Cable sizing

[thomaspotato]

Hi all,

I am trying to calculate a SWA 4 core that will be protected by a 250 amp switched fuse isolator. It is going to supply a 3 phase electric boiler that pulls 209 amps each phase at most. I'm filling in the parameters to an online cable calculator ,when asked for the voltage i've put 400 v, when asked for load in amps i've put 250 amps. Is this correct or should i be putting the total amps 627 because thats the total 3 phase load?

 

[nicebutdim]

I wouldn't trust any online tool with this calculation. No come back if specified cable is incorrect and that could be costly. Better to calculate yourself and know that all factors have been accounted for and result is correct. Load will be per phase as they taught you in college.

 

[thomaspotato]

I wouldn't trust any online tool with this calculation. No come back if specified cable is incorrect and that could be costly. Better to calculate yourself and know that all factors have been accounted for and result is correct. Load will be per phase as they taught you in college.

Yes you are probably right but can someone recommend a software i can use? 30 year since i was at college and i can remember the time it took to size a cable with all the factors

 

[DefyG]

There is plenty of software out there but at a cost!

If you haven't needed it for past 30 years, it wont be viable to buy a package for this one off project.

 

[mainline]

Hi all,

I am trying to calculate a SWA 4 core that will be protected by a 250 amp switched fuse isolator. It is going to supply a 3 phase electric boiler that pulls 209 amps each phase at most. I'm filling in the parameters to an online cable calculator ,when asked for the voltage i've put 400 v, when asked for load in amps i've put 250 amps. Is this correct or should i be putting the total amps 627 because thats the total 3 phase load?

The total current capacity of the system in amperes is the same as the per-phase rating when referring to a balanced three-phase system. Thus, a 200 amp 3 phase service provides a total of 200 amps across the three phases, not 600 amps..

 

[BradTW]

The total current capacity of the system in amperes is the same as the per-phase rating when referring to a balanced three-phase system. Thus, a 200 amp 3 phase service provides a total of 200 amps across the three phases, not 600 amps..

Nobody seems to be calculating the cable size only the fuse circuit breaker size. The current carrying capacity of the cable is not only dependant on the load like the fuse size is. It depends on the method of installation, proximity of other circuits environment etc. Also volt drop comes in to cable sizing not just current carrying capacity. Any or non of the above factors may be relevant dependant on the method of installation and length of run. Once you have the load current then you can decided what if any multipliers are relevant and calculate the actual cable c.s.a. and also the potential volt drop. Then you need to calculate maximum short circuit current carrying capacity for your cable and finally your earth cable size.