How to work a transformer calculation

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sparkyboys

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Ep x Ip = Es x Is voltage and current
Ep x Ts = Es x Tp voltage and turns in coil
Ip x Tp = Is x Ts Amperes and turns in coil

As well as many others on page 53 of the ugly's electrical reference book.

But the question i am asking is are these variables referenced with the numbers on a pad mounted transformer? and or poco transmission line? and or how these numbers and variables coincide with each other?

I am not sure if I am even asking the right form of question. I have very little transformer experience, but have been put on a totally new job that has transformers, and its just me(journeyman) and one of the master electricians in the company. He said we will be working together on these pad mounted transformers of this school which is 120/208 3ph. So other than just learning how to hook them up, i would like to better understand the mathematical side of the transformers so that i can get a better sense of what i am doing.
 
generally on site the only equation you need is EpIP= EsIs
or kVA in = kVA out

In theory a1 kVA transformer will deliver 1k VA to the load.
For practical applications transformers are considered 100% eff.

kVA=V.I.0 / 1000 (don' have a phase or theata sysmbol)
are rated in kVA because the nature of the load is unknown at the factory

also trans

I can not imagine a situation where you would.need to know the number of turns on the primary or secondary.

If you can ask a specific question regarding transformeres I am sure someone will help.
 
Transformers are the most un-understood peice of distribution equipment in the industry IMO, lots to know but not many that know it. I highly recommend you have a testing company test those after you install them.
 
zog said:
Transformers are the most un-understood peice of distribution equipment in the industry IMO, lots to know but not many that know it. I highly recommend you have a testing company test those after you install them.
Click to expand...

Can you put a size limit on that, cause I can tell you we install hundreds of dry transformers without hiring a testing company.:smile:

When I hear transformer I envision something like this

Acme%20Transformer.JPG
 
iwire said:
Can you put a size limit on that, cause I can tell you we install hundreds of dry transformers without hiring a testing company.:smile:

When I hear transformer I envision something like this

Acme%20Transformer.JPG
Click to expand...

Bob, I have tested houndreds of transformers that size, many new instalations require in the spec that everything is tested after instalation and before change of ownership. Sometimes the spec requires NETA certified testing companies and techs to do the testing, other times it just requires the equipment is just tested to the NETA specs. Most typical types of new builds that require this is schools and other gov facilities like prisons, labs, and contractors for the gov.

I bet most of those size you have installed were tested, but long after you were on to another job. However, this size is close to the smallest that get tested, more often we test padmounts, substation dry types, and oil filled power transformers.
 
generally on site the only equation you need is EpIP= EsIs (cpal)

Ok can anyone give me an example of how this formula is used when a commercial building has 120/208 3ph, starting with the primary?

or with a commercial building that has both 480/277 and 120/208 3ph? starting with the primary?
 
zog said:
I bet most of those size you have installed were tested, but long after you were on to another job.
Click to expand...

Nope, not normally. Well perhaps IR 6 months to a year down the road before we can get our final payment.

What types of facilities are you doing this at?

We do office buildings, retail, light manufacturing, pharmaceutical plants, universities etc The only ones I would expect the specs to require testing before energizing would be the pharmaceutical plants and universities.
 
sparkyboys said:
generally on site the only equation you need is EpIP= EsIs (cpal)

Ok can anyone give me an example of how this formula is used when a commercial building has 120/208 3ph, starting with the primary?

or with a commercial building that has both 480/277 and 120/208 3ph? starting with the primary?
Click to expand...

I work with a lot of 480 delta 208Y/120 transformers and most times the engineering is done for me but here is a pretty typical installation.

75 KVA 480-208Y/120 transformer

Primary side

75*1000/480/1.72= 90.3 amps on the primary.

Table 450.3(B) allows up to 250% rated over current device on the primary side if secondary protection is also installed. The panel on the secondary side is going to need protection as well so we can use one breaker for both assuming the panel is located near the transformer.

So 90 amps * 2.5 = 225 amps, we could use a breaker as large as 225 amps on the primary side of course that would also require primary conductors rated at least 225 amps as well.

Much more common is the use of a 125 or 150 amp breaker with conductors sized to match on the primary side of a 75 KVA transformer. Why run it so high above the transformers rating? Because if you use a 90 or a 100 amp breaker transformer inrush might immediately trip the breaker when you try to energize it.

Now the secondary side

75*1000/208/1.73=208 amps

Table 450.3(B) allows the secondary protection to be 125% of the secondary current and you can round up to the nest size if you want.

That being the case we can use the typical 225 amp 3 phase main breaker panels that are commonly available as the secondary protection.

Now all that said, there are as many ways to do this as there are electricians so you really need to go through Article 450 to get the straight dope. :smile:
 
sparkyboys said:
generally on site the only equation you need is EpIP= EsIs (cpal)

Ok can anyone give me an example of how this formula is used when a commercial building has 120/208 3ph, starting with the primary?

or with a commercial building that has both 480/277 and 120/208 3ph? starting with the primary?
Click to expand...

OK, you don't give much information, but with an assumed 100 kVA 12470-120/208 volt transformer:

Rated VA = Rated kVA * 1000 = 100000
Per phase VA = 100000/3 = 33333 VA = Ep?Ip = Es?Is

Dividing by Es: 33333/Es = Is
Secondary voltage to neutral = Es = 120 V
Rated secondary current in each phase = Is = 33333/120 = 278 A

Primary voltage to neutral = 12470/sqrt(3) = 7200 V

33333 VA = Ep?Ip

Divide by Ep: 33333/7200 = 4.67 A

Note that Ip = Es?Is/Ep (Allowing for rounding)
 
iwire said:
What types of facilities are you doing this at?

We do office buildings, retail, light manufacturing, pharmaceutical plants, universities etc The only ones I would expect the specs to require testing before energizing would be the pharmaceutical plants and universities.
Click to expand...

Yep, those are the ones I would expect to see testing for too.
 
tranformer

tranformer

VA = volts x amperes ...................1 PHASE

VA = √3 x Volts x Amperes............3 PHASE

1 KVA = 1000 VA

WATTS = VA x pf............................pf = power factor



for high voltage x'former, use megger tester for insulation testing.....
 
my 2 cents

my 2 cents

Watts is watts right. The question I would ask is what is the Load also just because you can apply 250% primary overcurrent protection does not mean it?s necessarily the best idea, you now have larger breakers / fuses, larger conductors, more space in a panelboard (possibly), the problem is like you stated with the new TP-1 xfmrs. Some manufactures can see in-rush greater than 21x primary FLA. But now you have to find out is that initial start-up (energization) or re-start (?hot? or ?cold?) start, also if you size the OCPD at 250% if there is a fault between the line on the secondary and the primary say line to line you might burn up the xfmr before the Primary OCPD trips because the trip curve will be well beyond the damage curve of the XFMR. I think (My opinion) that XFMR protection is becoming trickier as manufacturers have to comply with TP-1 and different manufacturers do it in different ways resulting in a wide range of inrush currents.
 
iwire said:
I work with a lot of 480 delta 208Y/120 transformers and most times the engineering is done for me but here is a pretty typical installation.

75 KVA 480-208Y/120 transformer

Primary side

75*1000/480/1.72= 90.3 amps on the primary.

Table 450.3(B) allows up to 250% rated over current device on the primary side if secondary protection is also installed. The panel on the secondary side is going to need protection as well so we can use one breaker for both assuming the panel is located near the transformer.

So 90 amps * 2.5 = 225 amps, we could use a breaker as large as 225 amps on the primary side of course that would also require primary conductors rated at least 225 amps as well.

Much more common is the use of a 125 or 150 amp breaker with conductors sized to match on the primary side of a 75 KVA transformer. Why run it so high above the transformers rating? Because if you use a 90 or a 100 amp breaker transformer inrush might immediately trip the breaker when you try to energize it.

Now the secondary side

75*1000/208/1.73=208 amps

Table 450.3(B) allows the secondary protection to be 125% of the secondary current and you can round up to the nest size if you want.

That being the case we can use the typical 225 amp 3 phase main breaker panels that are commonly available as the secondary protection.

Now all that said, there are as many ways to do this as there are electricians so you really need to go through Article 450 to get the straight dope. :smile:
Click to expand...

where do the 1.72 and 1.73 numbers from?
 
jghrist said:
OK, you don't give much information, but with an assumed 100 kVA 12470-120/208 volt transformer:

Rated VA = Rated kVA * 1000 = 100000
Per phase VA = 100000/3 = 33333 VA = Ep?Ip = Es?Is

Dividing by Es: 33333/Es = Is
Secondary voltage to neutral = Es = 120 V
Rated secondary current in each phase = Is = 33333/120 = 278 A

Primary voltage to neutral = 12470/sqrt(3) = 7200 V

33333 VA = Ep?Ip

Divide by Ep: 33333/7200 = 4.67 A

Note that Ip = Es?Is/Ep (Allowing for rounding)
Click to expand...

I have that exact calculation I need to do for a job at work except it is single phase, 3 wire, primary and secondary..oh and it's 120/240 secondary voltage
 
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