Transformer Ratio

[augie47]

This calculation has always given me a fit. I tend to get it backwards.... Will one of you math gurus go over it for me.
240.21(C)(1) says in part:
(1) Protection by Primary Overcurrent Device. Conductors
supplied by the secondary side of a single-phase transformer
having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

In another post, the OP has primary 208v 3phase(1/0 cu thhn) stepping up to secondary 480v (#2 cu thhn) With 150 amp ocpd in primary

Are his secondary conductors considered to be protected using the ratio rule ?

 

[jumper]

Too tired to do math, best I can do.:ashamed1:

http://forums.mikeholt.com/showthread.php?t=143653

 

[hurk27]

You can do it a couple ways and using the ratio is just one of them.

In the other thread you were correct as the transformer has a 2.3:1 ratio or in the case of using it in a step up mode 1:2.3

75,000 va /480/1.732 will give you 90.213 amps, now you can multiply this amperage with the 2.3 ratio and you will get 208 amps for the secondary or you can just calculate the 75,000 va /208/1.732 and it will give you the same 208 amps for the 208 volt side of the trany.

with the 125% allowance for the primary to protect the secondary will allow the primary to be protected by OCPD of 260 amps or less to meet the requirements in 240.21(C)(1) this should allow the inrush to not trip the OCPD when the trany is energized, but I'm very surprised that the inrush is not a problem by him using a 150 amp breaker as the lower voltage side of the trany is also a lower impedance which can give you a much higher inrush when using a transformer like this.

With 240.21 not allowing 240.4(B) to be used (next size up rule) the max breaker that can be used as per 240.6 will be a 250 amp and still meet the 125% allowance for a 75kva trany.

If the 150 amp breaker does hold then using the ratio method (150 amps/2.3) will limit the secondary load to 65 amps @480v which I think you have to use the 125% rule that will require conductors of at least a 81.25 rating but not sure about this?

Also 240.21(C) can not be used if the secondary is 4-wire 277/480 volt and there is 277 volt loads, this is because the current on the secondary could exceed the conductors ampacity before the primary OCPD would ever open, this is only for delta to delta or with single phase with a single voltage secondary (2 wire) if this was the case then you would have to have OCPD's on the secondary.

I get confused with some of this also so if I have somthing wrong let me know.

 

[hurk27]

I think I have to re-think this alittle?

If we protect a transformer at the 125% as allowed, should we also size the secondaries to the same 125%?

lets say we protect this 75kva transformer at the allowed 125% which is 260 amps which will allow a 250 amp breaker or fuses, using the ratio method this would allow the secondary conductors to be loaded to 108.7 amps before the primary OCPD would open if we had sized the conductors at the 480 volt rating 90.2 amps that would be the normal current availble from the secondary, it could be possible to overload the conductors? of course GFSC protection would still be met, but not overload, I guess I need to look deeper in the NEC to see if there is a requirment to size the secondary conductors at the transformer ratio of the primary protection?

Funny thing is (and I'm not good at math) is if I apply the 125% to the 90.2 amps of the secondary then it comes out to 112.75 amps, so the ratio doesnt comput in this case?

Anyone with any input on this?

 

[jim dungar]

If we protect a transformer at the 125% as allowed, should we also size the secondaries to the same 125%?

Yes.

You may be making it more complicated then it needs to be. We know the ratio of the transformer. We know that stepping up the voltage means the current must go down, so we need to divide by the ratio.

Determine the primary protective device; this requires you to use the 125% factor.
Take the primary protective device rating and divide by the turns ratio.
You now have the minimum rating of your secondary conductors.

You have already include the 125% factor, you have already upsized the protective device.

 

[hurk27]

Yes.

You may be making it more complicated then it needs to be. We know the ratio of the transformer. We know that stepping up the voltage means the current must go down, so we need to divide by the ratio.

Determine the primary protective device; this requires you to use the 125% factor.
Take the primary protective device rating and divide by the turns ratio.
You now have the minimum rating of your secondary conductors.

You have already include the 125% factor, you have already upsized the protective device.

Thanks Jim
Not that I try to over think anything :ashamed: but I wasn't sure about this.

Also when I do the math on a 208/480v 75kva transformer, as above I have 208 amps for the primary and 90.2 amps for the secondary, with a turn ratio of 1:2.3 so using 125% rating of the primary will require a 250 amp OCPD but when I divide this by my turn ratio of 2.3 I get 108 amps for the secondary, but if I try to add the 125% to the 90.2 amps for the secondary I will get 112.75 amps, just seem they should add up but they don't?:?

 

[augie47]

afraid I still can't wrap my brain around this.... anyone else want to give it a try...

 

[jumper]

afraid I still can't wrap my brain around this.... anyone else want to give it a try...

Let's try this. Last section.

http://ecmweb.com/code-basics/transformer-secondary-conductors

Note that multiplying by 1/3 is the reciprocal of dividing by 1.67.

Use 1.25 and 1/4 for xfmrs over 9A.

 

[david luchini]

afraid I still can't wrap my brain around this.... anyone else want to give it a try...

In the OP, you have a secondary conductor of #2 (ampacity = 115.) You have a secondary-to-primary voltage ratio of 480/208=2.30769. You have a primary OCPD of 150.

Your secondary conductor ampacity times your voltage ratio cannot be less than the primary OCPD (or your primary OCPD cannot be greater than the secondary conductor ampacity times your voltage ratio.) 115*2.30769 = 265.384. The secondary conductors are protected by the primary OCPD.

You could go as small as #6. Ampacity of #6 is 65... 65*2.30769=150. #6 would be protected by the primary 150A OCPD.

If you tried #8... 50*2.30769=115.384. The primary OCPD is greater than the secondary conductor ampacity times the secondary-to-primary voltage ratio, so #8 secondary conductors would NOT be protected by the 150A primary OCPD.

 

[jumper]

In the OP, you have a secondary conductor of #2 (ampacity = 115.) You have a secondary-to-primary voltage ratio of 480/208=2.30769. You have a primary OCPD of 150.

Your secondary conductor ampacity times your voltage ratio cannot be less than the primary OCPD (or your primary OCPD cannot be greater than the secondary conductor ampacity times your voltage ratio.) 115*2.30769 = 265.384. The secondary conductors are protected by the primary OCPD.

You could go as small as #6. Ampacity of #6 is 65... 65*2.30769=150. #6 would be protected by the primary 150A OCPD.

If you tried #8... 50*2.30769=115.384. The primary OCPD is greater than the secondary conductor ampacity times the secondary-to-primary voltage ratio, so #8 secondary conductors would NOT be protected by the 150A primary OCPD.

A transformer thread and David shows up.....who would have guessed.....I'm telling y'all he's got radar fellas.

David, you got this transformer stuff down pat.:thumbsup:

 

[augie47]

Yes he does
Thanks

 

[david luchini]

A transformer thread and David shows up.....who would have guessed.....I'm telling y'all he's got radar fellas.

Der?k, you've inspired me to add an avatar

 

[jumper]

Der?k, you've inspired me to add an avatar

Cool. We can be twins.

Me. In real time.