What does the OCPD Calculator return?

It returns the minimum protection current the device must carry, calculated from the design current you enter and, if you supply one, a continuous-load factor from your own standard. With no factor entered, the required protection current equals the design current.

Does it recommend a breaker or fuse size?

No. A standard device rating is only returned when a verified standard-rating series exists in OHMICLY engineering data. No such series is loaded in this version, so standard selection is reported as unavailable rather than proposed.

Does it check the device against conductor ampacity?

No. No verified ampacity table is loaded, so coordination between the device and the conductor is not checked. Interrupting rating, let-through energy and selectivity with upstream devices are also outside the scope of this version.

What does the maximum allowable rating comparison mean?

If you enter a maximum allowable protection rating, the tool reports whether the required protection current is at or below that value. It is a numeric comparison against your own input, not a compliance determination.

Protection

OCPD Calculator

Calculate the required overcurrent protection current from your design current, with any continuous-load factor you supply. Standard device ratings are shown only when verified rating data is available.

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

Protection current calculation

Applicable references

BNBC 2020
Bangladesh National Building Code
Overcurrent protection requirements applicable to installations in Bangladesh. Clause numbers are not shown because they have not been verified into OHMICLY.
NFPA 70 / NEC
National Electrical Code
Overcurrent protection and standard device rating requirements where NEC practice applies. Clause numbers are not shown because they have not been verified into OHMICLY.
Source

Method, formulas and limitations

Formulas used

Required protection current: I_protection = I_design × factor, where the factor is applied only when you explicitly enter one. With no factor entered, the required protection current equals the design current you supplied.
Maximum allowable rating comparison: When you enter a maximum allowable protection rating, the tool reports whether the required protection current is at or below that value. It is a numeric comparison against your own input, not a compliance determination.
Standard rating selection: A standard device rating is returned only when a verified standard-rating series exists in OHMICLY's engineering data. No such series is loaded in this version, so the tool reports standard selection as unavailable rather than proposing a breaker or fuse size.

What this version does not model

Code multipliers: continuous-load multipliers, motor-circuit protection limits and next-standard-size allowances are not applied.
Conductor coordination: the tool does not check the device against conductor ampacity, because no verified ampacity table is loaded.
Short-circuit performance: interrupting rating, let-through energy and selectivity with upstream devices are outside the scope of v1.
Device characteristics: trip curves, adjustable settings, thermal-magnetic versus electronic behaviour and manufacturer derating are not modelled.

About this overcurrent protection calculation

An overcurrent protection device protects the conductor and the equipment downstream of it against overload and fault current. Sizing it starts with a single number: the current the circuit is designed to carry. This tool takes that design current, applies any continuous-load factor you explicitly supply from your own standard, and reports the minimum protection current the device must be able to carry without nuisance tripping.

Everything after that number is code and product territory. The permitted rating depends on the conductor ampacity after correction and adjustment factors, on whether the load is continuous, on motor and transformer rules that differ between BNBC and NEC practice, on the interrupting rating required at the point of installation, and on the trip characteristic the manufacturer publishes. OHMICLY does not hold a verified standard-rating series, so this version deliberately does not propose a breaker or fuse size.

Use the calculated protection current as the minimum requirement, then select the device from the code applicable to your project and the manufacturer's catalogue. If you enter a maximum allowable rating — for example the limit set by the conductor you have already sized — the tool tells you whether your requirement fits below it, so an undersized conductor or an optimistic load assumption shows up before procurement.

Formula notes

Required protection current = design current x continuous-load factor, and the factor is applied only when the user explicitly enters one.
With no factor entered, the required protection current equals the design current supplied by the user.
A standard OCPD rating is returned only when a verified standard-rating series exists in OHMICLY engineering data; none is loaded in v1, so automatic device selection is reported as unavailable.
No code multiplier, motor-circuit rule, next-standard-size allowance or conductor-ampacity coordination check is applied.

Worked example — protection current for a sewing-floor sub-feeder

Scenario

Design current of the feeder: 48 A
Multiplying factor entered from your own standard where the load is continuous
Maximum allowable rating entered from the conductor you have already sized, if known

Steps, using the formulas on this page

1. Required protection currentIprotection = Idesign × your factor (no factor entered means Iprotection = Idesign)
2. ComparisonIprotection is compared with the maximum allowable rating you entered

The figure returned is a minimum requirement, not a device selection. OHMICLY holds no verified standard-rating series, so it will not propose a breaker or fuse size.

Bangladesh practice — common questions

Why does the tool not suggest a breaker size for a Bangladesh installation?: Proposing a rating would require a verified standard-rating series and the code rules that govern it — for example the overcurrent protection provisions of the Bangladesh National Building Code or the equivalent rules in the standard your project follows. None of these is loaded into OHMICLY, and inventing a rating would be unsafe, so the tool reports the required protection current and leaves the device selection to you and the code.
What else must be checked before a device is chosen for a factory panel?: Conductor ampacity after correction and adjustment factors, continuous-load requirements, motor and transformer rules, the interrupting rating required at the point of installation, coordination with upstream devices, and the manufacturer's trip characteristic. RSC and Nirapon electrical assessments routinely examine panel protection and coordination, so keep the supporting calculation with the panel records.

RSC & Nirapon audit resourcesPanel, protection and electrical safety checklists used in RSC and Nirapon assessments.Browse resources

Frequently asked questions

What does the OCPD Calculator return?: It returns the minimum protection current the device must carry, calculated from the design current you enter and, if you supply one, a continuous-load factor from your own standard. With no factor entered, the required protection current equals the design current.
Does it recommend a breaker or fuse size?: No. A standard device rating is only returned when a verified standard-rating series exists in OHMICLY engineering data. No such series is loaded in this version, so standard selection is reported as unavailable rather than proposed.
Does it check the device against conductor ampacity?: No. No verified ampacity table is loaded, so coordination between the device and the conductor is not checked. Interrupting rating, let-through energy and selectivity with upstream devices are also outside the scope of this version.
What does the maximum allowable rating comparison mean?: If you enter a maximum allowable protection rating, the tool reports whether the required protection current is at or below that value. It is a numeric comparison against your own input, not a compliance determination.