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Discussion Starter · #1 · (Edited)
Help please. Transformer conductor protection confusion. Down below is example problem, with solution, from NEC Handbook, relates to Article/Section 240.4. Can anyone please comment as to the reasoning for the conclusions reached in the solution, including the conclusion that the 50 amp breaker on the primary is the correct max size/rating for protection of the secondary conductor. ( why say that the sec/pri ratio that equals 0.5 means that the 50 amp breaker is right for protecting the 100 amp ampacity secondary conductor? I fail fully to follow/detect/understand the path of reasoning all the way through to the end conclusions. ).
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Application Example:
A single-phase transformer with a 2-wire secondary is provided with primary overcurrent protection rated at 50 amperes. The transformer is rated 480/240 volts. Conductors supplied by the secondary have an ampacity of 100 amperes. Is the 50-ampere overcurrent protection allowed to protect the conductors that are connected to the secondary? The secondary-to-primary voltage ratio in this example is: 240 480 = 0.5
Multiply the secondary conductor ampacity by this ratio: 100 amperes
Solution
0.5 = 50 amperes
Therefore, the maximum rating of the overcurrent device allowed on the primary of the transformer that will also provide overcurrent protection for the secondary conductors is 50 amperes.
( END )
 

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Over simplified. A single phase tranny output is directly proportional to it's input. The same cannot be said to occur with a 3 phase transformer.
 

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Because the transformer is only putting out 50A on the secondary side. You're being misled by the phrase:

Conductors supplied by the secondary have an ampacity of 100 amperes.

That just means that the conductors have 100A running through them. (Corrected as Wildleg suggested.)

Looking at the math in the example:

The wires can handle 100A

The transformer P/S Ratio is .5 because it is a single phase transformer. In this case you are first dividing the 240V by 480V...that gives you the .5 ...

Then you take the secondary conductor size and divide IT by that ratio...

100A conductor divided by .5 = 50A

The over-current protection on the primary is sufficient.
 

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Another thing to remember is to look to the appropriate sections of the NEC.

Article 450 is what you will use to size OCP for transformers.

Article 240 is what you will use to size OCP for conductors.

Both of the sections have to be utilized for a proper installation because you can't just use one section and assume that the OCP requirements are met to satisfy transformer protection and conductor protection.

Pete
 
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there is only one place you need to look for the answer to this question: 240.4(F)

you should always look in 450 as noted above, though, for answers specific to transformers.

the answer above that says there won't be 100 amps on the secondary is wrong. the current on the secondary is proportional to the current on the primary x turns ratio, minus transformer heat loss. or to put it another way, the power on the primary = power on secondary minus heat loss (and transformer inefficiency.)
 

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....

That just means that the conductors are big enough to carry 100A...it does not mean the transformer is PUTTING OUT 100A.
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this statement is so misleading and has no point ? if the primary can carry 50 and the turns ratio is 2:1, the secondary will potentially have 100 amps on it. The conductors must be sized accordingly and meet the rules of 450.3b and 310.16.
 

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this statement is so misleading and has no point ? if the primary can carry 50 and the turns ratio is 2:1, the secondary will potentially have 100 amps on it. The conductors must be sized accordingly and meet the rules of 450.3b and 310.16.

Corrected.

Edited Again.

The OP wanted to know how the Mathematics of the equation could make sense. This is a single phase transformer question...NOT rocket science. It's an EXAMPLE Wildleg...it's supposed to be simple.

Tossing out code sections doesn't help the guy get WHY the numbers match now does it. Maybe my example could have been more PRECISE by throwing in code references that he may or MAY NOT have understood...so I specifically stuck with the math OK?


I love LOVE when guys have to prove how specific and savy they are to everyone...OFTEN to the detriment of the person asking the question. Perhaps, in truth, the OP needed BOTH our input to sufficiently answer his question. But criticizing others who are validly helping the person does nothing to help the OP whatsoever.

Thanks for reading.
 

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Corrected.

Edited Again.

The OP wanted to know how the Mathematics of the equation could make sense. This is a single phase transformer question...NOT rocket science. It's an EXAMPLE Wildleg...it's supposed to be simple.

Tossing out code sections doesn't help the guy get WHY the numbers match now does it. Maybe my example could have been more PRECISE by throwing in code references that he may or MAY NOT have understood...so I specifically stuck with the math OK?


I love LOVE when guys have to prove how specific and savy they are to everyone...OFTEN to the detriment of the person asking the question. Perhaps, in truth, the OP needed BOTH our input to sufficiently answer his question. But criticizing others who are validly helping the person does nothing to help the OP whatsoever.

Thanks for reading.
but the conductors may very well have 100a on the secondary. how is your original answer helping to make it simple ?
 

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but the conductors may very well have 100a on the secondary. how is your original answer helping to make it simple ?
I wanted to simplify it to it's base numbers just so he could focus on the math.

I was wrong for the statement I made, changed it, left the math alone for him.
 
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