Transformer Primary Protection

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steve66

steve66

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Is there a standard for sizing circuit breakers or fuses on the primary of a transformer? Some percentage of the full load amps or something like that?

I have recently seen the ratio of 12x used for the ratio of inrush current to full load current. Does that sound right?'

If so, I'm having trouble finding breakers (and even fuses) that are certain to hold during the inrush.

Is it normal to use 1.5 or even 2.0 times the primary current for a circuit breaker size? And doesn't that mean your primary conductors have to be that much larger?

Any suggestions?

Steve
 
See 450.3 in the NEC. It looks like you are limited to 250% if you have both primary and secondary protection, 125% if you have only primary protection. That is assuming your transformer is under 600 volts.
 
steve66 said:
Is there a standard for sizing circuit breakers or fuses on the primary of a transformer? Some percentage of the full load amps or something like that?

I have recently seen the ratio of 12x used for the ratio of inrush current to full load current. Does that sound right?'

If so, I'm having trouble finding breakers (and even fuses) that are certain to hold during the inrush.

Is it normal to use 1.5 or even 2.0 times the primary current for a circuit breaker size? And doesn't that mean your primary conductors have to be that much larger?

Any suggestions?

Steve
Click to expand...

Are you protecting transformer with primaries <=600V?
Then you have two choices. 125% FLA with primary only protection, or 250% FLA if there is secondary protection at 125%.

If you are protecting a primary >600V you the same choices except the % are different. This is where you have heard the 12x number. Medium Voltage fuses for transformer primaries are chosen based on being able to hold 12X FLA for .1sec, as well as the 300% FLA.

For what it is worth.
You will notice there is a difference in the maximum size of a medium voltage fuse versus that of a medium voltage circuit breaker. This is due to how Time Current Curves of medium voltage fuses and "breakers" are drawn, which are not the same as for low voltage devices.
 
Under 600 Volts is what I had in mind. Generally, I'm thinking of a 480 to 208V transformer, in the 75 KVA to 150 KVA range.

Iwire:

I understand the NEC allows 250% of the primary rating, but I didn't think it was common to actually size the primary breaker that high. It seems like a waste to use a 200A circuit breaker and 3/0 wire on the primary of a 75 KVA transformer, since its rated primary current would only be 90 amps.

Jim:

I think I saw the 12x number in one of your posts, and also noticed that's what SKM software puts on their TCC by default. So using 12x rated current for 0.1 sec is not the right way to look at low voltage transfomers?

Thanks for all the replies so far.
Steve
 
steve66 said:
I understand the NEC allows 250% of the primary rating, but I didn't think it was common to actually size the primary breaker that high. It seems like a waste to use a 200A circuit breaker and 3/0 wire on the primary of a 75 KVA transformer, since its rated primary current would only be 90 amps.
Click to expand...

In my experience you are right.

We install 100s of 75s and the primary will typically be 125 to 150 if the design is done by an EE.

If an electrician does the design it will often be a 100 amp breaker on the primary and I have never seen one trip on start up.
 
iwire said:
In my experience you are right.

We install 100s of 75s and the primary will typically be 125 to 150 if the design is done by an EE.

If an electrician does the design it will often be a 100 amp breaker on the primary and I have never seen one trip on start up.
Click to expand...

So it sounds like the EE's are using about 150% of the primary FLA, and some of the electricians have been using about 110%.

I've generally been using about 125%. But I am starting to wonder if that is adequate.

Last week, I did see a 225 A breaker trip when a 150 KVA switched on, but the magnetic trip on the breaker was set to minimum. That may have been the first time that breaker has tripped in 5 years.

Steve
 
steve66 said:
some of the electricians have been using about 110%.
Click to expand...

Well yes, and no. :smile:

They are not trying to use 110% usually they are trying to use a 100 amp breaker that will fit in an existing panel without heavy modification.

A 125 or 150 is a 200 amp frame size and will not fit in most panels unless a sub feed kit is available for the existing.

It's strictly economics, not design. But we also do not like call backs and I can't say I have every had any when using a 100 for a standard 75 KVA.

Perhaps non standard transformers like K rated maybe more problematic.

Last week, I did see a 225 A breaker trip when a 150 KVA switched on, but the magnetic trip on the breaker was set to minimum.
Click to expand...

That makes sense.

That may have been the first time that breaker has tripped in 5 years.
Click to expand...

I bet your right.
 
steve66 said:
Under 600 Volts is what I had in mind. Generally, I'm thinking of a 480 to 208V transformer, in the 75 KVA to 150 KVA range.

Jim:

I think I saw the 12x number in one of your posts, and also noticed that's what SKM software puts on their TCC by default. So using 12x rated current for 0.1 sec is not the right way to look at low voltage transfomers?
Click to expand...

Steve,

12x FLA for .1sec is an industry standard (although I know of no trnasfromer that actually is this bad). So yes you could use this 12x to determine the required instantaneous (magnetic trip) setting of the primary protective device and then chose a device that aprovides this setting. But, in general it is not done that way until you reach larger transfromers (ie. over 100kVA).

Most small (<100A) thermal magnetic breakers have a fixed magnetic curve which overlaps the 10x handle rating, so, if the breaker is chosen based on the NEC minimum of 125%FLA (=1.25X) it is similar to chosing it based on 12.5X FLA.
 
Here's the way I have considered the purpose of transformer pri. OCP.

Most transformers have secondary protection. Doesn't the NEC require secondary protection if the secondary is 3ph4w or 1ph3w? If so the transformer is protected.
Then the primary breaker serves another purpose, that is in the unfortunate event that the transformer fails which quite likely may be a result from and internal winding failure or internal fault between windings and/or to ground.

The intent of the primary MCCB would then be to take the transformer off line and to isolate the failure preventing it from affecting the upstream distribution system. As such you really aren't "protecting the transformer" at all. You're coordinating the distribution system.

When a transformer pri. MCCB is viewed in this regard it may not be the thermal trip of the breaker that is the issue it is the instantaneous (magnetic) element that should be considered. One may view it as similar than MCPs for motors I would like to think that the breaker selected should be the lowest magnetic trip value as can be applied and still allow the transformer to be energized without tripping the breaker.

If one looks a art 430-52 for MCPs applying a pri. MCCB with a transformer that has a correctly sized sec. MCCB the pri. MCCB could be considered as short circuit protection in a similar manor. 430-52 allows an MCP to be set at 11x but no more than 13x or 17x by exception as I recall. The MCP is looking for a motor winding failure taking the motor off line to reduce the posibility of a fire and to limit motor damage. Its the OLR that provides overload protection for a motor as the secondary MCCB provides for a transformer.
 
iwire said:
Just to be clear, there is no NEC minimum.
Click to expand...

mea culpa,

I should have said NEC recommended minimum maxium vaule of 125%.

Actually "lowest" maximum sounds even better.
 
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