monkeyhead
Member
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.
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.