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I asked at college recently if when working out Ib (design current) do we use the nominal voltage (which will be a constant of 230v in most instances) or do we use the MI chosen voltage. As an example I've seen appliances rated at 3kW/240v according to the packaging. So is the design current 3000/240=12.5A or is it 3000/230=13A?
The answer the lecturer gave was you always work out Ib at the nominal voltage, ignoring what the manufacturer instructions said. I was a bit suspect of this answer, so Gods.....
EDIT: Bit more info as to why I'm asking. It might not make much difference in most cases, but take the example of a 9.5kW shower. If you work out Ib at 240v you can safely select In as a 40A breaker. However, work out Ib at 230v and you have 41.3A, which would require a 50A breaker and in turn would likely affect the size of the cable you have to install too.
The KW would be lower at 230 than 240.