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Im currently studying for my Construction Electrician Red Seal exam and I’ve been using the 2018 CEPE app.

I keep getting questions wrong regarding circuit breaker and fuse sizes due to the “manufactures chart” specs. My math is all correct but when it comes time to choose a breaker or fuse size I will answer incorrectly sometimes and it always says to refer to “manufactures charts” which is not in the code book.

For example I just had the following question : “What is the minimum size of non time delay fuse permitted to be used to protect a 25kVA, 600-120/240V, 1-phase, 3-wire dry-type transformer?”

25kVa/600= 41.6A

My math works out to “41.6amps x 1.25= 52 amps.

The answers I can choose from are:
A) 40A
B) 50A
C) 60A
D) 70A

I choose B) 50A because rule 26-254 1) states that a “circuit supplying the transformer shall be provided with overcurrent protection rated or set at NOT MORE THAN 125% OF THE RATED PRIMARY CURRENT”.

Then Rule 26-254 3) states “Where a value not exceeding 125% of the rated primary current of the transformer as specified in subrule 1) does not correspond to the standard rating of the overcurrent device, the next higher standard rating shall be permitted.”

Without having access to a manufactures chart, how am I supposed to know when subrule 3 applies?
 

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Professional Electrical Engineer, construction industry.
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The transformer manufacturer rating always equals the calculated value. Your math is correct. 25kVA, single phase, 600-120/240V transformer would have a maximum primary amp rating of 41.7A regardless of manufacturer. No chart required.
 

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I don't think it has to anything to do with manufacture rating.

Where a value not exceeding 125% of the rated primary current of the transformer as specified in
Subrule 1) does not correspond to the standard rating of the overcurrent device, the next higher
standard rating shall be permitted.

52A is not a standard size, therefore you would be allowed to size up. In practice, I would not do this. I would go down to 50A and not size a transformer that tight to the needed amperage. Mind you the bulk of the transformers I do are around the 10kva mark for controls, and we fuse both primary and secondary sides and it changes the fuse layout entirely. But that does not help your questions.

I really don't miss studying this questions.....
 

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It's a trade knowledge question with some code built in. The calculation is basic. key word is "NOT EXCEEDING"
 

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I do not understand how you consider this trade knowledge, that is pretty clearly code book. And yes, it does say exactly that not exceeding and then it says "THE NEXT HIGHER RATING SHALL BE PERMITTED" (caps makes it right, right?). I mean if you were going to go lower, you would say the next lower rating shall be allowed.

Trade knowledge to me, especially in the questions they ask, CANNOT be found in the code book. How to cut MC / Tek cable for one example. How to reverse a 3 phase motor, how to reverse a dc motor ect.
 

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I do not understand how you consider this trade knowledge, that is pretty clearly code book. And yes, it does say exactly that not exceeding and then it says "THE NEXT HIGHER RATING SHALL BE PERMITTED" (caps makes it right, right?). I mean if you were going to go lower, you would say the next lower rating shall be allowed.

Trade knowledge to me, especially in the questions they ask, CANNOT be found in the code book. How to cut MC / Tek cable for one example. How to reverse a 3 phase motor, how to reverse a dc motor ect.
Exactly why someone that studied the code book can't go in and get 100% on the CoQ exam. It contains trade knowledge questions. As for the caps, yes the next higher rating shall be permitted to use does not mean that it is the most correct. you would size down as to not overload the transformer. That is the trade knowledge. If it asked for the highest rating of OC you an use then yes, 60 would be correct. but its asking for minimum so its just a standard Math + read the rules question.

You stated this in your post previous to this I just wanted to reiterate.
 

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Well I agree that it is "Trade Knowledge" too, because unless you are out there working, you do not know what the standard sizes of fuses are. You need to know the book to get the calculated answer, but that is only the start.

Cheers
John
 

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As for the caps, yes the next higher rating shall be permitted to use does not mean that it is the most correct. you would size down as to not overload the transformer.
Just a quick counterpoint to this note. I never hesitate to upsize transformer breakers to the next higher size. Due to modern minimum energy efficiency ratings, there is more iron and magnetics in transformers than ever before. Impedance is down, weight is up, and parasitic heat loss is way down. The actual real-world kVA capacity is WAY above nameplate. Hence, if the code allows increasing the breaker to the next size, I say do it. Might as well get every last amp out of the equipment.
 

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Just a quick counterpoint to this note. I never hesitate to upsize transformer breakers to the next higher size. Due to modern minimum energy efficiency ratings, there is more iron and magnetics in transformers than ever before. Impedance is down, weight is up, and parasitic heat loss is way down. The actual real-world kVA capacity is WAY above nameplate. Hence, if the code allows increasing the breaker to the next size, I say do it. Might as well get every last amp out of the equipment.
Learnt something new, That's why I'm here (y)
 

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I disagree with the 'trade knowledge' with this.

Trade knowledge, would be knowing you had a 25kva transformer feeding a (ie) 25A load. You would protect accordingly.

Given the question, If you're installing a 25kva tranny and don't know what the loads are, max it out.
 

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Well I agree that it is "Trade Knowledge" too, because unless you are out there working, you do not know what the standard sizes of fuses are. You need to know the book to get the calculated answer, but that is only the start.
I would argue that with table 13, it is still technically code knowledge. The table shows those exact fuse sizes too. Now I would argue the trade knowledge comes from the fact that table isn't really referenced where I think it would help with that questions

Exactly why someone that studied the code book can't go in and get 100% on the CoQ exam.
I have met plenty of sparkies who cannot even terminate a wire, and yet have a full licence.

I had one wiring up a municipal dispatch center, and ask me how a relay works...I mean come on....and licensed. He now works at a hospital. Nice guy, but....yeah I don't hold much by the supposed test to qualify
 

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Here is one many guys forget or don’t know.

If you use 26-254(3) and go to the next higher overcurrent, you may also have to upsize the secondary conductors and bonding size. When you increase the overcurrent on the primary of a transformer that does not have secondary overcurrent you are also allowing more current on the secondary.
See 26-256(4)

For instance if you wired a 600/208 3 phase 75 kVA transformer with a 70 amp primary overcurrent, the secondary conductors and bonding would have to be rated for 201 amps.

The same transformer with a 90 amp primary overcurrent would need secondary conductors and bonding rated for 260 amps.
 
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