Transformer primary side protection

monkeyhead

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Sep 2004
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I've got a machine that appears to have it's transformers protected completely wrong. They've got a 7.5 kVA, 480v to 240v/120v, single phase step down transformer protected on the primary side with a 35A breaker. One the secondary side they have 3 seperate circuits, a 30A center-tapped 120V circuit, and two 30A 240V circuits.

I've always used the following formula on transformers.

Primary side: 1.5 * (VA_Rating / Primary_Votage)
Secondary: 0.8 * (VA_Rating / Secondary_Voltage)

So for this transformer, I'd end up with 25A breakers on both sides.

The bizarre thing, is that they have an amp rating of 15.6A printed on the schematics next to the transformer, but still have a 35A breaker protecting it.

Then they also use a a much smaller 250VA 120V to 24V step down transformer with a 10A breaker on the primary and nothing on the secondary. So if this thing shorts out on the secondary, it seems to me it will burn itself up well before the circuit breaker pops.

Am I totally missing something, or have I just come across a design flaw? The OEM has hundreds of these machines deployed so this seems surprising to me.
 
From what I can tell from NEC Article 450.3, it looks like they are allowed 250% on the primary if they also have secondary protection, which can itself be rated up to 125% or rated. But unless they have an overall secondary breaker i think they are incorrectlyt sized on the secondary.

Without any other information I'm gussing that the 120/24 transformer is covered by Note 3 from table 450.3(B).

Keith
 
Last edited:
Thanks Keith. I knew my calculations were on the conservative side, but I didn't realize that when you have both primary and secondary protection that you can step up the primary side protection to 2.5 times the rating.

They do seem to be violating Note 2 of Table 450.3(B) on the secondary.

On the smaller 250 VA there's no manufacture provided thermal overload protection, so I think they just blew it. The primary really shouldn't have any more than a 3A breaker based on this table.

I also need to verify what's in the enclosure... I've found other problems on these schematics, and popped open the control panel to find that everything is actually done right. Maybe they've got a guardian angel wherever they're getting their panels built that's keeping them safe.
 

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