# NEC Code: Transformer Overcurrent Protection



## jd5191 (Dec 29, 2020)

When selecting over-current protection for a transformer, the protection tables in Article 450 are broken up into transformers above 1000V and those up to 1000V. If you have, for example, a 4160V/480V transformer, do you use the "above 1000V" table for the high side and the "up-to-1000V" table for the low side? Or "above 1000" for both sides?


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## akyip (Dec 29, 2020)

From the practice problems that I've done, if any of a transformer's sides is greater than 1000 V, then you have to use the table for over 1000 V for both sides of the transformer (even if the other side is not over 1000 V).

If none of the transformer sides are rated over 1000 V, then you can use table for under 1000 V.

That is what I have been seeing with the practice problems I've been doing.

This is a good question. Hopefully someone with more experience than me can chime in on this


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## Byk (Dec 29, 2020)

jd5191 said:


> When selecting over-current protection for a transformer, the protection tables in Article 450 are broken up into transformers above 1000V and those up to 1000V. If you have, for example, a 4160V/480V transformer, do you use the "above 1000V" table for the high side and the "up-to-1000V" table for the low side? Or "above 1000" for both sides?


You will use over 1000V for primary and 1000V or less for secondary.

In your example (assuming any location and Z&lt;6%)

Primary:

Fuse would be rated at 300% of FLA or next higher standard size
Circuit Breaker would be rated at 600% of FLA or next higher standard size

Secondary:

Fuse would be rated at 125% of FLA or next higher standard size
Circuit Breaker would be rated at 125% of FLA or next higher standard size


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## akyip (Jan 10, 2021)

Byk said:


> You will use over 1000V for primary and 1000V or less for secondary.
> 
> In your example (assuming any location and Z&lt;6%)
> 
> ...


Hey, I'm circling back to this because I'm still a bit lost.

So let's say for example: a 3-ph, 13000 V / 240 V transformer.

If you were asked for BOTH primary and secondary protection for such a transformer, you have to use both tables even for just this one transformer? So the Table for Over 1000 Volts used on the primary side, and the Table for 1000 Volts and less on the secondary side?

I'm just asking and trying to confirm, because I've never dealt with a question like this before. Most of the transformer OCPD questions I've done only asked for primary only protection...

Another reason I'm a bit confused about this is because of how the tables are labeled:

Table 450.3(A): Maximum Rating or Setting of Overcurrent Protection for Transformers Over 1000 Volts

Table 450.3(B): Maximum Rating or Setting of Overcurrent Protection for Transformers 1000 Volts and Less

To me, based on the table titles, it sounds like you are actually supposed to use Table 450.3(A) for a transformer over 1000 volts, for both windings, even if only one winding is actually over 1000 V and the other isn't. And if the transformer in question does not have a winding over 1000 V, then you use Table 450.3(B).

Unfortunately for me I've never seen such a practice question like this, so I don't know...


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## Byk (Jan 10, 2021)

akyip said:


> Hey, I'm circling back to this because I'm still a bit lost.
> 
> So let's say for example: a 3-ph, 13000 V / 240 V transformer.
> 
> ...


You are correct, you will only use 1 table only. So for the example above 4160V/480V transformer you would use 430.3.A. 

just like you stated in your first response.  

I apologize for the confusion.


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