3 phase loading question

[Steve2381]

Hello all
I am working for a company and we are installing supplies to some industrial 3 phase heaters in a factory in Devon.
I am questioning the maths used by my boss to arrive at the SWA size to supply these heaters.

Seeing as I am not a wiz with 3 phase stuff (only just started working for this firm), can someone give me some pointers on calculating the load?
I know the basics, but would like to make sure I am right before rocking the boat!

We are supplying 4x 15000kw heaters - 3 phase.
What load is that going to be per phase, per heater? He has basically divided the 15000kw into 3 - therefore 5kw per phase, then divided it by 230v.
Arriving at 21.7A per phase.

That means with 4x 3 phase heaters on the distribution board, we are looking at around 88A per phase load.
That's a huge 348A load.

He has 'guessed' at a 35mm 4 core SWA for the 50M run. Not convinced any calcs have gone into that decision.

Thanks!

 

[Darkwood]

15000kw what you heating here the whole UK?

Show us your own attempt at calcs for said loading as choice of cable size and we will help you if needed... ive no problem with your question but you'd be surprised the threads we get just to hide the fact they want us to do all the calcs for them... not implying this is the case here but review your notes etc have a go and we will be happy to help.

PS 15000w may be easier on the supply cable than 15000kw

 

[ipf]

No, just Devon. 35mm should easily cover that with the 500mile run

 

[Paultrim123321]

348Amps on 35mm? You'l have a cable as a heater too if he carries on. Sorry I'm on my phone at the mo and out with family but just by the fact he hasn't used root3 in his calcs he is wrong. You need work out correctly three phase load using KW - Amps for three phase conversion first. Then you know your load. Then the fun starts, you need use many correction factors;thermal, grouping etc, length of run, how is he earthing the heaters? 5th core/SWA/seperate earthing conductor? These all need calculating.
As I said, sorry I can't give you an accurate answer at the mo but this needs more thought on your colleagues behalf.

 

[Spoon]

As Darkwood has said. Hope its 15kW heaters and not 15,000kW. You will need a big cable for that...

 

[Steve2381]

Ha... sorry all. Yes... 15kw heaters! Typing too fast for my own good.
Its not my job to be doing these calcs (and I am not up to the task as you can see!) I purely work for this firm and I am a little dubious as to their methods.
I come from a house bashing world... so 3 phase stuff is not my strong point.

All I know is the calcs I saw him scribble on the side of the drawings...... (15000/3)/230=21.7A
Seemed a little vague for my liking. No PF allowance or anything (do you need to allow for that?)

He then just announced 35mm 4 core SWA will be fine.

Judging by some of their other work to date.... I am not convinced.

 

[Marvo]

The heaters may have a power factor of 1 (linear load).

 

[Richard Burns]

If you have purely resistive heaters then it is possible that they are actually designed as three single phase 5kW heaters mounted in the same enclosure and using a different phase for each heater element.
Then your calculation given above is correct. 35mm² SWA can take about 125A and you have 88A so sounds like it should be OK, depending on installation method and length of run, etc.

This is making a lot of assumptions about the design of the heaters and the method of supply, so take it with a pinch of salt.
20kW of heating does sound a lot but then again I do not know the requirement for the heating.

 

[telectrix]

80A/phase approx. for the 4 heaters. 25 mm cable will handle that if there's not too many correction factors to allow for.

 

[ipf]

No probs with 35mm, although a bit oversized, off the top of my head.
Resistive load, presumably. Check your volt drop.

 

[Steve2381]

Thanks all.
The heaters are for some industrial air management system. Never installed one before.
Basically we supply an isolator and a third party connect to that.
I am guessing (but will try and confirm) that these heaters are simply a heater element per phase.
There are fans attached to the heaters, but these are separately supplied.

I think 35mm SWA will be a safer bet, just to cover any unknowns. Should not be too many factors to cover as its open clipped through an open roof space.

Again, any thanks all for your advice.
Only questioning this due to the last two complete cock-ups they have made on other jobs (not enough supply capability)

 

[Knobhead]

If it's 15,000KW let me know when you switch it on.

I'll watch it on this Dynamic Demand

 

[Megavolt]

Ha seems like your boss was right. What was these cock ups that have made you dout him ?

 

[JD6400]

If it's 15,000KW let me know when you switch it on.

I'll watch it on this Dynamic Demand

Tony , what a top site . That one has gone straight on my favorites bar ! unk:

 

[Steve2381]

Cock ups... wow, where to begin....

Installing a single phase board in a factory where most of the machines were 3 phase.
Running a huge bundle of cat5e cables in the same trunking as the supplies to the office ring circuits (for about 25M)
Installing a 3 phase SWA and isolator in a garage, and telling the customer to order a 3 phase oven... then I turn up to connect the oven and its only single phase at the incoming.
Frying the entire AV system in a house by fiddling around with things he doesn't understand behind a AV distribution system... and accidentally switching the voltage to 110v (how the hell he did that I don't know).

... just a few things make me worry

 

[Guest55]

he sounds a total liability lol.
start looking for a new job before he drags you down.
;-)

 

[micknew]

Your a right pair then.

 

[Tiny Spark]

Think ide be looking for another company in that case!

 

[Steve2381]

Yep... already looking around!

 

[Megavolt]

How is he still in business did he pay to but any of it right ?