Three phase calculation

[Simonslimline]

I see it was a waste of time coming on here not even one decent answer

You won't get far with that kind of attitude to be honest. Try showing your calculations first and people will be more willing to help out.

As has already been said, it would be a good idea to get access to the trainee section on here, seeing as you are an apprentice.

 

[Lucien Nunes]

40.41Amp

Too many SF. Do you really know the current to within +/- 10 milliamps using the information given?

 

[Jonspark]

Too many SF. Do you really know the current to within +/- 10 milliamps using the information given?

weve already got

p=28000
u=400
pf=1
and 1.732
so according to the formula isn't that correct, or have I made a mistake some where?

 

[Lucien Nunes]

The answer is numerically correct but as this is possibly a student asking I was suggesting the good practice of quoting the correct number of significant figures in the answer for the accuracy of the given data. 28kW could be 27,500W or 28,499W making the current anywhere between 39.7 and 41.1A at 400V, yet you gave it specifically as 40.41A i.e. between 40.405 and 40.415A. Call me picky if you like.

The funniest one I saw was in a plant handbook where a pump was described as being driven by a 1491.3996W motor. Whoever had the job of doing the metric conversions did not recognise a 2 hp motor when he saw one.

 

[HandySparks]

The answer is numerically correct but as this is possibly a student asking I was suggesting the good practice of quoting the correct number of significant figures in the answer for the accuracy of the given data. 28kW could be 27,500W or 28,499W making the current anywhere between 39.7 and 41.1A at 400V, yet you gave it specifically as 40.41A i.e. between 40.405 and 40.415A. Call me picky if you like.

The funniest one I saw was in a plant handbook where a pump was described as being driven by a 1491.3996W motor. Whoever had the job of doing the metric conversions did not recognise a 2 hp motor when he saw one.

I know what you're saying, but I don't think you should draw ANY conclusion from the number of significant figures shown.

For example if I bought a 3m length of conduit, I wouldn't expect it to be anywhere from 2.5m to 3.5m. And if the conduit was described as 3000mm long, I wouldn't be surprised if it was actually between, say, 2990mm and 3010mm.

You either have to make some reasonable assumptions (which is what all of us do, most of the time), or refer to a relevant standard for manufacturing tolerances, or ask the originator of the figures what the tolerance is in that particular case.

 

[Lucien Nunes]

Your example is a good one and your final point hits the nail on the head. Applying it to the quoted answer would suggest to me that the 4 digit precision given is not appropriate to the source data, regardless of the number of SF in the question. I was just hammering out the implications of this longhand...

 

[DPG]

Let's just call it 40 eh. Daz

 

[Knobhead]

40.41Amp

I suppose you thought you’d look clever in the eyes of the OP by doing his work for him. Your problem now is all that have taken part in this thread have quite the opposite opinion.

 

[Besoeker]

I see it was a waste of time coming on here not even one decent answer

This from Tel:

P=√3VIxpf

isn't just a decent answer. It is the entire answer.

 

[Jonspark]

I suppose you thought you’d look clever in the eyes of the OP by doing his work for him. Your problem now is all that have taken part in this thread have quite the opposite opinion.

Just give em the answer, if they don't want to do the work themselves it'll come back to bite em one day.
That's my view, like it or lump it [emoji6]

 

[HandySparks]

...For example if I bought a 3m length of conduit, I wouldn't expect it to be anywhere from 2.5m to 3.5m. And if the conduit was described as 3000mm long, I wouldn't be surprised if it was actually between, say, 2990mm and 3010mm....

And if the conduit is described as 300cm long, it's probably from Homebase, because retailers think that the general public need units that are as close as they can get to 'inches' !

 

[telectrix]

and whats wrong with inches. easily measured from end of thumb to 1st joint? bloody french centimeters, based on an inaccurate measurement by ratbag revolutionaries that chopped the heads off anyone with a brain.

 

[TheHUNTED]

Just give em the answer, if they don't want to do the work themselves it'll come back to bite em one day.
That's my view, like it or lump it [emoji6]

So what about the poor customer who has most likely searched for an electrician to come and wire a new machine in for his business, but ends up getting somebody who can't be bothered to work out the load. If that's the case I'd hate to see the rest of the install.

 

[telectrix]

to be fair to OP, he is an apprentice. he should change his status on his posts.

 

[DPG]

Probably a homework question. Daz

 

[Deleted member 72666]

and whats wrong with inches. easily measured from end of thumb to 1st joint? bloody french centimeters, based on an inaccurate measurement by ratbag revolutionaries that chopped the heads off anyone with a brain.

It's not just me then that uses his thumb to measure multiples of 1½".

 

[DPG]

I use something else

 

[Deleted member 72666]

I use something else

Lol, is that what you were doing?

 

[telectrix]

i have to admit to using something else to measure feet.

 

[GMES]

I use something else

i have to admit to using something else to measure feet.

You know what they say about guys who shout the loudest!!