Selection of Transformer and Cable Rating for A/C Units

You should have a separate circuit for each A/C unit...You need a distribution panel 150 amps and 3 ea 40 amp breakers and circuits for each unit....The minimum and maximum circuit size should be marked on each unit, and/or specified from the manufacturer....You should draw a diagram and label all components, including distance and location for all components....You need to know the power factor of the transformer...also need single or three phase designation....

http://www.aeicables.co.uk/literature/CurrentRatings.pdf

P_(kW) = S_(kVA) × PF

http://www.rapidtables.com/calc/electric/kva-to-kw-calculator.htm

http://www.csemag.com/single-article/selecting-sizing-transformers-for-commercial-buildings/4efa064775c5e26f27bfce4f0a61378e.html

thank you for your quick response.

Here is a suggestion. You've said nothing about the method of installation, cable lengths or ambient temperature. If you don't know what you're doing, then delegate the task to a qualified Electrician, who will select and install the correct materials and give you a certificate in line with your national electrical standards upon completion of testing that will satisfy your fire insurance company's most stringent requirements.

Don't wait around here! There are plenty of threads about cable sizing on here and they all say the same. I ought to report this to Admin as a duplicate thread, really.

Thank you for your advise ,

the installation method is In free air or on perforated cable tray .

cable length is 40 mtrs , ambient temperature is 45'c.

Then you have all the information you need to carry out the calculations in line with the applicable national electrical standard. Why waste time coming here?

Some of the values of interest are not calculated, but rather are looked up in tables according to decision procedures spelled out in local/regional regulations. You should obtain a copy.

Your local regulations may differ, but wiring to motors is often sized for 1.25 x FLC, taking into account ambient temperature and conductor grouping.

IIRC, time-delay fuses can be rated at 1.75 x FLC, and inverse-time circuit breakers at 2.25 x FLC.

All of this depends on the specific local regulations.

It's all in

Free access NFPA codes and standards

an electrician not only knows how to read this but also understands just how close the voltage can range above or below that actual line voltage to the machine. personally I'd ride on the high side.

How many phases?

In the UK, 380V would signify 3-phase live to live voltage, with live to neutral (single phase) being 220V.

In that case, splitting the 3-phase at the distribution board using single breakers sized for the individual units would be the simple solution. No transformer required.

Although theoretically possible, no normal 16kW 220V load would be single-phase.

Here in saudi all line to line is 380V and line to neutral is 220v for industries.

apart from the other household , commercial,building side is line to line is 220v and line to neutral is 110v.

now i have 380V 3 phase but the load is 220v 3phase 60hz.

how can i be take the single breaker to split the 3 phase.

"how can i be take the single breaker to split the 3 phase." You just don't get it, do you? As Crabtree suggest, hire an electrican and have it done properly so you don't burn the place down!

i know man it can't posible to do that ,

i am asking to him how he suggested me like this.

What we have, is failure to communicate.

I give up! Between #13 & #14 there's a total break down in communication. I foresee the Op as the next Darwin's Award Winner. Let's just hope he doesn't take anyone else with him!

Hi

Your original post did not state location, or phase inputs.... Or why 380V units were not obtained to match the supply available. Does the air con supplier not have these? It would appear simpler to exchange the units before fitting them?

Mohammed,

Going back to your original question. A current of 40 amps at 220 volts is 40 x 220 = 8800 watts = 8.8 kW single phase compared to your statement of 16 kW. Also 40 amps per line at 380/220 volts 3 phase is 40 x 220 x 3 = 26400 Watt = 26.4 kW.

.............so there is a fundamental error between the current, voltage and power you told us.

So that we can avoid mis-understanding, please give us the data from the "rating plate" on the load equipment (a rating plate image has been posted in reply, as an example, already).

67model

Thank you for your suggestion.....

that is old model but it's modified we have only compressor unit specification available later we checked by practically by means of applying load to one unit only.

As per my question let it assume 3 phase 220 v 40 amp(fla)

For kW conversion the theoretical formula is used

kW=V*I*pf*1.732/1000 assumed pf is 1

If you use this you will get 15.24 kW .

That's the value I entered in my question.

Thank you again for your kind answer

According to convention, 380/220 V is a 3 phase supply with 380V line to line and 380/√3 = 220V to neutral. This was the transformer you specified in your first post.

The supply you meant appears to be 220/127 V 3 phase, hence confusion.

The posts about need to have distribution board with separate circuit breaker and cables to each unit are good advice.

Each motor probably takes 6 times rated current during start and voltage at motor must exceed 80% despite voltage drop in primary supply, transformer and cables.

Assuming 48 kW transformer rating with 4% impedance, starting just one motor will draw twice rated transfo current causing 8% voltage drop in transfo.

Assuming 2% volt drop at rated motor current in cables transfo-motor will give 2%x 6 = 12% drop.

Voltage drop of 8% transfo + 12% cable = 20%.

So any time two motors start together will cause a start failure.

40m of cable with 2.54 volts drop @ 40 amp looks like 25 sq.mm copper or more.

There comes a time when too much info. could lead to a disaster. Without the actual data plates info., it's a shot in the dark for anyone. And why I nominate the OP for the next Darwin's Award. Again we should not suggest any remediation on this thread. (Students who get to college but aren't quite ready for college work need remediation.)

From your question, its very obvious that u lack d skill to do it, first u need to have d primary knowledge to do d job, then we can guide u here on some few stuff to make d job perfect, u are putting your life n d safety of a equipment at risk by trying to do of yourself, get a qualified engineer n learn from him.

Dear all,

thank you for your appropriate suggestion ,

my project has finished and also the a/c units are working.

materials used

35 KVA transformer

150 ampere main DB

63 ampere breaker individual a/c units.

25 mm2 xlpe copper cable for wiring.

know under load 43-45 ampere is taking each unit on breaker.

thank you all.

Glad to hear you got it working! Thank you for letting us know - often one does not know if a CR4 "poster" solved their problem. Do you have measured voltage at motor and current during start - when one motor is already running from transfo?

One thing I did not, till now, think about (you never did give picture of motor rating/connection plate).

This is that many standard motors have two equal windings per phase (6 terminals).

Consequently, a motor with 220V windings can use a 220V (line to line) 3 phase supply with Δ connection or 380V (line to line) with Y connection.

But I expect you already found motor had 3 terminals.

67model

According to The Saudi Arabian Distribution Code in this area are two rated low voltages 380/220 and 220/127 V. See:

http://climateobserver.org/wp-content/uploads/2014/09/Saudi-Arabia_Electricity-Distribution-Code-2008.pdf

So one of three a/c unit 40A 220 V S=SQRT(3)*220*40=15.24 kVA

3 units S=3*15.24=45.72 kVA

In my opinion 80kVA is 78% more than is required. However, if it is the next standard rated transformer it could be o.k.

The transformer rated current is 121.54 A [380 V] and 150 A /20 kA has to be o.k.

The 380 V[1/0.6 kV] cable ampacity has to be equal or more than 150 A.

Then, according to IEC 60364-5-52-where 90oC insulation rated is permitted 3*50 mm^2 [192 A for 30oC ambient corrected for 45oC 167 A] one single 3 conductor cable on a perforated tray copper conductor, 3 conductors loaded, XLPE insulation.

Since NEC does not permits 90oC 2/0 awg is required for 75oC insulation 45oC ambient.

2/0=67.4 mm^2 [3*70 mm^2 copper conductor XLPE insulation]

For outgoing cables of 40 A each-if they are all 3 on the same perforated tray IEC 60364-5-52

Tableau A.52-17 for 3 touching cable a 0.7 derating factor has to be employed.

Then 0.87*0.7=0.609 total derating factor I required 65.7 A. Then 3*25 mm^2 copper XLPE it is required.