Sizing Fuses for Transformer

Hi Everyone,

I am a student and am new here and I have a question that is not a plc topic. I would like to know how to go about sizing fuses on the primary side of a transformer. The transformer is 10KVA. the primary voltage is 480V three phase and the secondary is 200V three phase. I fused the secondary based on the load. The primary already has a 30 amp breaker. My task is to put a disconnect switch with j fuses before the breaker. My question is would the fuses be of the same value as the breaker? My answer came out to 30.05 amps. My calculation is as follows - 10000/(480*1.732). I was told to calculate my fuses based on the secondary load. I am not sure about this and would like a second opinion. Thanks.

I wanted to update my calculations. I got 30.05 because I am basing my fuses at a 250% rating. (10000/(480*1.732))*2.5

Hi and welcome to the forum. I have been an electrician for years and finally got into programming.
What I would advise you to do is look in the most current edition of the NEC. I had last used the 2014 edition for my journeyman license renewal. Transformers are Section 450. Read carefully, because there are other sections it will refer you to reference regarding feeders for transformers, branch circuits and numerous tables that are sprinkled about the code book. Hope this helps you and again welcome to the forum.

redeye1234 said:

Hi Everyone,

I am a student and am new here and I have a question that is not a plc topic. I would like to know how to go about sizing fuses on the primary side of a transformer. The transformer is 10KVA. the primary voltage is 480V three phase and the secondary is 200V three phase. I fused the secondary based on the load. The primary already has a 30 amp breaker. My task is to put a disconnect switch with j fuses before the breaker. My question is would the fuses be of the same value as the breaker? My answer came out to 30.05 amps. My calculation is as follows - 10000/(480*1.732). I was told to calculate my fuses based on the secondary load. I am not sure about this and would like a second opinion. Thanks.

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Seems like a bad idea to fuse the primary side of the transformer with fast acting Type J fuses. I'd put those on the secondary side. Ideally, you'd want to limit the primary side fuses to 125% of the transformer's rated FLA, but magnetizing inrush current on power up could be as high as 12 times FLA.

saultgeorge said:

Hi and welcome to the forum. I have been an electrician for years and finally got into programming.
What I would advise you to do is look in the most current edition of the NEC. I had last used the 2014 edition for my journeyman license renewal. Transformers are Section 450. Read carefully, because there are other sections it will refer you to reference regarding feeders for transformers, branch circuits and numerous tables that are sprinkled about the code book. Hope this helps you and again welcome to the forum.

Click to expand...

You guys have this covered pretty well as is. I also am a licensed electrician and installed a hundred or so transformers over the years, and had many inspected by the AHJ.

The single most important thing about overcurrent/short circuit protection for the transformer is to FUSE/CB the secondary exactly at 125% of the transformers output rating.

Your load may be smaller than the transformers secondary output, but you need to analyze your load separately and protect your loads, and feeders downstream of the transformer base on their individual requirements.

Again to repeat, the protection of the transformer is 125% of the secondary rating.


And again you are also correct the primary fusing/CB is really just short circuit protection if there were to be a RARE failure on a transformer with the correct secondary protection. Yes, the NEC NFPA70 would allow 250% of primary rating of the transformer.


You do want to make sure that the primary fuse/CB can handle the inrush of the transformer, not in a function of amperage, but in a function of TIME-DELAY. A LPJ Class J fuse actually is a good choice to get through the inrush, or a D Curve breaker.


Do NOT use class CC fuses, or fast acting fuses, or supplementary circuit protectors on the primary.

The NEC got this one right. There are hundreds and hundreds of transformers that last years and years, all due to proper protection of their secondary.

Thank You

Plastic said:

...and had many inspected by the AHJ.

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Only "many"? I smell side jobs here...

rootboy said:

Only "many"? I smell side jobs here...

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Actually, I NEVER did a side job. As an industrial electrician, working many Saturdays, and Sundays due to the need to make installations during non-manufacturing schedule.... The pay and overtime is more than enough.

Also, side jobs represent real liability risk.

The transformers that were inspected by AHJ were permitted jobs as installations to the industrial premises, that are covered under the scope and jurisdiction of building code authorities.

The majority of the transformers installed were part of the industrial process machinery, and not looked at by inspectors. But these transformers had to satisfy safe, sufficient, at 100% uptime requirements of our customers.

Back to the OP. 200v secondary is u usual. If merely a homework question, the just do the math 10,000/200/1.73*1.25 for your secondary protection.

The most common secondary would be 208/120v WYE 4-wire secondary. Another common secondary would be 240 Delta output.

Curious? Is the 200 just a math problem, or are you approaching an actual installation.

NFPA 70 NEC requires the secondary of your separately derived system to be Bonded to facilitate the activation of your short circuit protective device during ground fault circumstances.

Your secondary is allowed to be un-bonded if you have purposeful engineering reasons, but you must provide addition ground fault indication devices, and display plaques indicating that an un-grounded system is present.

Regards

redeye1234 said:

... The primary already has a 30 amp breaker. My task is to put a disconnect switch with j fuses before the breaker. ...

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Why would you add a fused disconnect AHEAD of the primary protection circuit breaker? You need one OR the other, but not both.

jraef said:

Why would you add a fused disconnect AHEAD of the primary protection circuit breaker? You need one OR the other, but not both.

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It's very common in industrial plants (almost without exception). The disconnect is stabbed into the buss duct, and fused there. Then there is the ground level disconnect with its fuses and/or breakers.

A common mistake that new engineers make is to fuse the buss plug disconnect at the same value as the equipment. Don't do that. Fuse it to the max so that the fuses on the ground level are the ones to go and not the ones that require the buss to be shut down to replace the fuses.

Plastic said:

Actually, I NEVER did a side job. As an industrial electrician, working many Saturdays, and Sundays due to the need to make installations during non-manufacturing schedule.... The pay and overtime is more than enough.

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I'd have never made it through my apprenticeship without them.

These days I tell people that I'm a "Handyman". One that carries a Master's license...

I haven't had to deal with inspectors for over thirty years.