Single Phase Center Tapped Transformer OCPD sizing

[minerman]

Hello, I have a single phase 480V primary / 240V/120V center tapped secondary transformer rated at 37.5kVA. Right now it is protected with a 200A CB on the primary and a 400A CB on the secondary.

These seem too large to me. Can someone back me up?

Primary Current = 37500VA/480V = 78.125A
Max Primary OCPD = 2.5 x 78.125A = 195.3125A

Secondary Current = 37500VA/240V = 156.25A
Max Secondary OCPD = 1.25 x 156.25A = 195.3125A


Am I correct about all of this? I am actually not sure if I should be using 120V or 240V for my secondary calculation.

Thanks!

 

[charlie b]

I agree with your math. It is appropriate to use 240, and not 120, for the secondary calculation. Please note that you can't upsize the primary from 195 to 200, because table 450.3(B) does not have a Note 1 (or similar note) on the primary side. So the limit on the primary OCPD is 175 amps. You can upsize the secondary from 195 to 200, but you can't go higher than that.

 

[petersonra]

I would have probably put 100A and 200A breakers in. maybe 200A on both sides. never noticed that one can't upsize to the next breaker on the primary side. seems odd. UL508a allows you to go to the next size on both sides.

 

[minerman]

Thank you. I brought this up to my boss; told him I thought it is wrong. He justified the sizing by using 120V on the secondary and upsized both based on that. He said, he thinks its ok and that we are not going to change it. So, now I have an ethical dilemma. :thumbsdown:

 

[charlie b]

I usually take 125% of both the primary and secondary currents, and then take the next higher standard OCPD. So I haven't run into the situation of going up from the 250% primary current before.

 

[petersonra]

you brought it up to the right person. your ethics dilemma has been satisfied IMO.

It does not seem to me that there is any immediate danger to anyone from this setup.

is this being inspected? the inspector may well notice.

sometimes things just happen this way and there is little you can do about it. it is not always worth expending any of the limited personal capital one has available over this kind of thing. Sometimes we have to save it for when there is a more urgent issue at stake.

BTW, what size wire did you run?

 

[minerman]

you brought it up to the right person. your ethics dilemma has been satisfied IMO.

It does not seem to me that there is any immediate danger to anyone from this setup.

is this being inspected? the inspector may well notice.

sometimes things just happen this way and there is little you can do about it. it is not always worth expending any of the limited personal capital one has available over this kind of thing. Sometimes we have to save it for when there is a more urgent issue at stake.

BTW, what size wire did you run?

We haven't actually got it installed yet. But, I'm planning on running 3/O for the primary (#4 ground) and 2-3/O per phase (#3 ground) on the secondary based on these sizes. I'm actually designing the wiring right now, that's how it got brought to my attention.

 

[minerman]

you brought it up to the right person. your ethics dilemma has been satisfied IMO. <br>
<br>
It does not seem to me that there is any immediate danger to anyone from this setup. <br>
<br>
is this being inspected? the inspector may well notice.<br>
<br>
sometimes things just happen this way and there is little you can do about it. it is not always worth expending any of the limited personal capital one has available over this kind of thing. Sometimes we have to save it for when there is a more urgent issue at stake. <br>
<br>
BTW, what size wire did you run?

<br><br>We haven't actually got it installed yet.  But, I'm planning on running 3/O for the primary (#4 ground) and 2-3/O per phase (#3 ground) on the secondary based on these sizes.  I'm actually designing the wiring right now, that's how it got brought to my attention.

 

[minerman]

Does this explaination make sense? I am still trying to wrap my head around this. This is for a 120VAC panelboard, 2 legs and a nuetral. So it is ok to calculate the secondary at 120V? To me it seems like the kVA rating for the transformer would be based on the entire set of windings, so we should use 240VAC?


Use 460 for primary and 120 for secondary.
We never use 480 for design calculations
Primary is 81.2 amps x 2.5 = 203 amp. The breaker could actually be 225 amps.
Transformer has no 240 Vac circuits. It only uses 120 vac circuits. The transformer has a catalog rating of 312 full load amps at 120 vac.

​

 

[petersonra]

Does this explaination make sense? I am still trying to wrap my head around this. This is for a 120VAC panelboard, 2 legs and a nuetral. So it is ok to calculate the secondary at 120V? To me it seems like the kVA rating for the transformer would be based on the entire set of windings, so we should use 240VAC?


Use 460 for primary and 120 for secondary.
We never use 480 for design calculations
Primary is 81.2 amps x 2.5 = 203 amp. The breaker could actually be 225 amps.
Transformer has no 240 Vac circuits. It only uses 120 vac circuits. The transformer has a catalog rating of 312 full load amps at 120 vac.

​

It could be a 120V only PB. More likely it is a 240/120 PB.

I seem to recall that the code requires one to use the "standard" voltages of 480 and 120 V as opposed to the actual voltages for doing calculations.

 

[petersonra]

Does this explaination make sense? I am still trying to wrap my head around this. This is for a 120VAC panelboard, 2 legs and a nuetral. So it is ok to calculate the secondary at 120V? To me it seems like the kVA rating for the transformer would be based on the entire set of windings, so we should use 240VAC?


Use 460 for primary and 120 for secondary.
We never use 480 for design calculations
Primary is 81.2 amps x 2.5 = 203 amp. The breaker could actually be 225 amps.
Transformer has no 240 Vac circuits. It only uses 120 vac circuits. The transformer has a catalog rating of 312 full load amps at 120 vac.

​

I seem to recall that these type of calculations are done using the "standard" voltages of 120, 240, and 480 regardless of any measured voltages.

I would not be real worried about 200A primary side CB. it is so close that no one is likely to care, although I trust Charlie enough that if he says you cannot round up on the primary side, then I believe you can't.

It is possible that they could wire the PB up as only 120 only, but I think it is more likely 240/120 since you indicated it was a three wire PB. In any case, I can't see anyway to wire up a main CB PB as 120V only that would be legal either in this case.

I think they just did it wrong based on what you are telling us. It is not the first such mistake someone has made and has resisted correcting when it is pointed out to them. It won't be the last.

 

[petersonra]

<br><br>We haven't actually got it installed yet.  But, I'm planning on running 3/O for the primary (#4 ground) and 2-3/O per phase (#3 ground) on the secondary based on these sizes.  I'm actually designing the wiring right now, that's how it got brought to my attention.

thats a lot of pricey wire.

 

[minerman]

I seem to recall that these type of calculations are done using the "standard" voltages of 120, 240, and 480 regardless of any measured voltages.

I would not be real worried about 200A primary side CB. it is so close that no one is likely to care, although I trust Charlie enough that if he says you cannot round up on the primary side, then I believe you can't.

It is possible that they could wire the PB up as only 120 only, but I think it is more likely 240/120 since you indicated it was a three wire PB. In any case, I can't see anyway to wire up a main CB PB as 120V only that would be legal either in this case.

I think they just did it wrong based on what you are telling us. It is not the first such mistake someone has made and has resisted correcting when it is pointed out to them. It won't be the last.

I would love it if I could find the article about using standard voltages. This is often a question of mine. I've always used 460 for motor calculations, because it has 460 in a lot of the motor data tables, but have always tried to use 480 for everything else.

The panelboard has a 2-pole circuit breaker that gets connected to the two opposing sides of the transformer. The panelboard's nuetral gets attached to the center tap of the transformer. So, it looks like we will have two 120V to nuetral sources that are 180 deg. out of phase with one another. So, I guess you could call the PB, 120V only, but I think the transformer secondary itself, has a voltage across the windings of 240V. That is why I'm thinking 240 needs to be used in the calc.

thats a lot of pricey wire.

We seem to be good at using a lot of pricey wire. Luckily, this is a short run, but that is the main reason I started paying close attention to the sizing of the OCPD's.

 

[jim dungar]

I would love it if I could find the article about using standard voltages.

The closest are:
100 Definitions of Voltage, Nominal (which refers to ANSI C84.1)
220.5(A) Voltages (which is used for load calculations)

 

[minerman]

The closest are:
100 Definitions of Voltage, Nominal (which refers to ANSI C84.1)
220.5(A) Voltages (which is used for load calculations)

You guys make my day. You're so damn helpful. Thanks for doing what you do.

 

[hurk27]

The problem I see is you have a center tapped transformer that has windings rated at 240 volts on the secondary at 156.25 amps for 100% load, if this transformer had two separate windings (4-wire) where X1-X3 and X2-X4 could be paralleled together for 120 volts only then I would say your boss would be correct as the parallel rating of these paralleled windings would be 312.5 amps.

With the above said let me add why his thinking is in error:

Just because you only will be connecting 120 volt load to a 240/120 volt panel does not mean you do not have 240 volt loads, because of the center tap balancing neutral you will still have 120 volt loads that are in-fact in series across 240 volts, the neutral is only there to maintain a balance of 120 volts when one side of the 120 volt loads is not equal to the other side, we all know what happens when we loose a neutral.

With that known and the fact your transformer is 240 volt rated for it current you must use the 240 volt rating not 120 volts.

As was mentioned installing conductors for the 400 amp rating makes no sense cost wise on a transformer only rated for 156.25 amps so by using a 400 amp disconnect on the secondary will unnecessarily cause a cost over run for this job that could be avoided when a 200 amp disconnect and rated conductors is the right choice.

I also question if a 400 amp rated panel is also going to be used as you can not protect a 200 amp rated panel @ 400 amps, talk about over engineering:blink:

As also said for a safety point of view, the 400 should protect for bolted fault depending upon the length of the feeders from the transformer to the panel and the impedance of the transformer, but will not protect for transformer overload, if 400 amps is needed then a much larger transformer is needed, so if your boss wants to go with this, as long as you have no liability of the job, then its on his shoulders.

 

[mike1061]

Hello, I have a single phase 480V primary / 240V/120V center tapped secondary transformer rated at 37.5kVA. Right now it is protected with a 200A CB on the primary and a 400A CB on the secondary.

These seem too large to me. Can someone back me up?

Primary Current = 37500VA/480V = 78.125A
Max Primary OCPD = 2.5 x 78.125A = 195.3125A

Secondary Current = 37500VA/240V = 156.25A
Max Secondary OCPD = 1.25 x 156.25A = 195.3125A


Am I correct about all of this? I am actually not sure if I should be using 120V or 240V for my secondary calculation.

Thanks!

I don't understand this at all. Where are the red highlighted numbers coming from
Thanks
Mike

 

[hurk27]

I don't understand this at all. Where are the red highlighted numbers coming from
Thanks
Mike

For the 250% see Table 450.3(B) for primary protection of a transformer

For the secondary see 240.4(F) and 450.3 + table 450.3(B) for the 125% requirement

 

[mike1061]

Thanks for the reply. It's very clear.
It just seems wrong, to protect conductors at 250% of there rating.
Thanks
Mike

 

[david luchini]

Thanks for the reply. It's very clear.
It just seems wrong, to protect conductors at 250% of there rating.
Thanks
Mike

Table 450.3(B) doesn't say anything about protecting conductors, only about protecting the transformer.

For the transformer in the example, 450.3(B) would permit a 175A c/b on the primary side of the transformer. The conductors would have to be protected by that 175A c/b, or 2/0AWG minimum.