacity check is not automatic and must be verified against your applicable code.

Worked example — sizing a sewing-floor feeder cable

Scenario

Three-phase 400 V feeder, load current 100 A at 0.9 power factor
One-way run 150 ft (about 46 m), copper conductors, one per phase
Three current-carrying conductors, ambient temperature 30 °C
Correction and grouping factors taken from your own standard (1.00 each in this example)
Design voltage drop limit from the project specification: 3 %

Steps, using the formulas on this page

1. Design current after your factorsIdesign = I ÷ (k_temp × k_group)
2. Voltage drop check for each listed sizeVdrop = √3 × I × (R·cos φ + X·sin φ), with R = ρ × L ÷ (A × n)
3. SelectionThe smallest listed size whose drop % is at or below your entered limit

The size returned satisfies voltage drop only. Check it against the allowable current from your own ampacity table, then against short-circuit withstand and protective-device coordination before it is issued for construction.

Bangladesh practice — common questions

Can I use this size directly for a BNBC-assessed installation in Bangladesh?: No. This version selects on voltage drop alone. A conductor for a project assessed against the Bangladesh National Building Code must also satisfy current-carrying capacity after correction and adjustment factors, short-circuit withstand and coordination with the protective device. Use this result as the voltage drop input to that wider check, not as the final size.
Which correction and grouping factors should I enter for a Bangladesh factory?: Enter the factors published in the standard your project is assessed against, for the actual ambient temperature and installation arrangement on site. Bangladesh factory ceiling voids and cable routes frequently run well above a 30 °C design ambient, and bunched cables on a common tray need a grouping factor, so both values should come from your standard's tables rather than being assumed.

RSC & Nirapon audit resourcesCable, panel and installation checklists used during RSC and Nirapon electrical assessments.Browse resources

Frequently asked questions

What determines the required cable size?: Design current, installation method, ambient temperature, grouping, permitted voltage drop and protection against fault current all influence the selected conductor size.
How does this calculator choose a size?: It evaluates every conductor in the selected size system in ascending area order and recommends the smallest one whose calculated voltage drop is at or below the design limit you enter.
Does it check current carrying capacity?: Not automatically. OHMICLY does not hold a verified ampacity table, so no allowable current is assumed. If you enter a base allowable current from your own table, the calculator applies your correction factors and compares it with the load current.
Where do the derating factors come from?: They are values you enter from the standard applicable to your project. The calculator never invents temperature correction, grouping or installation method factors.
Can I use it for copper and aluminium?: Yes. Both copper and aluminium are modelled, in metric mm2 sizes as well as AWG and kcmil sizes.