Voltage Ratio

[A/A Fuel GTX]

I'm looking for the formula to calculate primary to secondary voltage ratio as required in 240.21(C) (6).....:-?

 

[billsnuff]

480/277 = 1.733/1

 

[infinity]

Primary to secondary would be 480/208 = 2.31

 

[A/A Fuel GTX]

Thanks....somehow I figured it would be more complicated

 

[ryan_618]

Primary to secondary is 480 to 120, not 480 to 208. We are comparing (creating a ratio) a phase to a phase, not a phase to two phases. Assuming a delta wye transformer.

 

[iwire]

Primary to secondary is 480 to 120, not 480 to 208. We are comparing (creating a ratio) a phase to a phase, not a phase to two phases. Assuming a delta wye transformer.

Ryan I don't understand why we would use 480 to 120.:-?

I am not saying your wrong only that I just don't get it.

My gut says it would be 480 to 208. :-?

 

[ryan_618]

Voltage ratio is the same as volts per turn on a transformer. If you know the primary voltage (on a given phase), and you know the turns ratio of the transformer, you can figure out the secondary voltage. The voltage of the secondary in a wye connected system is a function of the single phase voltage. The line to line voltage is simply a matter of how you configure the phases (delta or wye).

Draw a delta, and tell me the voltage of any one phase. It will be 480. Draw a wye and tell me the voltage of any one phase. It will be 120. Connect three of the 120 phase together in a wye configuration and you can get 208 phase to phase, but this doesn't change the turns ratio (pr voltage ratio) of the transformer.

Lets see how the engineers kill me on this one...

 

[coulter]

...Lets see how the engineers kill me on this one...

Not this one. Was just reading along - not paying any attention - just nodding my head. Hit ryans post, blinked, saw the windings in my head and thought, "Yep, right on, that's the way they are wound - glad one of us was awake."

carl

 

[winnie]

Gonna disagree here.

For purposes of 240.21(C)(6), the ratio that we care about is that of input voltage to output voltage, not of any individual windings, but of the entire transformer or bank feeding the conductors in question.

It is quite true that the voltage ratio of individual phase coils in a 480 to 208/120 transformer is 4:1. But the ratio of current flowing in the primary supply conductors to the secondary output conductors is 2.31:1. The reason for these two different ratios is that the current on each primary supply conductor gets divided between _two_ primary coils, but the secondary comes out of _one_ coil.

Thinking on this a bit further, if you have a transformer bank, then internally to the bank you might have conductors which are subject to a 4:1 voltage ratio, but once everything is combined and you have your 3 output phases, IMHO the ratio is 2.31:1

-Jon

 

[brian john]

480/';120 is 4 to one when TTR'd it is off that by a bit like 4.012

Think of it as three separate transformers 480 to 120

A-B at 480 to A-N 120
B-C at 480 to B-N 120
C-A at 480 to C-N 120

Think of the delta 3 lines (3 transformers) only connected at the common phase and the wye 3 lines (3 transformers) connected at the neutral.

I have a drawing somewhere????

 

[iwire]

Man, I thought I was just starting to understand it and Jon throws a wrench in the works. :grin:

It's all good, by the time this thread is done I will have learned a few things.

 

[A/A Fuel GTX]

Man, I thought I was just starting to understand it and Jon throws a wrench in the works. :grin:

It's all good, by the time this thread is done I will have learned a few things.

Glad I could help, Bob.:wink: Gotta love this forum, the knowledge that is shared here is priceless.

 

[jghrist]

I've got to throw in with infinity. There's a turns ratio and a voltage ratio. The voltage ratio is the ratio of ?-? voltages of the transformer. The turns ratio is the ration of individual windings.

 

[infinity]

I've got to throw in with infinity. There's a turns ratio and a voltage ratio. The voltage ratio is the ratio of ?-? voltages of the transformer. The turns ratio is the ration of individual windings.

The handbook has an example and they use 480/208 volt ratio.

 

[Smart $]

Gonna disagree here.

...IMHO the ratio is 2.31:1

FWIW I concur :grin:

 

[iwire]

There's a turns ratio and a voltage ratio.

Now I am getting it. :smile:

 

[Smart $]

Now I am getting it. :smile:

Sounds like you're not totally convinced...?

 

[billsnuff]

so, 240.21.c.6.1 is 2.31 x (primay OCP/3)= min ampacity secondary conductor, right?

 

[infinity]

so, 240.21.c.6.1 is 2.31 x (primay OCP/3)= min ampacity secondary conductor, right?

That's the way I read it.

 

[Smart $]

so, 240.21.c.6.1 is 2.31 x (primay OCP/3)= min ampacity secondary conductor, right?

That's the way I read it.

Provided its a 480 primary, 208Y/120 secondary transformer. Other configurations will likely have a different ratio. Take 480 primary, 240/120 3? 4W secondary transformer as an example...