3 Phase.. calcs and bit rusty!!

[DurhamSparky]

been asked to price up and install some cable for a farm...etc..

my 3 phase calculations are a bit rusty so trying to brush up....

NEDL Have dropped a 50kva supply in on 3x35mm incomers..!

first part of job is to wire a TP&N Isolator to there meter tails so they can sign job off and allow me and the farmer to then proceed with deciding what he wants..!

just checking im right here... Kw = KVA / PF ..... im led to believe that NEDL told the farmer PF is 0.98 so quite negligible in this scenario...

so deciding on what size Isolator to install i would assume that

50000/230 = 217A
217/3 = 72A per phase

which ties in nicely with NEDL paaper work of 70 Amps per phase (correct me if im going wrong.. please)

as for the isolator i will be fitting a 100A TP&N (any body got preference on brand???)

never been asker to work out the design of 3 phase circuits so its all new and i dont want to screw it up hence getting head back in books again to read up on stuff that you brain forgets!


seems pretty easy and self explanatory but just rusty!!

 

[Knobhead]

72A @ 400 / 230V Bang on
Want a copy of the spread sheet?

 

[DurhamSparky]

please!! might help...!! been over 5 years since i looked at 3 phase calcs!! and then i was drunk!

 

[Knobhead]

Sent, I take it your on nights

 

[DurhamSparky]

oh yes... im ermmmm manning the phone and keeping look out...! my turn to sleep tomorrow!

 

[Knobhead]

Just don’t do my mates trick. He found great place to kip, on the cable trays in the loft of a switch room, nice and warm. He rolled over, fell of the tray, through the ceiling, landed on the switchgear, fell off that and landed on the floor in a heap. Off work for a month with a bad back! I’m just glad I wasn’t the shop steward at that time.

 

[ezzzekiel]

can i have copy of spread tony got some 3 phase work coming up and been a long time since i touched it

 

[truckster]

CAn I have one too, 3ph comes along every so often and I always have to start re-learning

 

[Knobhead]

PM me with your e-mail addresses

 

[johnboy6083]

i worked it out slightly differently steve, but got pretty much the same answer. im gona write it down here, as it may help others, if im right.
if im wrong, then others will realise im an idiot.

kva= kw x power factor, so this means alos that kw= kva/power factor

kw= 50,000/ .98 = 51020 = 51.02kw.

I= P/V = 51020/400= 127.55A

127.55/1.732= 73.6 A per phase.

 

[Knobhead]

Hate to say this John but usable power from a transformer is lowered by the worsening power factor. So it goes down to 70.7A @ .98PF.

 

[Knobhead]

I’ve just altered the 3Ph spreadsheet to incorporate PF for transformers. Let me know if anyone wants a copy.