XFORMER SIZE, voltage drop

No 15 hp pump would use 200A even if 240 volts was the line voltage. why don't you just get the power from the hp? hp multiplied by 736 will give you watts 15x736=11040watts. what concerns the amperage it would be watts divided by (240x3) that will give you 11040/(240x3)=15.33A per phase neglecting the power factor.

If you have the power factor divide the amps by that. for example if your cosine phi is .75 the amp will be 15.33/.75=20.44A per phase.

But for choosing a transformer you need to consider that you cannot load it coninuously at 100%. 60-80% maximum continuous load is optimal.

one more thing all motors draw much more current at start up then they do running normally. these are called transient currents which last for a fraction of a second or sometimes for a few seconds. this current can be up to 3.5 times the nominal current. so your transformer should be sized big enough to tolerate the start up current.

As for the 100A for the lights you should divide your load equally ( as much as possible ) between the 3 phases. that is called balanced load. as it is not wise to have 1 phase loaded and the others underloaded as you're gonna have to buy a larger transformer.

Again you didn't mention 100A on each phase or all 3 summed up.

I can help you more if more details are posted

The major issue that you will have, is the voltage drop during the start of the pump. This depends on the starting method employed, but if you assume that the pump is full voltage started, (Direct On Line) then it will draw around 6 times it's rated current. This will cause 6 x the voltage drop fro the duration of the start.

You need to now the impedance of the cables and the impedance of the transformer to calculate the voltage drop.

Typical transformer impedances are in the order of 5 - 6%. That means that you will have a 5 - 6% voltage drop at full load of the transformer.

Best regards,
Mark Empson

http://www.LMPhotonics.com | http://www.LMPForum.com

Hello GISdude

In addition to the good advice given above:

To assist the pump starting, and reduce lighting "flicker", plus reduce power factor, ensure that Power Factor correction capacitors are connected as closely as possible to the pump motor.

If you don't have PF correction capacitors at the pump motor, at the instant of starting, current may be much in excess of 10x the run current for the pump motor.

With PF capacitors fitted, the instantaneous starting current is as Marke stated above, around 5x the run current for the pump motor.

The next thing you could do, is to use an electronic soft starter for the pump, to minimise starting current peaks.

Of course we don't know what type of pump you are using, you may have a well 2,000 feet deep, with the pump at the bottom, starting against a 2,000 foot head of water - because we don't know, as you have not specified the pump situation.

It is normal to install a transformer with spare capacity, to allow for losses, transient starting currents, and future increase in load - normally allow at least a 133% larger transformer than you need at the time.

Advise further please, with

Kind Regards....

Hi Sparks,

What you describe, is a very good practice when the customer lets you do the right thing. (and is willing to pay for it...)

As far as our GISdude is concerned, Why do we have to guess? what is his primary voltage? does he have or is he Planing to have tapes on his transformer or not? he isn't saying anything.

If I may guess, the 200A and the 100A have nothing to do with the real loads but rather they are probably the maximum distribution panel capacity as he reads it off the panel's door or wherever.

Wangito.

What are you talking about?

A 15HP pump motor, and I mention PUMP because it is considered as heavy load start, will consume 20Amps/φ running, and 5-8 times this during start.

Use soft start system of some kind, and KVAR's to correct cosine φ.

Oh. by the way, what is your primary voltage? and if you already have 240V why do you need a transformer?

Very vague question to put it mildly.

Wangito

Hello wangito

<"Oh. by the way, what is your primary voltage? and if you already have 240V why do you need a transformer?">

In the absence of a reply from GISdude his Supply is probably from an 11kV or HV equivalent distribution line.

Thus the need for an 11kV/240V transformer on-site.

As a matter of interest, in a similar situation when cables were already installed and were too small for the current taken by a larger pump yet to be installed, we have installed a pair of 3 phase local transformers, from 400V to 1.1kV then via the buried cable at 1.1kV, to a 1.1kV to 400 V transformer at the pump end, along with a soft starter and PF correction capacitors.

Although the pair of transformers were dearer than the replacement cable cost, because we did not have to excavate and replace the existing cable, overall costs were cheaper.

Thus the "line or cable loss" was far less at the higher voltage, but if you do this "trick" it is important that your line or cable is designed for that higher voltage.

If GISdude replies, perhaps he may care to clarify his particular installation.

Kind Regards....

The starting current due to pump motor starting if dol method is allowed will be 6 times the rated current.However the duration for which it persists will be low as pump is a low inertia load and can accelerateto rated speed in about 2 sec then the voltage will recover.On the other hand if star /Delta starter is used (which is more common) the voltage and current is reduced and the peak occurs during the change over whih is for fraction of second.Considering the distance cable sizing is very important so that u don't have a large voltage drop at the receiving end.

Where are you located?If in the USA, then you should be a licensed electrician to do this type of work, and you would know how to do the proper calculations.

Seeking advice for an ad-hoc installation is dangerous and foolish, and illegal.

All single motors must be calculated at 125% of nameplate rating.If multiple motors are used, the largest must be figured at 125%, and the rest at 100%.

A rule of thumb for transformer to power a motor is 1 KVA per Hp.This allows for starting current, and a reasonable amout of safety margin.

How much do you know about bonding and grounding detached garages or shops?

A ground rod, or not? A bonding strap or not?

Where do you use a GFCI breaker? AFCI breaker?

I thought so.

As for calculating the house load, you must know the square footage for general lighting load, what type of heat/cooling will be used, and many other factors.

My advice? Get a qualified licensed electrician to do the work before someone gets hurt.

Hi guys. I'm sorry I wasn't detailed enough for the OP.

I am on a 12kV primary distribution side (USA) with a 15kVA XFORMER that is feeding 120/240V to an EXISTING 2500 sq ft house w/ a 200A main breaker, AND I have an old 15 hp pump that has been there about 30 years. What I need to do is add 2 additional lights for this detached garage. This will add an additional 100A main breaker to the load. That is it for my load.

I know the rule-of-thumb for a motor is 1hp per kVA, this is why I'm concerned about the additional lights that are being added. The garage IS ABOUT 400 feet away, this is why I'm concerned about Flicker/VD - will I need to replace the XFORMER to a 20kVA or possibly 25kVA? If this is the case, the customer will need to pay for the upgrade.

Thanks for the reply's.

GISdude,

I suggest you check the NEC (National Electrical Code, NFPA-70) at a local library and read study Article 220, with particular attention to §220.3. There are a lot of unknowns which need answering before your questions can be answered. However, this article will ask for all these unknowns.

Regarding the code letters on the motors, these tell you the general range of starting current compared to running current. See Table 430.7(B). The term "locked rotor" means that the motor shaft is not turning, which is the condition with the greatest "inrush current". Others posting on this forum have said the inrush is 3.5x or 6x or 10x the running current. Well, the answer is "possibly any of the above and perhaps none of the above". Your motor's nameplate will tell you which one.

You must have enough voltage at the motor's terminals when it is starting to get it to run. If the distance is large, every trick you can use to decrease the starting current is good (such as the power factor correction capacitors or soft starts mentioned).

I agree with others, that if you have any doubt or are unfamiliar with this stuff, get your work and calculations checked by a competent master electrician or electrical engineer.

Regards--JMM

You make reference to a "customer", and by that I deduct that you are doing this work for hire.If you are not a licensed electrician, you are breaking the law, and to instruct you would make anyone who gives advice an accessory to the crime.

If you are a licensed electrician, how did you pass the exam with so little knowledge?

This is basic electricity 101.

You are aware that you will be liable for any damages that occur as a result of your work, even many years later.Insurance companies are very persistent and determined.You could lose your house,any property you may now own or aquire later, and even pull prison time if your negligence or ignorance causes personal injury of loss of life.

Proceed at your own risk.

Hello again GISdude

I sure hope you are properly licensed.

A 2500 sq ft house, with electric cooker and all the appliances which are used these days, is going to require at least a 25kVA transformer on its own.

If you add that pump, the details of which you give only the motor size, but not the type mechanically, nor the head of water it pumps against, the pump is going to require a 10kVA or 15kVA transformer all on its own.

So, the combined transformer requirement is going to be 35 - 40 kVA, and to allow future expansion, install a 50kVA transformer.

From much personal experience, that's what you require.

Also use a soft starter, plus the PF correction capacitors, at the motor terminals if possible.

Once the iron core and copper windings of the transformer are increased, along with the soft starter plus PF correction, those blinking lights should be a thing of the past.

In stating the above, ensure that your 12kV HV distribution supplier is able to provide sufficient power, (not just voltage), at the transformer primary connection.

In some cases, if you increase the transformer size, all you do is drag down the primary HV, because the 12kV network is insufficient to supply the connected load.

It is always possible, of course, that a loose joint or bad connection exists in the 12kV HV supply network, which would aggravate an already bad situation at the LV Load side.

Kind Regards....

I have always heard the longer the run the heavier the cable should be that way you don't have the flicker. With it being at full load of course and once the pump motor has filled the res. full of operating press. say 50 psi.

You got to be smart when your curious. - Gator