Breaker size to reverse feed a 30 kva transformer?

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zinsco

Member
Location
San Diego, CA
Customer has a 30 KVA, 3-Phase, 480v Delta - 208Y/120v transformer. It is reverse feed with a 60 amp breaker on the 208v side and #10 wire coming from the 480v side feeding a machine. I don't remember the fuse size at the machine.

They called because the machine lost power. I found the 60 amp breaker tripped and priced the job to replace the transformer because I thought the transformer was bad.

I've done some research tonight to find out that the inrush when you reverse feed a transformer is really high so I'm thinking that that 60 amp breaker is to small and that is why it tripped.

What size should the breaker be on the 208v side to reverse feed this transformer and handle the inrush?
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
zinsco said:
Customer has a 30 KVA, 3-Phase, 480v Delta - 208Y/120v transformer. It is reverse feed with a 60 amp breaker on the 208v side and #10 wire coming from the 480v side feeding a machine. I don't remember the fuse size at the machine.

They called because the machine lost power. I found the 60 amp breaker tripped and priced the job to replace the transformer because I thought the transformer was bad.

I've done some research tonight to find out that the inrush when you reverse feed a transformer is really high so I'm thinking that that 60 amp breaker is to small and that is why it tripped.

What size should the breaker be on the 208v side to reverse feed this transformer and handle the inrush?
Click to expand...

The 208 volt side is rated 83 amps so not that surprising a 60 is having trouble.

The code would allow up to 250% of that 83 amps as an overcurrent device rating. (212 amps) I would think a 100 amp would do it but I am really guessing so wait for someone more knowledge on that.

Is there overcurrent protection on the 480 volt side?

How are they dealing with the 480 volt side, corner grounded delta or ungrounded with ground-fault indicators?
 
Z

zinsco

Member
Location
San Diego, CA
Well, like I said, I don't remember the fuse size in the disconnect at the machine but #10 was ran so I'll go with 30 amps on the 480 side. Also, I don't believe I saw any neutrals which I think is required if you reverse feed.

The thing is, they have had this for a number of years without trouble. Also, I was able to get the 60 amp breaker to hold once.
 
Z

zinsco

Member
Location
San Diego, CA
The 208 volt side is rated 83 amps so not that surprising a 60 is having trouble.

The code would allow up to 250% of that 83 amps as an overcurrent device rating. (212 amps) I would think a 100 amp would do it but I am really guessing so wait for someone more knowledge on that.

Is there overcurrent protection on the 480 volt side?

How are they dealing with the 480 volt side, corner grounded delta or ungrounded with ground-fault indicators?
Click to expand...


This is a better response, sorry.

I've never reverse feed a transformer before so I didn't know if the numbers changed, but if they are the same as feeding it the normal way then ya, a 60 amp breaker wouldn't stand a chance. The thing is, they have had this for a number of years without trouble. Also, I was able to get the 60 amp breaker to hold once.

As far as the 480v side, I don't remember the fuse size in the disconnect at the machine but #10 was ran so I'll go with 30 amps on the 480 side. Also, I don't believe I saw any neutrals which I think is required if you reverse feed.

I guess I'll have to feed it with 100+ amps. With the huge inrush with reverse feeding a transformer, does anyone know if I'll have to go up to 200 amps?

Also, is that only inrush load? I mean, what about the panel it's feed from? If I put that big of a breaker in there will I be looking at overloading the panel?

Don't think I could even fit a 200 amp 3 pole in there but probably a 150 or something.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
zinsco said:
... The thing is, they have had this for a number of years without trouble. Also, I was able to get the 60 amp breaker to hold once.

...

I guess I'll have to feed it with 100+ amps. With the huge inrush with reverse feeding a transformer, does anyone know if I'll have to go up to 200 amps?

...
Click to expand...
Working for years on a 60A breaker indicates an major upsizing is not required. In fact, trips could just indicate some "rating" degradation of the existing breaker. You could possibly replace it like for like and get several more years without a trip.

Then again, maybe not. Borderline installations can be shifty.
 
Z

zinsco

Member
Location
San Diego, CA
Smart $ said:
Working for years on a 60A breaker indicates an major upsizing is not required. In fact, trips could just indicate some "rating" degradation of the existing breaker. You could possibly replace it like for like and get several more years without a trip.

Then again, maybe not. Borderline installations can be shifty.
Click to expand...

Ok then, if I just replace it I could at least go with a 70 I think. Has #6 thhn so if I go with the 75 degree column which is 65 amps and next trade size I could get it to 70. Agree?

I am wondering how a 60 amp could have been holding when a 30 KVA transformer is at 83 amps on the 208v side. Any explanation on this?
 
Smart $

Smart $

Esteemed Member
Location
Ohio
zinsco said:
Ok then, if I just replace it I could at least go with a 70 I think. Has #6 thhn so if I go with the 75 degree column which is 65 amps and next trade size I could get it to 70. Agree?
Click to expand...
Agree... and a lot simpler fix (assuming it'll work) than replacing the transformer.
 
T

templdl

Senior Member
Location
Wisconsin
iwire said:
The 208 volt side is rated 83 amps so not that surprising a 60 is having trouble.

The code would allow up to 250% of that 83 amps as an overcurrent device rating. (212 amps) I would think a 100 amp would do it but I am really guessing so wait for someone more knowledge on that.

Is there overcurrent protection on the 480 volt side?

How are they dealing with the 480 volt side, corner grounded delta or ungrounded with ground-fault indicators?
Click to expand...
Yes, that 60a breaker is an issue all right. In addition, to add insult to injury the LV windings of the common stepdown transformer sare sound around the core first with the HV windings over the top which makes it aa convenient to access the HV taps.
Being that the LV are now being the windings that are being feed the percentage of the FLA will be greater than those when used when being used as a sterp down transformer because the LV windings are right next to the core. HV Inrushes current of transformers of this size are commonly at lease 11x the pri FLA. The inrush increases with energy efficient desdigns, K-factor etc.
When that transformer is used as a step up that inrush will be greater than 11x due to the fact that the LV wdgs are wound right next to the core. As it is extremely important to not be shy when sizing the breaker which feeds the transformer when secondary protection is as lso provided.
The only other thing that messes with your brain is the taps which are now sort of backwards because they an now of the load side.
 
Z

zinsco

Member
Location
San Diego, CA
templdl said:
Yes, that 60a breaker is an issue all right. In addition, to add insult to injury the LV windings of the common stepdown transformer sare sound around the core first with the HV windings over the top which makes it aa convenient to access the HV taps.
Being that the LV are now being the windings that are being feed the percentage of the FLA will be greater than those when used when being used as a sterp down transformer because the LV windings are right next to the core. HV Inrushes current of transformers of this size are commonly at lease 11x the pri FLA. The inrush increases with energy efficient desdigns, K-factor etc.
When that transformer is used as a step up that inrush will be greater than 11x due to the fact that the LV wdgs are wound right next to the core. As it is extremely important to not be shy when sizing the breaker which feeds the transformer when secondary protection is as lso provided.
The only other thing that messes with your brain is the taps which are now sort of backwards because they an now of the load side.
Click to expand...

Well, what should I do then?
 
H

Haji

Banned
Location
India
zinsco said:
what should I do then?
Click to expand...

Get the current time characteristic of the 60 A breaker and study: if the current corresponding to 0.15 second is greater than 12*83=996A, then your breaker is okay and its many years of operation without problem is not due to chance.
 
R

rlundsrud

Senior Member
Location
chicago, il, USA
Is this transformer dedicated for this machine or is it feeding a panel board that feeds additional equipment? If it is dedicated for this machine, what are the load specs for it?
 
Z

zinsco

Member
Location
San Diego, CA
The breaker is a square D QO360 and yes the transformer is dedicated to the machine. I don't know the load of the machine right now and does anyone know the time/current characteristic for a square D QO360?
 
Z

zinsco

Member
Location
San Diego, CA
Haji said:


Get the current time characteristic of the 60 A breaker and study: if the current corresponding to 0.15 second is greater than 12*83=996A, then your breaker is okay and its many years of operation without problem is not due to chance.
Click to expand...

So, I don't understand how to read the graph to figure this out. I called up Square D and talked to advanced tech support and found this out:

The 60 and 70 amp breakers are going to be between like 350 and 910 amps corresponding to 0.15 seconds. If I go up to 80 amps it will be more like 710 to 1500 amps and a 100 amp breaker will not really be any better.

It's only like 7' x 3 of wire to change it to #4 from #6 and go with the 80 amp, so that's the plan now. Agree?
 
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