On Sunday, 12 November 2017 14:48:27 UTC, Johnny B Good wrote:
On Sun, 12 Nov 2017 04:00:43 +0000, John Rumm wrote:

On 12/11/2017 01:55, Johnny B Good wrote:

A gander at the rating plate should tell him all he needs to know
(notably, the maximum power rating and the nominal voltage - typically
since harmonisation, 230v). In some cases, the label may even show a
second higher wattage rating for 240v. In any case, since the heating
elements are a fixed resistance value, it's a fairly trivial task to
calculate the 240v loading from the 230v figures if more precision is
needed.

Quite allot of stuff specced for the UK market will quote the power
assuming its running on the genuinely manly 240V rather than the nominal
girly 230 euro volts.

So 2kW @ 240V suggests a load of about 29 ohms. So the actual power
could be anything from 1608W to 2207W if you swing across the full
permitted nominal 230V range of 216V to 253V

Indeed! As a matter of fact, I just spotted a response by ARW to this
thread just after posting wherein he considerately quoted the attribution
by Whiskey Dave who had posed the question as to why a test load of five
such heaters had failed to trip the 32A circuit breaker on a lab bench
supply of ,oddly, 220v.

Why oddly at 220V I don't think I've seen it much higher.


To reiterate, WD had asked us to guess the actual wattage in spite of
getting what I thought in the circumstance of the supply dropping from
220v down to a mere 202 volts (presumably with all five heaters plugged
in -

Yes all on ONE phase from ONE MCB breaker.


it would be extremely worrying indeed if this voltage drop was the
result of just one heater), a rather accurate 1.6KW reading on his
inexpensive Maplin energy consumption meter. A drop in power from the 2KW
(presumed to be specified for a 230v supply) level down to 1.6KW on a
202v supply is exactly what one would expect.

That's about it.


Assuming a 1.6KW load at 202v, I calculate a heater resistance value of
25.5 ohms which neatly ties in with a 230v rating of 2KW. The amperage at
202 volts is a mere 7.92. For five such heaters, this represents a total
loading of 39.6A which won't immediately trip a 32A circuit breaker.

Yes this is what we found we didn't expect immidiate unless all heaters were switched on at the same time,it was a trial to see what would happen if someone strted up everything at the same time, but it was over 2 hours before it tripped out.


As ARW succinctly pointed out, the problem would appear to be a wiring
issue (compounded imo, by the strange choice of 220v rather than the more
typical 240v of a UK mains supply). If the bench supply had stayed at the
220v level WD's test setup would have resulted in a current of 43.13A
which may still not have tripped the breaker in a timely fashion.

Well the idea was that we didn't want the power tripping during a lab or lecture so tried running worse case senerio with what we had to hand.



Even assuming that the mention of a 220v supply was a typo for 230v,


which it wasn't my office with ONE heater at 700W , 212V dropping to 211 when the heater switches to 1.7KW.


the
current would have only increased to 45.1A on 230v, rising to 47A on a
240v supply. WD really needed to use 6 or 7 such heaters to test trip
that 32A circuit breaker. TBH,

I didn;t have 6-7 heaters at the time just 5 .



I'm amazed that WD couldn't figure all of
this out instead of making a fool of himself in this NG by asking such a
foolish question to which he already had all the data he needed to
provide the answer he sought.

That wans't my original question that is why, and you've avioded the original Q.

If yuo have a 2KW heater switched to full on what is it;s power consumption adn of course you''ve got that wrong it's NOT 2KW not 1.6KW not even 1.4KW not even 1KW but 700W .

SO like opur so calledheating engineers must have done hopw many 2KW heaters does it take to raise the temperature from 14C to 16C.

How dos that change is the 2KW heaters donlt give out 2KW but only 700W how does that change teh calculation.


Now the original heating system was put in in the 50s , the engineers told the college that they system wasn't designed for such a long run through 2-3 departments i;e the heating conduit path was too long to maintaine the temperture. This is why the lectures offices get really hot so they academics open their doors and windows and by the time the heating conduit gets into the lab the air is almost cold if not colder than the outside temperature..

All we need is a heating engineer that knows stuff, both theory and in practice
It was 12C @ 9am, now 16C @ 13:45.

This is with 5 of 2KW heaters and 2 of 1.5KW heaters.

So if yuo wanted the demp to be 16C before the 10am class how many heaters would we need in total. ?