Sizing 3phase Transformer (Motor application)

If you are sizing a transformer for a fire pump, that would appear to be a minimum requirement. Section 695 deals with fire pumps.

Correct. However, the NEC is primarily concerned with safety, and sets forth minimums to establish reasonable safety. However, conformance with these minimums does NOT guarantee that the system will function acceptably.

Again, safety in the case of a fire pump generally means running the motor to destruction, if necessary. Not much sense in shutting down the motor if the building is still ablaze. Saving the motor and sacrificing the building is poor economy.

This is an engineering issue, rather than a code issue. It really depends on how much voltage drop you are willing to tolerate during the starting period.

In very round numbers, in the case of a transformer feeding a single motor load, the voltage drop ( in percent) will be equal to the transformer impedance multiplied by the percentage of transformer full-load rating drawn during the starting period. There are many assumptions built in to this statement, but these assumptions are usually valid enough.

For example, let's take a transformer of 5% impedance with rated secondary current of 100A. If you connect this to a motor which draws 600 LRA it is reasonable to assume (to a first approximation) that the transformer secondary voltage will drop 30% during the starting period. At first glance this looks like a problem, but as far as the motor is concerned it probably doesn't matter as long as you have enough starting torque at the reduced voltage. Remember that reduced voltage starting techniques routinely start motors on 50 percent voltage and even less. The transformer won't care as long as the starting period is only a few seconds.

The problem you may encounter is that control devices; relays, contactors, etc. tied to the secondary of the transformer may not work properly under the low voltage conditions. If this is the case it is probably most economical to run the controls from a separate transformer so that they are not exposed to the low voltage during the starting period.

If your transformer is feeding only a single motor, and you can tolerate the reduced starting torque, you calculations should be OK.

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I agree. Too often the NEC is used as a design manual. It is not a design manual. The NEC only contains minimum design rules for providing a minimum margin of safety for some installations. Engineers seldom design to minimum standards from my experience. Engineers should do electrical designs and electricians should stick to installing these systems. I prefer to have a competent engineer doing the design job. It just makes the job easier for everyone involved.

Reply to
Gerald Newton

is standard:


  • 7A) *208 *

transformer 480/208V

part of the


The motor is designed to start that way...the voltage drops regardless. If you were to design as though locked rotor was constant everything would be unviably massive.


It was always that way. If you fused or overloaded for LRA you would have no protection on the 99.999% dominant run cycle... you do use time delay fuses though in m motor circuits in order to handle surge (where fuses are used instead of breakers)...breaker and OL devices are heat sensitive with a mass that must heat up before they trip so you get a built in time delay for start current.

Phil Scott

transformer etc. via

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Reply to
Phil Scott

sometimes I

piece of equipment

tasked with

books, sites,

industrial environment.

What works for me across a broad spectrum is to observe broadly and ask yourself if you see anything wrong with the system you are observing, You can do this even driving by some places..and what could be done better...there is always something... or differently and what the advantages and disadvantes of the myriad options are... that thinking will help a lot... then read the trade journals especially the ads...bogus ads and legitmate ads, right or wrong the ads raise key issues in the industry in condensed form for your consideration.

Then when you design a system look at from all perspectives, later expandability...isolation of loads..reduction of arc flash hazards, problems in the larger grid and how you can design your plant to be affected the least by those.

the more integrated your thinking across these broad spectrums the better engineer you will be... then constantly look for your own find you can do 5 or 10 iteration of all the design options before arriving a truly good one... My first is usually in error.

My 10th iteration (most of which I do with calcs stage one, then pencil on scratch paper) is the best...then I move it to CAD... the oposite of what most managers want...they want the first iteration on CAD and for that to be largely the final iteration...

.... thats actully not workable however in complex environments and most environments these days, even a hot dog stand is complex if you consider all factors. Most however cant see many factors so they think they are seeing it all...and with great confidence proceed to produce yet another disaster.

Phil Scott












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Reply to
Phil Scott

I did this calculation both ways but I don't know which one is standard: given 142 HP 3phase motor SF 1, 208V, LRA 2702. FLA 359 7A Transformer

Using NEC (695.5? don't have my code with me) [1.25* (359A + 7A) *208 *

1.732] /1000 = 164.82 kVA

From this I conclude that I'll need at least a 225kVA transformer 480/208V

My question is why didn't they factor in the LRA of 2702 as part of the Transformer's size? The high in-rush will affect the transformer's output voltage.

The NEC did go on to use the LRA to calculate overcurrent protection.

Long story short, I tried the calculation another way:

by calculating the short circuit capacity of a 225kVA transformer etc. via the link below it worked also.

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Any assistance will be appreciated. Thanks, KIP (Knowledge Is Power)

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| I agree. Too often the NEC is used as a design manual. It is not a design | manual. The NEC only contains minimum design rules for providing a minimum | margin of safety for some installations. Engineers seldom design to minimum | standards from my experience. | Engineers should do electrical designs and electricians should stick to | installing these systems. I prefer to have a competent engineer doing the | design job. It just makes the job easier for everyone involved.

Diverting from the original topic a bit: who should do the design when it is a case of electrical systems being designed for a home?

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| Also loads are calculated to the last btu (when the load is | hundreds of millions of btu's and the calc takes weeks... a | total waste of time but that is virtually always demanded).. | then the primary and often surge loads, crucial back up and | redundancy requirements are ignored that if one | 10 dollar relay fails... an entire semi conductor facility | gets hot and has to shut down loosing tens of millions of | dollars a day. thats not uncommon. why? the UBC doesnt | address those isues (nor should it). that range of idiocy is | at least 95% prevelant at one level or another with most | projects Ive seen... only a few experienced old timers take | the broader conditions into account.

I guess this is why in almost every place I have worked, the air conditioning systems have been inadquate. Then we get lectured on not trying to make any adjustments in the system.

| We are running low on competent engineers of all stripes these | days... the issue was mentioned also in the superb 'arc flash' | post a week or two ago. The problem is most engineers think | they are competent when most are not even close.

Yet so many that are competent are unemployed or underemployed, with their work going to India. It's not the engineers who are to blame; it's the corporate investors who try to squeeze every bit of profit from the least level of investment.

| I am retained over the years on industiral projects, usually | projects in deep financial or technical trouble...all of | course caused by idiocy.. the larger the corporation the | farther south it seems to get. | | I start with a meeting... someone in their group (always one | of the vested interests, often the group responsible for most | of the disaster) hired me. And I am always presented as a | premier expert in my field (which is smoke, there are many | people a lot brighter than myself). but thats how I am | introduced...its part of a corrupt managements strategy.

They couldn't afford someone even brighter. But you're probably asking for just the right amount of money to keep on the job.

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there are many reasons...designing to the NEW performance specs of the equipment is a problem, and that IS mandated by law the equipment ages, and compressors wear, and heat exchange surfaces foul and corrode performance can easily drop

20 to 30%...thats virtually never figured into the job. You are also required to design for a 2% hot day...not peak hot days... and compressor efficiency drops 2 to 3% for each rise in outside air temp above the peak rating 97 def F... at 110 deg F performance is off another 30% believe it or not... you can look this up on the mfgrs own charts...add aging to that and you have a problem.







There are many causes you have listed one of the leading causes..I had to go back to contractng as rates fell by 60% in inflation adjusted dollars and the phone quit ringing..these days prison guards, police and fire dept grunts make about twice the average engineers wage. (80 to 150k in SF..thats beat cops.. the dept heads over 250k)






thats a big factor. But that business is now deader than road kill two weeks out... My rates for that kind of consulting were running about the same as the client companies dept heads. And they hated to pay even that. Top level consulting rates were 3 times that, $2,000 to $3,000 a day. These companies were paying BS degreed staff with 10 years experience 20 dollars an hour.. 2 bucks more than union grocery clerks.... and a fourth of what I make now slinging wrenches, screw drivers and a own hours.

But even that is getting tighter. Wages for the trades in this area run 25 to 50 dollars an hour. the engineering jobs tend to peak a little under that.

Self employment is the way to go. at 50 to 100 dollars an hour. (you need to put on a show at 100 dollars... some projects can double that return)

in the engineering business fakers could squeeze construction its a lot harder to fake, you have to be at least marginally competent...near the top you have to be utterly competent or you get eaten alive by the compeition.

As the baby boomers retire, competence is getting scarce... a day I longed for... but I had not counted on utter idiocy in management...that blunts the edge some. I think my new tactic is going to be to cease advising the client on technical issues in order get the work, just quote his unqualified disaster.... thats soul destroying though. Big city life. Utter cynicism.

Away from the cities, reality precludes much of that idiocy.

Phil Scott



Reply to
Phil Scott

Thanks for all your words of wisdom. As a "green" engineer sometimes I second guess myself. Most of my cases I'm handed a used piece of equipment (after the fact of voltage configurations, hp etc.) and tasked with designing power etc. Besides field experience, what are some books, sites, and tools I can utilize to help strenghten my practical applications knowledge in an industrial environment.

Thanks all,


Reply to

Point well taken :)

Thanks again, KIP

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By Roe's Practical Electrical Project Engineering book, a 150KVA transformer would provide enough power.

Reply to
Gerald Newton

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