Transformer secondary

[bwyllie]

For a K-Rated 75KVA transformer, with 150A primary and a 225A secondary protection what is the required secondary feeder size? I believe it should be 5-300kcmil(3-phase, 2 parrell neutrals), based upon a 200% neutral and also the neutral being considered a current carrying conductor and thus applying an 80% derating of all conductors since there will be 4 current carrying conductors in the conduit. Thoughts..

 

[ryan_618]

Re: Transformer secondary

What is the load on the secondary conductors?

If it is more than 208 Amps, you will need 250KCMIL. If the load is less than 208A, you can use 4/0.
90 degree column ampacity of 4/0=260A 260*.8=208A, which does not exceed the 75 degree column rating [110.14(C)].
240.4(B) permits the next standard rating to be used.

[ July 14, 2004, 02:12 PM: Message edited by: ryan_618 ]

 

[dereckbc]

Re: Transformer secondary

What is the input and output voltage? I am assumming 480/208. If so, try these numbers.

Primary OCPD = 125-Amps
Secondary OCPD = 200-amps
Seconday conductors = 4/0 (3-Phase, 1-N)

 

[taylorp]

Re: Transformer secondary

bwyllie:

It sounds like you are describing a transformer installation where you have high harmonic loads on the neutral???

Quote:
"2 parallel neutrals based upon a 200% neutral"

Is this right???

 

[bwyllie]

Re: Transformer secondary

1) Yes, transformer is 480V:208Y/120V

2) Yes, this is an office space.

 

[dereckbc]

Re: Transformer secondary

bwyllie, there is no specific answer to your qustion. Your primary OCPD can range from 1-amp to 225-amp. The secondary OCPD can range from 1-amp to 250-amp. Assuming you want to get the most out of the transformer (FLA) you choose 225/250 respectively.

I have designed a lot of data centers which are rich in harmonics, more so than an office environment. And in those 15 + years I have rarely ever seen the neutral current exceed a phase current. Of those few times as I recall the highest was 110% on a lighting panel, never on a PDU. What I am getting at is you can size the neutral to 200% if you want, but IMO, and mathematical 141% is the most you would ever want to go. Personally I use 100% on data, and 141% on lighting. So I suggest the neutral be the same size as the phase conductors.

If you go with a 250 OCPD on the secondary you will need a 250 KCM minimum. If voltage drop from length is a concern you can go larger. Good Luck...Dereck

 

[ryan_618]

Re: Transformer secondary

I beleive you could go as high as a 300A OCPD on the secondary, if you really wanted to. 450.3(B) allows 150% of the rating, with the next standard size rule being permitted.

75000/208/1.732=208A
208*1.25=260A
260A, next standard size=300A

I'm not saying its a great idea, but you could do it. Remember to comply with 408.16, however:

408.16 Overcurrent Protection.
(A) Lighting and Appliance Branch-Circuit Panelboard Individually Protected. Each lighting and appliance branch-circuit panelboard shall be individually protected on the supply side by not more than two main circuit breakers or two sets of fuses having a combined rating not greater than that of the panelboard.

 

[lady sparks lover]

Re: Transformer secondary

I disagree in one aspect...*lady sneaking into the convo*

[ July 15, 2004, 02:08 PM: Message edited by: lady sparks lover ]

 

[ryan_618]

Re: Transformer secondary

Originally posted by lady sparks lover:
Would you not have to upgrade 125% for 5 currect carrying conductors, and then upgrade for 125% for the secondary side over currect protection rating?

Hi Lady. You would have to size the OCPD first then size the conductors to match it, after applying an 80% ampacity adjustment off of the 90 degree column, with the final ampacity not to exceed the 75 degree column of your conductors. 125% is the maximum you can set the OCDP for the secondaries, based on 450.3(B).

 

[dereckbc]

Re: Transformer secondary

Originally posted by ryan_618:
I beleive you could go as high as a 300A OCPD on the secondary, if you really wanted to. 450.3(B) allows 150% of the rating, with the next standard size rule being permitted.

75000/208/1.732=208A
208*1.25=260A
260A, next standard size=300A

[/b][/QUOTE]Table 450.3(B) Secondary Protection, Current of 9 Amps or More = 125%. However I agree a 300-amp OCPD would be the max allowed under 240.4(B), but I am conservative, I would use 250 OCPD.

 

[lady sparks lover]

Re: Transformer secondary

Originally posted by ryan_618:

Originally posted by lady sparks lover:
Would you not have to upgrade 125% for 5 currect carrying conductors, and then upgrade for 125% for the secondary side over currect protection rating?

Hi Lady. You would have to size the OCPD first then size the conductors to match it, after applying an 80% ampacity adjustment off of the 90 degree column, with the final ampacity not to exceed the 75 degree column of your conductors. 125% is the maximum you can set the OCDP for the secondaries, based on 450.3(B).

Hi Ryan,

But does this does on account for harmonic loads. Take a look at what I copied from the NEC handbook

[ July 15, 2004, 04:45 PM: Message edited by: lady sparks lover ]

 

[ryan_618]

Re: Transformer secondary

Hi Lady. I think he already addressed the harmonic issue by using a K rated transformer and a 200% nuetral. After that, everything should be treated just like a normal transformer, with the exception of the ampacity adjustment for the current carrying conductors.

 

[lady sparks lover]

Re: Transformer secondary

Originally posted by ryan_618:
Hi Lady. I think he already addressed the harmonic issue by using a K rated transformer and a 200% nuetral. After that, everything should be treated just like a normal transformer, with the exception of the ampacity adjustment for the current carrying conductors.

Hi again Ryan,

My design group must be overdoing it, because for harmonic loads:

</font>

• <font size="2" face="Verdana, Helvetica, sans-serif">200% neutrals</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">derated conductors for feeders that have 4 or more CCC</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">isolated ground (althought this is for noise)</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">K-factor transformers</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">we multipy 125% for the OCP and derate to 80%for 4CCC</font>

<font size="2" face="Verdana, Helvetica, sans-serif">

Most of the panels are fed from those transformers which are K-rated. I guess, because those loads are strictly computer loads. For some reason, I tend to think it's a bit much, but what do you think??

Although, some transformers have still overheated, it hasn't happened very often.

[ July 16, 2004, 10:22 AM: Message edited by: lady sparks lover ]

 

[ryan_618]

Re: Transformer secondary

Originally posted by lady sparks lover:
For some reason, I tend to think it's a bit much, but what do you think??

I'll tell you the truth, I think your next statement answers your question...

Although, some transformers have still overheated, it hasn't happened very often.

If your transformers aren't failing and your customers are OK with paying for it, I think you're doing the right thing. I can't think of many things would have a greater negative impact on a building (electrically speaking of course) than a transformer failure. I think if its not broken don't fix it!

I have heard of problems with people using K-rated transformers that are too high and having problems created by it, so that might be something to consider.

 

[lady sparks lover]

Re: Transformer secondary

Originally posted by ryan_618:

Originally posted by lady sparks lover:
[qb] For some reason, I tend to think it's a bit much, but what do you think??

I'll tell you the truth, I think your next statement answers your question...

We had on instance were some cheesy, bottom of the basement, thinks he knows it all contractor decided not to use a K-rated tranformer, and didn't use 200% neutral and it blew the tranformer up, and set the building on fire. It cost him for being dishonest and stubborn. He'll never contract in this area again!!

Sorry, no offense to electrician or contractors this guy just was some guy who thought he knew best!

 

[ryan_618]

Re: Transformer secondary

Thats too bad, but it sounds like he got what was coming to him. A quick question....for a typical ofice environment, what K rating do you normally use? Also, do you use 200% panels or just normal panels?

 

[lady sparks lover]

Re: Transformer secondary

Originally posted by ryan_618:
Thats too bad, but it sounds like he got what was coming to him. A quick question....for a typical ofice environment, what K rating do you normally use? Also, do you use 200% panels or just normal panels?

For an office, I would use K-4, but depending on the application I'll go up to a K-13. I think with a 200% neutral, and 80% deration, a K-4 might do it.

Yes, I usually get a panel with the 200% size neutal lug terminal installed.


Lady

 

[dereckbc]

Re: Transformer secondary

Originally posted by lady sparks lover:

For an office, I would use K-4, but depending on the application I'll go up to a K-13. I think with a 200% neutral, and 80% deration, a K-4 might do it. Lady

Patty, just curious, have you ever seen or witnessed a neutral go beyond 100%? The reason I ask is I can count the times I have on one hand, and of those few times it was never over 110%. I work strictly with data and telephone equipment which IMO has much higher harmonic potential than an office enviroment. It appears to me the highest percentage neutral currents I have seen are in 277 lighting panels.

 

[lady sparks lover]

Re: Transformer secondary

Originally posted by dereckbc: Patty, just curious, have you ever seen or witnessed a neutral go beyond 100%? The reason I ask is I can count the times I have on one hand, and of those few times it was never over 110%. I work strictly with data and telephone equipment which IMO has much higher harmonic potential than an office enviroment. It appears to me the highest percentage neutral currents I have seen are in 277 lighting panels.

You know Dereck, I haven't measured a technology panel over that, but all I can say is in the work that I do (mainly schools with technology panels with serve only computers), there is always at least 100%. I would think that would be enough to warrant:


1. you'll have 4 CCC in one conduit.

2. distorted voltage wave patterns.

If none else, the 80% derating is applicable.

It was my thought that the K-rated Xfmr is just insulated at the cores to prevent over heating, but does little to correct the distorted waves forms caused by harmonics.

I could be wrong you know...

 

[lady sparks lover]

Re: Transformer secondary

Originally posted by dereckbc:

Originally posted by lady sparks lover:

For an office, I would use K-4, but depending on the application I'll go up to a K-13. I think with a 200% neutral, and 80% deration, a K-4 might do it. Lady

Patty, just curious, have you ever seen or witnessed a neutral go beyond 100%? The reason I ask is I can count the times I have on one hand, and of those few times it was never over 110%. I work strictly with data and telephone equipment which IMO has much higher harmonic potential than an office enviroment. It appears to me the highest percentage neutral currents I have seen are in 277 lighting panels.

Hey Dereck,

To add, do they use the <10% THD ballast for the HID and fluorescent fixtures, because usually that takes care of that? That might be it, because using the old <20% THD never worked for us.


Lady