Design Current
First thing to do is calculate the design current. (Ib) I=Watts/Volts. You do this so you can find out the total current.
Apply Diversity Factors (where applicable)
Diversity is described in the onsite guide Appendix A as a way to determine the maximum demand for an installation and includes the current demand for commonly used equipment. For example if in a domestic household there were 8 lights on a circuit, not all the lights would be on at the same time therefore the expected load will not reach the maximum amperes. The On Site Guide states in Table A2 that when completing a lighting circuit in small hotels, boarding houses or guest houses the diversity is 75% of total current demand. Therefore before calculating current using I=Watts/Volts you must first calculate 75% of the power. In some final circuits water heaters (thermostatically controlled),floor warming installations and thermal storage space heating no diversity allowable this is because there is no drop in the current demand with these. Select Circuit Protective Device (In) A circuit protective device or circuit breaker is used within a circuit to make, carry and break normal loads current. It is also used to break any abnormal currents e.g. short-circuit currents. Once current (Ib) is calculated then refer to Part 4, Table 41.3 in IET Wiring Regulations. The circuit breaker (In) has to be equal to or greater than design current (Ib). The current has to be able to pass through the circuit breaker it is used to protect the circuit from damage caused by overcurrent or short circuit. Circuit breakers have to comply with BS EN 60898. Calculate Tabulated Current Tabulated current (IT) comes into factors when cables are bunched together or embedded in insulation or in high ambient temperature (room) temperatures. …show more content…
All the correction factors can be found appendix 4 table 4D5. A healthy circuit will always produce a certain amount of heat. When the heat builds up and can’t escape, the resistance will increase and will cause the PVC to melt and could catch fire. If the tabulated current is more than (Ib) you will need to increase the conductor size, instead of a 10mm conductor you might need to use a 16mm, it will create less heat as the cable will be larger so the current is below its capabilities. You can find the rating factors (Ca) for ambient air temperatures other than 30 degrees in table 4B1.The formula for tabulating current is It= In/ca so the protective device divided by the correct correction factor.
Select Cable
The cable (Iz) has to be greater or equal to than (It) Tabulated current. Now you would go to appendix 4 table 4D5. There is 6 different methods to choose. Example it could be reference method C, clipped direct or method 102 in a stud wall in thermal insulation with cables touching the inner wall. You would go down the list and pick the conductor that is capable of carrying the current. If the conductor is greater than 16mm you will need to go to table 4D2A.
Confirm Voltage Drop Meets Requirements The voltage drop is to determine the volt drop across the circuit. To calculate the volt drop = (mv) mini volts x amperes (ib) the design current x the metres of the cable run, and then divide them by 1000. So mv/a/m 1000 For low voltage installations directly from low voltage