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Tarrel
Joined: 29 Nov 2011 Posts: 2447 Location: Rossshire, Scotland

Posted: Wed May 01, 2013 11:44 am Post subject: 


adam2 wrote:  Tarrel wrote:  Actually, current would be my biggest concern with voltage fluctuation. Would I be correct in thinking that, say a 3kW load was running at 250V, the current flow would be 12 Amps, but if the same load was running at 200V, the current flow would be 15 Amps? If so, cabling that was underspecified, or "on the limit" could be at risk of overheating or fire if the voltage dropped.
Correct me if I'm wrong. 
It depends on the type of load.
A simple resistance heater will use LESS current on a lower supply voltage. There is no question of overloading wiring.
If the heater uses 12 amps at 250 volts, then at 200 volts it will only use 9.6 amps.
The heat output would be much reduced, to about 2KW.
The problem would arise if the reduced heat output was noticed and compensated for by the use of additional heaters.
Consider an area heated by a pair of 3KW heaters, at 250 volts they will each use 12 amps, or 24 amps in total.
Now suppose that the supply voltage drops to 200 volts and each heater uses 9.6 amps, or 19.2 amps in total, no problem.
But what if someone notices that the space is not sufficiently warmed and uses a third heater ? Each of the three heaters now uses 9.6 amps or 28.8 amps in total. That might be a problem if the original wiring was only just suitable for 24 amps.
In theory the worst that could happen would be a blown fuse, but in practice a problem might occur.
Other loads such as switched mode power supplies use constant power, and at a lower supply voltage will use more amps.
3KW worth of IT equipment and electronic lamp ballasts will indeed use 12 amps at 250 volts, and 15 amps at 200 volts.
A properly designed installation should be designed accordingly, but in practice it might be a problem in cases that are already marginal.
When assesing the current demand of an installation, I presume an actual voltage of 250 volts for heaters as this is close to the maximum that can reasonably be expected.
In the case of switched mode power supplies then I presume an actual voltage of 200 volts as this is about the lowest that can reasonably be expected. 
Thanks for that. I've learned something this morning! _________________ Engage in geoengineering. Plant a tree today. 

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