| 1 | Confirm the nominal system voltage | Country or project voltage, phase arrangement, frequency, and earthing system. | Breaker voltage rating must be equal to or higher than the system voltage. Common low-voltage systems include 120/240 V single-phase and 230/400 V three-phase. | A 230/400 V, three-phase installation generally uses 230 V line-to-neutral and approximately 400 V line-to-line. | Verify the local electrical code and utility supply before selecting the enclosure and breakers. |
| 2 | Create a complete circuit schedule | Loads such as lighting, sockets, HVAC, water heating, pumps, ovens, and machinery. | Record each circuit's load type, rated power, operating voltage, phase, and quantity instead of estimating from the number of rooms. | Separate lighting, general-purpose outlets, kitchen equipment, and motor loads into clearly identified circuits. | Ask for a circuit directory and permanent labels inside the box. |
| 3 | Calculate current from power | Rated power in watts or kilowatts and the applicable voltage. | For single-phase loads, use I = P ÷ (V × power factor). For three-phase loads, use I = P ÷ (√3 × V × power factor). | A 2,300 W resistive load at 230 V draws approximately 10 A before applying code-required design margins. | Use the equipment nameplate rating and do not size a breaker from wattage alone. |
| 4 | Apply continuous-load requirements | Expected operating time and the applicable definition of a continuous load. | Many electrical codes require continuous loads to be included at 125% of their expected current. The exact rule depends on the jurisdiction. | A 12 A load treated as continuous may be evaluated at 15 A for design purposes where the 125% rule applies. | Confirm the requirement with a qualified electrician or the authority having jurisdiction. |
| 5 | Account for starting and inrush current | Motor, compressor, transformer, LED driver, and power-supply starting characteristics. | Short-duration inrush may require an appropriate trip curve, but the breaker must still protect the conductors and equipment. | A pump may have a substantially higher starting current than its running current; use the manufacturer's data for selection. | Do not simply install a larger breaker to stop nuisance tripping. |
| 6 | Match the breaker to conductor ampacity | Conductor material, cross-sectional area, insulation temperature rating, installation method, ambient temperature, and grouping. | The protective device rating must not exceed the allowable conductor ampacity unless a specific code exception applies. | A circuit using a conductor permitted for 16 A protection should not be fitted with a 25 A breaker merely because the load occasionally rises. | Have cable sizing and breaker coordination checked together, not separately. |
| 7 | Check short-circuit interrupting capacity | Prospective short-circuit current at the installation point. | The breaker's interrupting rating must be equal to or greater than the available fault current at its location. | If the calculated prospective fault current is 6 kA, a device rated below 6 kA is unsuitable for that position. | Request the required short-circuit rating, such as 6 kA or 10 kA, from the project designer or utility data. |
| 8 | Select the correct number of poles | Single-phase or three-phase supply, neutral switching requirements, and local code. | One-pole, two-pole, three-pole, or four-pole configurations must match the circuit conductors and isolation requirements. | A three-phase load may require a mechanically linked three-pole protective device so all phases disconnect together. | Confirm whether the neutral must be switched and whether shared-neutral circuits require linked poles. |
| 9 | Choose protection functions by risk | Shock risk, wet areas, outdoor circuits, surge exposure, sensitive electronics, and fire-safety requirements. | Depending on the design, the box may need residual-current protection, overcurrent protection, surge protection, or arc-fault protection. | Outdoor socket circuits may require additional residual-current protection according to local regulations. | Specify trip sensitivity, test requirements, coordination, and the required protection standard before ordering. |
| 10 | Allow capacity for future expansion | Expected additional circuits, spare ways, enclosure dimensions, busbar capacity, and service rating. | Reserve physical spaces and electrical capacity for future loads without exceeding the main service or busbar rating. | A panel with 24 usable ways may reserve 4 to 6 ways for future circuits, subject to the project design. | Confirm enclosure rating, ingress protection, heat dissipation, spare terminals, and maximum allowable circuit count. |