What's up, everybody? Welcome to another episode of Let's Ask Paul, the
podcast where you get to ask me Paul Abernathy anything you want about the National
Electrical Code, all things electrically related, and if selected, I will respond
in a podcast much like today, or I could respond in an email.
All you gotta do is again, go to Paul abernatty dot com, submit
your information, and we will define me either way get you an answer.
Now. I get hundreds of these, so again bear with me. We'll
get to them as quick as we can. But I love getting them,
so I do appreciate you folks sitting in your questions. Just do me a
favor. Make sure you specify the code cycle that your question is based around,
and be as very detailed and descriptive as possible, because, as you
know, with a lot of things, there can be certain variables, and
we don't want to make sure that we get it wrong because of a variable
that we leave out, so we want to want to make sure that we
do it right. So please, by all means, if you're going to
submit something to us, make sure that you submit the details involved. Okay,
thanks again for following me, folks and listening again. You can listen
to these podcasts on any of your favorite listening platforms, Apple Podcasts, Google
Podcasts, Breakers, Spotify, Deezer, our Heart Radio. So many ways
to listen. And another way to listen is simply downloading our free mobile app
and listening from our actual mobile app on your phone. That's one way to
listen. So if you want to get the mobile app, you don't go
to the Google Store, Apple play Store or whatever they are. You don't
do that. We don't do that. I don't want to give them companies
any more money than we have to. So what you do is you just
go to our website fast tracks System. Then that's tr X Okay, not
track, it's TRX so fast trackssystem dot com all one word, and you'll
see in the navigation you'll see mobile app. You just go to it and
you click on it. If you're on your desktop and you go there and
you see a bar code, a QR code, you can just scan it
with your phone. It'll ask you do you want to load it onto your
homepage? Answer yes, We will not load any software into your phone.
It is simply a link to our mobile web based app and on your phone
it works just like an app, but we update it and that way it's
always relevant and we're always adding new things, new calculators, new resources.
There's a chat feature you can post pictures of an install and you're like,
hey, what do you think about this? And it's only available for the
members who have the mobile app, So again it's very nice, a very
knit group there that have the mobile app. Always remember, folks, you
got to register your app though, so once you get it, you got
it. You'll see on there about your access. You got to create your
account on the app, so people are from forgetting to do that. But
once you get the app, you got to register your app. So the
instructions are right there in the app. You just click the icon. It
talks about the registration and getting your account for the mobile app. Once you're
set up, then you're good to go. Okay, So don't let it
confuse you, all right. So ongoing episodes, we've did quite a few
episodes we've been doing based on this question that was submitted to me. We
answered about parallels, series circuits, we did combinations, we did OHMS law,
we've done all of that. We talked about how to formulate the horsepower.
We have different episodes that are available. They'll be titled correctly so they'd
be easy for you to differentiate those over in whatever platform you want to listen
to us on. Again, we're on Spreaker, a Deezer Apple podcast,
Google podcast. I like to listen on Spotify personally, but you do whatever
you do. And so this episode, we're going to be focusing and it's
not gonna be a long one, but we're gonna be focusing on transformer sizing.
And again, this is gonna be difficult to do kind of in a
podcast, but we're going to do our best to do it and try to
convey it what we're talking about here. Okay, So sizing a transformer.
So we're gonna do a single phase transformer and they're gonna do a three phase
transformer, and kind of give you some basic synopsis of how this would work,
okay, so that you get kind of an understanding on what you may
or may not do. Now, we'll also talk about kind of how to
calculate the primary and secondary amps, because that is probably more in line with
what you would get on an exam for those that are preparing for an exam.
But I also want to start out with a more practical application of when
you have specific loads in how you would size your transformer. So this is
again a very practical, high level version. We're not getting into a bunch
of extra nuanced variables. This is the basic information. And of course we
have a video series it's available that goes deeper into transformers than this, but
I want to kind of give you that. So I also will tell you
that if you do have the Uglies Book, this is the versions that is
in the Uglies Book. So I want to give credit where credit's due.
I didn't want to reinvent the wheel. The Ugliest Book is an amazing resource
if you've got it, but again it's not translated into easy listening. So
that's what the episode today is trying to convey. All Right, So the
first thing sizing a transformer a single phase transformer. So let's say we size
a four hundred and eighty volt primary and it's going to be a two forty
one to twenty volt secondary transformer. Okay, so for eighty primary two forty
one twenty secondary single phase transformer. For the following single phase incandescent. I
mean, I don't know if anybody uses incandescent anymore. But anyway, incandescent
lighting loads consisting of forty eight recessed luminaires, each rated two ampiers. Okay,
oh, each rated two ampiers and it's one hundred and twenty volts.
Each fixture or luminair has one hundred and fifty watt lamp. Okay. Now,
these luminaires can be evenly balanced on the transformer. It's a single phase
transformer. But you know what, we do not use the lamp wattage here,
because that's the first thing I want to convey. We're trying to calculate
and we're trying to get the VA value. We're not going to be using
the wattage. Okay. First thing we want to make it clear. Now,
what we got in the question is that each one of these are two
ampiers each, okay, and here's forty eight of them. Okay. So
what we want to do is we want to find out what the VA is
for each one of these based on the amps, based on the vaults.
Okay. That's why it's so important for you to understand Ohm's law, right,
And hopefully you went back and listened to that podcast I did on Ohlms
law, because this is straightforward, If every one of these luminais are two
ampiers times one hundred and twenty volts, okay, so again two times one
twenty and that is two hundred and forty vault ampiers or va. And how
many did we have. We had forty eight of these things, okay,
So two forty times forty eight that equals eleven five hundred and twenty va.
So that is what our lighting contribution is on this transformer. Okay, this
single phase transformer the secondary pay. So in that scenario, we now want
to get that into kVA. Well, obviously we know eleven five hundred and
twenty is va. That in order to do to get this into kVA,
we just simply divide that by thousand, okay, So I'm gonna divide that
by thousand, and that as eleven point five to two kVA. So when
you go looking at single phase transformers and I need to find one that is
at least eleven point five to two kVA, well, chances are the one
the next one you're going to find larger than that is going to be a
fifteen kVA transformer. Okay, So that's what you would have. So keeping
keeping that in mind, that's exactly how you would do it. Now,
it should go without saying that when you actually physically do this in the panel,
the distribution or balance it making sure you're evenly balancing on each phase of
this transformer. Because it is to forty volt one twenty, so you have
two legs that are one twenty, so you want to balance them out.
So that means that in this case we had forty eight luminaires, I'm going
to put twenty four on Leg A and it's single phase, so we'll just
say Leg A and we'll do twenty four on Leg B. So again we're
going to evenly distribute these loads on the between the different phases, and of
course then that means the neutral is going to see what the imbalance, which
in this case is nothing right for these these are going to be balanced out
for this single phase transformer. Now, one thing to remember that kVA,
whatever the primary is, is also the secondary. So I mean, if
it's fifteen kvas, which you choose, and it's fifteen kVA across the board,
okay, whether it's primary secondary, it's a fifteen kVA transformer, it
just so happens that the load we're dealing with is going to be placed on
the secondary. But we calculate it out at the fifteen kVA and that's the
transformer mi of them that we would need. Okay. Now moving on to
this equation where it would be a three phase transformer, then we're going to
incorporate some three phase loads here, So sizing this transformer for these loads,
and again remember these loads are on the secondary. That's the whole point is
we're picking up these loads, that is taking it from like in our single
phase four eighty volts, we're dropping it down because we need one to twenty
volt loads to handle or lighting loads that we had. So in the three
phase, if you had a primary of four hunder and eighty volts and you
had a secondary of two hundred and forty volts. Again, three phase transformer,
which is again a polyphase transformer, it says to supply one three phase
twenty five kVA process heater. Okay, so it's going to be using all
the legs in a three phase, but then it has a one single phase
five kW unit heater. Now it's not going to be something that you can
balance that five kW amongst the other phases because it's only using one phase.
So there's no way you're going to be balanced to be able to balance out
this transformer. So what do we typically do with that? Since that five
kW is only going to be on one phase, it adds loads directly,
and if you were to do it that way, you run the risk of
undersizing your transformer. So it is very common practice to even though it's only
five kW on the one leg or the one phase, you really want to
do it times three, so it's like the five kW on each phase in
order to balance it out. Okay, that's very common practice. Whether you
do this or not, that's just a very common practice. So it'd be
five kW times three would be fifteen, and then you have the original twenty
five kVA which is for the three phase right, so it'd be twenty five
plus fifteen, which comes up to be a total of forty kVA. And
of course, at that point, if you go look at your transformer three
phase transformers, probably the next side that meets it or exceeds that forty kVA
would be a forty five kVA transformer. That makes sense, Okay, that's
probably what you're going to do. Pretty much in the field, that type
of thing we don't want to run the risk of undersizing our transformer, okay,
all right. So the next thing is, okay, so what if
we know the transformers kVA and remember it's fifty kVA whether it's primary or secondary.
That you know the old saying wats in wat's out. Watts is watts
whether it's primary secondary, watts equates to what also the va. Okay,
So if it's kVA, it'd be the same as killowatts. If it's VA,
is the same as watts, So they're synonymous here, okay. So
let's kind of work this out. So what if we have a transformer now,
and we want to work out the amps on primary side, and we
want to work out the amps on the secondary side. Okay. So let's
say we have a four to eighty two forty volt single phase fifty CAVA transformer
with a total Z impedance of two percent, which is marked on the side
of the transformer, okay, and that's what's being installed. It's not a
very efficient transformer, but it's two percent impedance, okay, which is the
Z all right, And so it's being installed. So we need to calculate
the primary and secondary amps and we can even calculate the short circuit current at
this transformer. Yes, we can, all right, so let's do it.
So what's the formula? So to find the total current on the primary
which is the I for current in P. That's usually what you have,
an I and then a little P that's the primary current. The formula is
Cavea times one thousand, and we really don't need to do that if you
already know that it's fifty kVA. You know the K is one thousand,
so you know it's fifty thousand. But anyway, to stay consistent, we'll
do kVA, which is fifty times a thousand, and if you've heard me
tell you before, in this formula, the top portion is the numerator,
the bottom portion is the denominator. Only reason I'm doing it this way is
to pink this mental picture. It's kind of like a fraction. You know
what goes on the top and you know what goes on the bottom, right,
So that's kind of why I do this. So the top part is
going to be fifty thousand, and then the bottom part is the E primary,
which E is the voltage and P being the primary voltage. So this
would be four under and eighty volts, right, that's the primary, So
fifty thousand at that point you're saying, okay, that's the primary voltage.
But what we're going to do is we're do fifty thousand divided by or numerator
divided into the denominator, which is for eighty. Because the voltage for the
the primary is four eighty, so that would be fifty thousand divided by four
eighty and that is gonna give me one hundred and four point one six.
And you know we can drop anything that is under point five, so we're
dropping it. So that as one hundred and four amp years on the primary
side for this transformer at that impedance at two percent impedance. Okay, that's
what we've got. Now the impedance really doesn't come into play yet. That's
only gonna be when we do the short circuit. But anyway, just throw
it in there so you understand that's what that transformer is based on. Now
doing the secondary, remember the numerator, the top part doesn't change, the
KVEA doesn't change. It's the same whether it's primary secondary. Don't get lost
in that. So it's still fifty thousand. The only difference here is the
secondary voltage is two forty, right, So we're gonna do fifty thousand,
okay, and we're gonna do fifty thousand and divided by two forty, and
that gives me two hundred and eight point three. Drop the point three,
so it's two hundred and eight for the secondary. So this transformer, this
fifty kVA transformer, has a primary amperage rating of one oh four in a
secondary amperage rating of two zero eight. Okay. Now we want to know
what is the short circuit current rating that we're dealing with here, Okay.
So in order to do that formula, you simply and again we're using the
secondary because that's where our loads is. That's where we're going to be dealing
with, you know, with with with everything that's going to be connected on
the load side of it, okay, or the secondary side. So the
short circuit ampires for that secondary, and that's referred to as the is s
C. Okay, is the in the formula is the current from the secondary
divided by the impedance that's given to us in the equation in this case was
two percent. So how do you remember what we always do with percentages?
We want to get it down to decimals. Start at the right and move
two spaces to the left, so two percent becomes start from the right,
move it two spaces to the left, it becomes point zero two. Correct.
So now it's a simple equation to find the short circuit ampurating. Okay,
right here at the secondary is two toh eight. Okay, that's the
amps that was from the secondary. We already calculated that divided by point zero
two and that is ten thy four hundred ampiers. And so that is what
your short circuit ampiers are. Okay, right there at that secondary. All
right, Now there is apps out there just to throw this out there.
Cooper Bustsman has a great app that will really, you know, work all
this math for you and all that as you continue down. But again,
that is basically how we would do it for a single phase transformer. Okay,
Now what about a three phase Okay, let's look at this differently with
a three phase. Now, like always, since we're dealing with three phase,
we're gonna be bringing in the square root of three, which is one
point seventy three to two. But I'm gonna remind you if you want to
use simply one point seventy three in your equations. You are perfectly okay to
do that. It's gonna work out the same, and it's not gonna be
enough variable when you start doing the roundings that it's gonna make a difference.
But you do you, and I'm gonna use one point seven to three today.
But remember square root of three is actually one point seven three two.
You want to use the whole value. That is perfectly fine to you.
It's not gonna make a difference in the equation. And if you don't believe
me, work it out, you'll see it's not gonna make a difference,
bait, once you do all your rounding and everything you're supposed to do.
All right, So let me give you the next the next one here,
So we need to size and find out what the primary and secondary amverage current
is for a three phase transformer. So it's a four eighty two oh eight
vote three phase transformer. It's one hundred kVA transformer, and its impedance is
one percent, okay, and it's being installed. So if this is the
kind of question that you get on an exam, it's basically just wanting you
to determine what the primary amps and what the The secondary amps are pretty easy
and straightforward. It's simply the same as it was before for the single phase.
The only difference is in the denominator portion the bottom. You're going to
be adding the one point seven three or one point seventy three to two if
you desire, in order to come up with the lower portion. Okay,
So let's kind of work this one out. So I think we all know
that one hundred kVA. Think of the k as a thousand, So this
is one hundred thousand va, and that goes in the top portion, which
is the numerator, and then the bottom portion was for eighty okay, for
eighty times one point seventy three two, and that would give you up four
eighty times that Goness calculator for eighty times one point seventy three two. If
that's what you're using, that's going to give you eight thirty one point three
six. You're more than happy to drop the point three six and use one.
I just use eight thirty one. It's going to be fine. If
you want to use the whole amount, that's fine. Two. I'll do
both if you want. Now, if I do this same thing for eighty
times one point seventy three. You'll see that the value is eight thirty point
four, okay, which again you wouldn't round up, so you'd be using
eight thirty. At the end of the day, you're going to see that
it's not going to make a big difference in the load. I'll show you,
all right, So let's use the first one. Well, we did
the whole amount of the four eighty times. It is one point sevent three
two. So again four eighty times one point seven three two was eight thirty
one thirty six. So we're gonna do one hundred thousand, okay, divided
by eight thirty one point thirty six, and that gives me one hundred and
twenty point two eight. We dropped the point two eight, so it's one
hundred and twenty ampiers. That is the primary current. Okay. Now,
just to show you it's no different, I'll do the four eighty times one
point seven three instead, and that's eight thirty point four. So I'll do
one hundred thousand divided by eight thirty point four, and as one hundred and
twenty point four, we dropped the point four, it's still one twenty okay,
So just wanted to make it clear For all you purists out there that
say it makes a difference, it's not going to make a difference. Okay,
but you do you personally. I'm doing this because I'm following the way
that you're the the what is this the the ugliest book goes. But I'm
just trying to as I'm throwing these tidbits out there. Uh, it's not
gonna make a difference. If you want to use one point seventy two,
and again I try to kid students into that mode. That's what you're gonna
be using. The other thing that we do is people know that without having
to think about it. They know when they're doing transformers, if it is
for eighty, then you're gonna use eight thirty one. If it's a two
to eight, you're gonna be using three sixty. So that typically people already
know that and then they kind of gravitate towards that. So anyway, you
get the gist of what we're doing here. Okay, so one twenty primary
for this one hundred kVA transformer with one percent impedance. So now let's do
the secondary, same thing, one hundred thousand on the top. Remember that
doesn't change primary to secondary. That you know, watts or watts va volt
dampiers is the vault dampiers. Okay, it doesn't change. So we're going
to do one hundred thousand, and in this case it's three sixty because that
is two eight times one point seven three two is three sixty point twenty five
six. So you could drop the point twenty five six and just use three
sixty. But also for you others that want to know what it is at
just one point seven three, then you can do the times one point seven
three and that is three fifty nine point eight, which you would round to
three sixty. So in that one, it's just three sixty. Okay,
use three sixty, all right, So one hundred thousand divided by three sixty,
which is the two eight square rooted of three uh, and that is
two hundred and seventy seven point seven, which is going to round okay,
point five and greater amps. We're going to round to two seventy eight.
So I have a primary of one twenty and a secondary of two seventy eight.
Now, in order to find the short circuit ampur rating at that secondary,
we use the same formula we did before. The only difference here is
now it's one percent versus in the last one it was two percent, so
one percent would be Take that one, move two decimal spots to the left
at a decimal and you got point zero one. That is one percent.
So it's two seventy eight, which is the secondary current divided by a zero
point zero one, and that gives me twenty seven eight hundred ampiers and that
is the short circuit ampiers at that secondary Now again, if you want to
get further down secondary terminals and you want to get on down, then you
know there is a method that is in the ugliest, but it's also an
app you can get which is called the Cooper Bustman app, and it's a
great app to have on your phone. Shout out the Cooper Busman. They
do a great job, so they're always at your fingertips. When you would
want to do one of these kind of calculations and have it, maybe you're
doing available fault current calculations, or you got to do labeling or whatever.
Usually the utility will give you that information to the service or even the value
at the transformer. Then you can calculate that out from the transformer to your
service. YadA YadA, YadA. But anyway, that's a good start for
you to be able to figure all this out and give you a good start,
and it also allows you to get the numbers that you can use to
plug into your your cooper busman things like that based on whatever transformers that's sitting
there, based on the stuff that's from the name plate, the impedance,
the KBA, the three phase, the voltage, all those type of things.
You can be savvy enough to calculate this stuff out. Okay, so
that's kind of all we really wanted to talk about in this episode, and
so it's kind of basics of transformer sizing. I tend to only see that
on exams. The basic examples that I gave you, don't overthink it.
Now. Using these values, obviously are now going to have to size your
protection for your transformer. So you're going to need to go to four fifty
and you're gonna be learning to size the primary protection of the transformer and possibly
needing secondary protection or you have casions where the primary will provide the protection to
the transformer both primary and secondary protection. And then of course you're sizing your
conductors and all this kind of stuff. That's for a totally different episode.
Hopefully you've watched my videos on Transformer calculations. We have a whole series of
that available over on fast tracks Tube. That's our video platform, and again
yes it's not free. Sorry, you've got to pay for it, but
again that helps us do what we do, and you know what, you
should be wanting to pay for good education, So we try to give you
the best we can, and that's why we put together that video platform and
it's growing every day. New videos are being added all the time, and
over the years, that's going to be our repository for really just great videos,
and eventually all of our podcasts will be moved to there as well,
so it's gonna be a one stop shop and it's gonna be awesome. All
right, folks, Hopefully you got something out of that, and again just
simply talking single phase and three phase transformers. Until next time, stay safe,
God bless, and remember you can listen to all of our podcasts or
even submit your own request over at Paul Abernathy dot com and be sure to
get our mobile app at our website, or you can even go to n
ec chat dot com and get it directly from there as well. Till next
time folks stay safe, God bless both both fast, nothing b