How to work out min cross sectional area of live conductors?

[Simonslimline]

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.

 

[hightower]

The KW would be lower at 230 than 240.

I know, hence why I'm asking if we strictly must use 230 to work out design current even if the MI states it's been rated at 240?

 

[Andy78]

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.

If you want to work out current drawn at different voltages then.....

As the resistance of the element is the constant you would work out that and then apply it to the voltage. Using your example of 3kW at 240V (sorry about the clumsy sum but I don't know how to type a squared symbol)

R=V*V/P
240*240/3000
R=19.2

P=V*V/R
230*230/19.2
P=2755

So 3000W at 240V is equivalent to 2755W at 230V. You can then work out the current drawn at each voltage.

In practice you would use the manufacturers higher rating as Ib.

 

[hightower]

If you want to work out current drawn at different voltages then.....

As the resistance of the element is the constant you would work out that and then apply it to the voltage. Using your example of 3kW at 240V (sorry about the clumsy sum but I don't know how to type a squared symbol)

R=V*V/P
240*240/3000
R=19.2

P=V*V/R
230*230/19.2
P=2755

So 3000W at 240V is equivalent to 2755W at 230V. You can then work out the current drawn at each voltage.

In practice you would use the manufacturers higher rating as Ib.

That's the answer I was looking for.

 

[Simonslimline]

That's the answer I was looking for.

Sorry mate i slightly misread what you typed. Waking up slowly i think.