Design Current?

[Dannywhite]

how do i work out the design current of a circuit? ive been given the voltage, Ze and PFC..

its the highways depot assignment but they havent given me the power of any of the circuits.

 

[Andy-1960]

If that is all you have been given I don't believe you can..

 

[telectrix]

as above. you need the wattage of all loads on the circuit as a start.

 

[Richard Burns]

You could work out the design current if you have the resistance of the loads, but Watts or kilowatts is more common. Even if you have been given a figure of 12 light fittings using B22 incandescent bulbs you could assume a maximum of 100W per bulb.

I would expect that there is some clue somewhere about what will be connected to the circuit otherwise you cannot proceed with the design.

 

[DavidSpark]

As the assignment expands it's has questions relating to planning final circuits for the installation so at some point you do work them out.

 

[Dannywhite]

how do i work out the design current for lighting then? it says the high bay lighting ia 150w each and 12 lights on that circuit..

also sockets i think we just assume the wattage for it like theres going to be a max of 3kw on being plugged in or something. anyone help with formulaes for each?

 

[telectrix]

how do i work out the design current for lighting then? it says the high bay lighting ia 150w each and 12 lights on that circuit..

also sockets i think we just assume the wattage for it like theres going to be a max of 3kw on being plugged in or something. anyone help with formulaes for each?

you have 12 lights @ 150watts each, so your load is 150x 12 = 1800 watts. using ohms law, I = P/V = 1800/230 = 7.827 amps but that's not all. you also need to allow for inrush currents.

 

[DavidSpark]

In regards to the lighting circuit, how would you think you'd work it out?

What type of power circuits can you have for socket outlets as shown and described in BS 7671?

 

[Richard Burns]

For socket circuits you could look in appendix A of the on site guide that gives ideas for the calculation of the demand for various circuits, however it is not overly accurate and may give rise to high demands.
If you consider the anticipated use of the circuit considering the area in which it is installed and the likely type of equipment to be used you should be able to approach a reasonable estimate of the current demand.
If you were to assume that there were 3kW plugged into every socket you would rapidly overload the circuit. For instance a 20A radial circuit would be potentially overloaded with just two 3kW appliances but common use means that the circuit is not overloaded because the appliances are not all used at the same time.
This is the application of diversity which is critical to the possibility of designing sensible circuits.
Have a think about how you would go about this and come back with your ideas and we can give you more feedback.