Calcs for supply currents to a delta connected appliance.

Let's see if I've understood correctly...


you've got three single phase loads, a heater connected say L1-L2, another heater connected say L2-L3 and a light (transformer) connected say L3-L1.


It's all designed for 480V L-L but you've connected it directly to a 415V supply (no 415:480V transformer) and measured the currents in L1, L2 and L3.


If the loads were pure and constant resistances then increasing the voltage from 415 to 480 would indeed increase the current by a factor 480/415 - so that probably is a reasonable first approximation.


In practice though there are likely to be some complicating factors - heaters typically aren't constant resistance but have a lower resistance when cold, which then increases as they heat up - when running on 415V rather than 480V they're probably not quite as hot as they should be so their resistances might be lower than for normal 480V running - so the calculated currents for 480V might be an over-estimate. Similarly for lighting if it's of a filament type. The lighting transformer might also introduce some variation - especially so if it's of one of the electronic varieties and so attempting to present a constant load rather than a constant impedance to the supply.


    - Andy.

Let's see if I've understood correctly...


you've got three single phase loads, a heater connected say L1-L2, another heater connected say L2-L3 and a light (transformer) connected say L3-L1.


It's all designed for 480V L-L but you've connected it directly to a 415V supply (no 415:480V transformer) and measured the currents in L1, L2 and L3.


If the loads were pure and constant resistances then increasing the voltage from 415 to 480 would indeed increase the current by a factor 480/415 - so that probably is a reasonable first approximation.


In practice though there are likely to be some complicating factors - heaters typically aren't constant resistance but have a lower resistance when cold, which then increases as they heat up - when running on 415V rather than 480V they're probably not quite as hot as they should be so their resistances might be lower than for normal 480V running - so the calculated currents for 480V might be an over-estimate. Similarly for lighting if it's of a filament type. The lighting transformer might also introduce some variation - especially so if it's of one of the electronic varieties and so attempting to present a constant load rather than a constant impedance to the supply.


    - Andy.