Voltage Ratio
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ryan_618
Senior Member
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- Salt Lake City, Utah
I still think turns ratio and voltage ratio are one and the same. Here is an interesting article on the subject (not saying it is right or wrong, as I know not who wrote it): http://www.tpub.com/content/doe/h1011v4/css/h1011v4_49.htm
It is one and the same regarding single phase and delta-delta or wye-wye three phase transformers. Delta-wye (and wye-delta) transformers would be, shall we say, special cases. Let's examine available transformer secondary current for the delta-wye instance...
Transformer: 480V delta primary, 208V wye secondary
Primary OCPD: 100A
How much secondary line current is generated when 100A primary line current passes through the primary windings? Note the answer is in amperes and not dependent on the usage voltage.
Primary OCPD: 100A
How much secondary line current is generated when 100A primary line current passes through the primary windings? Note the answer is in amperes and not dependent on the usage voltage.
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nafis
Senior Member
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- Palestine,tx
hope that is what you looking for
hope that is what you looking for
Secondary phase current = 480V * 100 A / 120*1.732 = 231 A
thanks
hope that is what you looking for
Secondary phase current = 480V * 100 A / 120*1.732 = 231 A
thanks
To show the math more correctly, it would be...
480V ? 100A ? √3 = 120V ? ?A ? 3
480V ? 100A ? 120V ? √3 = 231A
...or...480V ? 100A ? 120V ? √3 = 231A
480V ? 100A ? √3 = 208V ? ?A ? √3
480V ? 100A ? 208V = 231A
480V ? 100A ? 208V = 231A
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
That's what came to my mind.
nafis
Senior Member
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- Palestine,tx
Amen brother.
ryan_618
Senior Member
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- Salt Lake City, Utah
I spoke to a freind of mine that teaches for NFPA, and whose opinion I hold in rather high regard. He told me I am dead wrong, and that you guys are right. 
*Not that I don't respect people's opinion here as well, but I wanted to be able to speak to someone about it, not just type it.
*Not that I don't respect people's opinion here as well, but I wanted to be able to speak to someone about it, not just type it.
That's what I finally figured out. The coils are wound just the way you daid, but that's not the current flow the NEC is concerned with. The current ratio is as your buddy said.
carl
- Location
- New Jersey
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- Journeyman Electrician
Ok, so can anyone explain why we need to do this calculation and what does it really mean when using secondary conductors up to 25' long?
It's correct to a first approximation.
For a practical transformer, voltage ratio and turns ratio are not quite the same.
The calculation relates the secondary current to the primary current for comparison of the secondary conductor ampacity with the primary protection rating. I don't understand the multiplication by one-third the rating of the overcurrent device, though. Seems like it should be the other way around. Say you have a 4160-480 transformer with a 100A fuse in the primary. You would need an ampacity of 100?4160/480/3 = 289A for the secondary. If you actually had 300A in the secondary, you would only have 35A in the primary and the fuse wouldn't come close to blowing.
Don't forget the conductors must terminate at a single ocpd with a rating that is not more than their ampacity...
...but I too wonder why multiply by one third. Does anyone **know**???
weressl
Esteemed Member
Of course. The transformer is wound as follows:
There is a steel core with three legs. Each phase windings of the Primary and Secondary are wound on the same core leg. Each winding has a ratio that is the number of turns around the core. If the primary voltage is 480V and the turn ratio is 4:1 then the secondary voltage will be 120V. When you connect the three winding into a three phase system you still have 120V on each coil, but when you common one leg of each winding the voltage summary between the free legs of two coils will be (sqrt)3*(leg voltage) or ~208V. Of course this is only true if the Primary side is a three phase system and the winding is connected to each other in delta.
weressl
Esteemed Member
Care to elaborate?
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
Paragraph 240.21(C) (6) is part of the section of code relating to 'tap conductors'. Tap conductors are permitted to violate the general requirement that conductors be protected at their _supply_ end.
Tap conductors all require some sort of OCPD at their supply end, but this OCPD may be a large multiple of the conductor ampacity. Tap conductors are generally limited in length, and different OCPD to ampacity ratios are permitted in different circumstances.
As I read 240.21(C)(6) this provides a method by which the concept of a tap conductor may be extended to situations where the OCPD is on the primary side of a transformer, and we are evaluating the protection requirements on the secondary side.
-Jon
Tap conductors all require some sort of OCPD at their supply end, but this OCPD may be a large multiple of the conductor ampacity. Tap conductors are generally limited in length, and different OCPD to ampacity ratios are permitted in different circumstances.
As I read 240.21(C)(6) this provides a method by which the concept of a tap conductor may be extended to situations where the OCPD is on the primary side of a transformer, and we are evaluating the protection requirements on the secondary side.
-Jon
I'll try.
If you increase the load on the output of a transformer, its output voltage will drop due to its impedance. Thus, the input to output voltage ratio will change.
If the transformer is designed to produce rated output voltage at rated load, then the primary to secondary turns ratio has to be slightly lower than the stated voltage ratio to achieve this.
At least in the USA, transformers are not designed to produce rated voltage at rated load.
So, on load you get less than rated output voltage.
Thus, the voltage ratio changes between no load and rated load.
The turns ratio doesn't.
That was my point.
weressl
Esteemed Member
Exactamundo! Yet you seem to miss the same at the other post where the inrush current puts the transformer beyond its design limits thus sufficiently dropping the voltage to dimm the lights and not to do so when the transfomer is pre-loaded.
Incidently - as you pointed out - the difference between the V/V and T/T ratios is insignificant.
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