On 29/11/2017 10:48, whisky-dave wrote:

Anyone with any insight would have known that 202/240 should give you the fraction that you use to multiply by so :-

No it doesn't.
It is worrying that you are apparently in an electronics teaching lab.

A heater of 2KW at 240V will consume 2000W
A heater of 2KW at 202V will consume 1683W

So if it's consuming 700W already we have 1683-700= 983W NOT 1300W that you expected.

He is more nearly right that you are. Anyone that understands the basics
of Ohms law will know that the power dissipation in a resistor R is I*V

= V^2/R

The power dissipated in a fixed resistor R scales as the *square* of the
applied voltage V all other things being equal.

Nominal 2kW at 240V = R = 29
So on a 202V supply it will consume 202^2/29 ~ 1.4kW

R won't quite be constant but it won't vary by all that much either.
R might be a percent or so lower at 1.4kW since the element is cooler.

Cheap and nasty resistive electric heaters are usually configured
series, single and parallel to present loads of V^2/2R, V^2/R and 2V^2/R
to the mains being low, medium and high heat settings respectively.

You perhaps ought to worry if your lab mains voltage is as low as you
say it is something somewhere must be dropping nearly 30V.

--
Regards,
Martin Brown