calculating power

[securespark]

What's the joke?

 

[ericmark]

liampope:

1300 watt divide 230 = 5.7A

1300 watt divide 240 = 5.41A

the more voltage with the same current is LESS amps as there are more volts to divide the amps by!

Oh dear seems maths is not a strong point?

Watt = Joule per second but in electric AC circuits considered as volts x amps x power factor correction in DC of course no power factor correction.

Rated watts is rated volts x rated amps so impedance = rated volts² divided by rated watts.

So 1300 watt = 230²/1300 = 40.69Ω
So 240v at 40.69Ω = 5.9A or 1415W

There is a big difference between rated watts and actual watts and even my calculation is not really spot on because the resistance of tungsten changes with temperature so if you measure a bulb with an ohm meter and then try calculating the current you will find when measured the figures do not match.

Discharge lights are very bad with over voltage. I found out to my cost. I was to light some tunnels and use 60W 110V fittings. To keep within the 16A supply I should be able to use 30 fittings but to be on the safe side I was just 25 so should have been 13.6A with a Loadmaster MCB (No magnetic trip on these) there should have been no problem. 2 hours latter called as lights had gone out. Use clip on measure 25A no wonder it had tripped. So I tested one lamp in work shop and I found the volts were more like 120v so using same as above total load should have been 14.9A (0.6A each). Because these were 110 volt there was an auto transformer and it had 110v and 127v tapping so by changing to the 127v tapping the current dropped to the 0.55 to 0.6 amp expected so first 20 set to 127v and last 5 to 110v as volt drop on cable would stop last few firing.

I mention this as an example of how relying on calculations can also get one into trouble and in hind site I should have used my clip on in first place.

Modern units may be switch mode HF units an not have this problem but I would say if one expects the lights to be all left on then a 6A MCB should only have a 4A calculated load. However one would hope all lights would not be left switched on so it should be OK.

However the calculations are only level 1 or 2 and to complete C&G 2330 is level 3 so I would have thought this would have included at least imaginary numbers and ohms law should be easy peay!

I will admit when asked to work out power factor correction I have to get out my text books and I hope I have a PC with one of the maths programs installed but rated watts to amps should be simple to any second year apprentice.

 

[SUNRAY]

Sereral years ago I had to drill loads of holes in plasterboard on a building site and accidently took a 240V drill with me (wrong drill in the box) so I scrounged a yellow plug from a site electrician. When he realised what I intended doing he snatched the plug back and using a few @*+# type words explained to me that 110V tools run at a higher current than 240V and my drill would blow up because it couldnt cope with the extra current.

What a fool he was, I did it, it worked surprisingly well, while cutting holes in plasterboard I hardly noticed any difference apart from less noise than usual.

 

[ban-all-sheds]

oh my lord this has become a joke .listen i can do ohms law

No you can't.

my question was based around the lights being on 12v transformers and basically just making sure my sums were correct which is all i was doing just making sure !

Don't judge everybody by your intellectual standards and think you can get away with b*****ks like that.

You need to make sure that you've done a simple calculation like that correctly when you have been doing a 2330 course for 2 years?

FFS.

 

[SUNRAY]

Some of this is just too frightening. The scary bit is when they have been out in the field on their own for 5 years and still cant cut the mustard.

 

[ceraunic_level]

I am only a student but this is my understanding.

VP / VS = IS / IP


230 / 12 = 4.16 / 0.21


P= V X I

12 X 4.6 = 55.2W (19 X 55.2 = 1048.8W) 1048.8 / 12 = 87.4 I

230 X 0.21 = 48.3W (19 x 48.3 = 917.7W) 917.7 / 230 = 3.99 I


The current is high on the secondary of the transformer compared to the Primary. This is why you have a short cable connecting transformer to lamp.

The primary current is what you would use to calculate max demand which is 3.99 I

I THINK thats right but stand to be corrected

 

[bernardgreen]

I am only a student but this is my understanding

Nothing wrong being a student, so say "I am a student"

Correct but not clearly expressed

12 X 4.6 = 55.2W (19 X 55.2 = 1048.8W) 1048.8 / 12 = 87.4 I

is far better on three separate lines,

12 X 4.6 = 55.2W

19 X 55.2 = 1048.8W

1048.8 / 12 = 87.4 A ( 87.4 A not 87.4 I )

It also helps to add notes
VP / VS = IS / IP
where
VP = volts Primary
VS = volts Secondary
IP = current in primary
IS = current in secondary

Writing 3.99 I means 3.99 times some unknown current I

Writing 3.99 A means a defined current of 3.99 Amperes

 

[chapeau]

And even better still would be to show the derivation of

VP / VS = IS / IP

from fundamental law(s) of physics.

Though that is probably degree level stuff in todays dumbed down educational world

 

[LiamPope]

I THINK thats right but stand to be corrected

Only problem I can see is that your 4.16 IS (50/12), became 4.6 in your power calc - typo? Your 0.21 IP (50/230) ought to have been rounded to 0.22. That's why your watts figures are so different and you appear to have a >100% efficient transformer .


Anyway, sorry for helping to cause so much confusion! I should clarify that I glossed over/missed the fact the OPs 50w lamps were 12V. My comments were more general anyway, but perhaps I should have qualified they apply to a simple passive load (i.e. lamps). It has correctly been pointed out that where the load is, for example, a SMPS for SELV lighting, it will behave closer to a fixed power load. So everyone's right in one way or another . Though I get the impression some may not fully understand exactly when they are right.

Liam

 

[ricicle]

What's the joke?

Well, I think the circuit is grossly overloaded already.

Those lamps at 12V will be drawing in excess of 79A.

At that rate, you should have each fitting on its own 6A breaker.

To make switching a little easier, you could pair them up & fit a 10A breaker for each pair.
__________________

 

[spark13]

bas , let me first start by saying that if you had spoke to me like that face to face man to man you would be picking your teeth up by now and i very rarely lose my temper but you think that HIDINGbehind a computer and coming across as this person who basically dislikes everybody who you presume are not of the same intelligence ! you take this as a sign of strength and people on here just take that crap not me mate .
the big reason i posted this question was to see how friendly and approachable this site is and fair play the majority are very good but yet again u showyourself to be somebody who gets their kicks blasting people and all behind a keyboard. i can do calcs and will have u know am doing very well at college but i dont need to justify myself to u or anybody else on a message board which should be a place to ask questions before acting on ideas !!

 

[ban-all-sheds]

bas , let me first start by saying that if you had spoke to me like that face to face man to man you would be picking your teeth up by now

I expressed incredulity because I was, and still am, incredulous that someone could be 2 years into a 2330 course and still not understand Ohm's Law, or not understand it well enough to have confidence in their ability to do simple sums.

And for that you want to commit GBH. How nice.

and i very rarely lose my temper but you think that HIDINGbehind a computer

I'm not hiding.

and coming across as this person who basically dislikes everybody who you presume are not of the same intelligence !

You tried spinning a line which was patently ludicrous. You obviously thought it would work; I presumed because to your powers of reason and intellect it seemed believable.

I suggested that you shouldn't make the assumption that everybody else had that level of intellect.

It isn't dislike, but even it it were it wouldn't make me want to resort to violence because I'd run out of other options.

you take this as a sign of strength and people on here just take that crap not me mate .

That's because you're a nasty little thug.

the big reason i posted this question was to see how friendly and approachable this site is

So did you come here and lie as some sort of test?

i can do calcs and will have u know am doing very well at college

Just so long as they don't expect you to be able to cope with Ohm's Law, eh?

but i dont need to justify myself to u or anybody else on a message board which should be a place to ask questions before acting on ideas !!

No you don't, but as you ask questions which beggar belief, and threaten criminal violence against anybody who expresses disbelief, I hope we'll not be seeing you around here again.

 

[spark13]

i never threatened anybody the fact that u think u can talk to people however u want and expect to just roll through people without any repercussions to me is the act of a bully ! your response tells me that u never normally get pulled up on your posts which i think is sad considering the way u talk to people and this is the reason i speak out, ur a person whos posts reflects your behaviour and your overall lack of respect to anybody bar yourself!
i never and i hope never will address people in the manner that u do and like i say i STRONGLY suspect that your are not that blunt or dare i say BRAVE enough to approach people in that manner in the REAL WORLD which is why i think your computer is sadly your way of gaining a shot in the arm for yourself on a nightly basis

 

[ban-all-sheds]

i never threatened anybody

bas , let me first start by saying that if you had spoke to me like that face to face man to man you would be picking your teeth up by now

the fact that u think u can talk to people however u want and expect to just roll through people without any repercussions to me is the act of a bully !

You've been doing 2330 for 2 years and you don't know the relationship between volts, amps and watts?

So what's bullying about that?

Tell me - how many weeks into a 2330 course do they cover off the simple aspects of Ohm's Law in single-phase circuits with unity PF?

Stop blustering about how hard done by you are, and answer that simple question - how many weeks in?

oh my lord this has become a joke .listen i can do ohms law

No you can't.

What's bullying about that, given that you had to ask this:

fuse is 6 amp mcb i already have 3x100 watt lights, but in kitchen i have 20 spots which are on transformers 12v 50w bulbs can anybody help to calculate my current ampage in this circuit please???

?

Stop blustering about how hard done by you are, and answer the simple question - why did you ave to ask that?

my question was based around the lights being on 12v transformers and basically just making sure my sums were correct which is all i was doing just making sure !

Don't judge everybody by your intellectual standards and think you can get away with b*****ks like that.

What's bullying about that?

Do you really expect people to just accept without comment the assertion that after 2 years in college doing 2330 you are still incapable of working out (3x100 + 20x50)/230?

Stop blustering about how hard done by you are, and answer the simple question - why can't you work out (3x100 + 20x50)/230?

your response tells me that u never normally get pulled up on your posts which i think is sad considering the way u talk to people and this is the reason i speak out, ur a person whos posts reflects your behaviour and your overall lack of respect to anybody bar yourself!

Respect?

bas , let me first start by saying that if you had spoke to me like that face to face man to man you would be picking your teeth up by now

Sorry - not now - I don't respect violent sub-humans.

i never and i hope never will address people in the manner that u do and like i say i STRONGLY suspect that your are not that blunt or dare i say BRAVE enough to approach people in that manner in the REAL WORLD

You're wrong.

I guess in the real world I don't have to deal with thugs who resort to violence when they run out of any civilised and rational ways to argue their case.

which is why i think your computer is sadly your way of gaining a shot in the arm for yourself on a nightly basis

So what is it for you then?

What is it for someone who 2 years into a 2330 course still can't work out (3x100 + 20x50)/230?

 

[spark13]

quite clearly i can work out the sums and admittedly posted a ridiculous question which i did realise v quickly and somewhat embarassingly and you over using the same quote over and over is a sign you cannot defend the acquisations put to you without throwing the same old line back in time and time again very sad very childish
ive seriously had enough of this now the very last thing i want to do is argue every post and be very childish about this .i merely have and always will stand my ground and fight my corner only wen i feel somebody is trying to be-little-bully-embarass watever u wanna call it that is my feelings u have ur own which u have already stated but by carrying this on i believe i am turning into something i really dont want to be ,behaving like something i dont want to be and generally behaving like U believe it or not i read ,respect, ur comments at times and would like to leave it at just that and nothing more more than that look forward to ur and other peoples posts