Transformer OCPD Design

infinity

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New Jersey
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Journeyman Electrician
I have a 3Ø, 75 KVA 480-208Y/120 step down transformer. Primary is protected at 125 amps secondary is protected at 300 amps. All conductors are the correct size for the OCPD's but are the OCPD's properly sized for this transformer?
 

Dsg319

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West Virginia
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Wv Master “lectrician”
Shouldn’t secondary be 250ampere OCPD? since secondary current comes out to be 208amps x 125%=260.

Or is there an exception allowing to round up to the next standard size OCPD?

This is without getting my book out and using the Mike Holt electrical toolbox app.
 
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jim dungar

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The 125% is a maximum value. You can use a smaller value but you cannot round up to the next standard size.
208 x 1.25= 260 therefore 250A is acceptable.
 

don_resqcapt19

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Illinois
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retired electrician
The 125% is a maximum value. You can use a smaller value but you cannot round up to the next standard size.
208 x 1.25= 260 therefore 250A is acceptable.
As long as the conductors are protected at or below their ampacity, this is a complaint installation as secondary protection of the transformer itself is not required.
 

jim dungar

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Wisconsin
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Retired Electrical Engineer - Power Systems
As long as the conductors are protected at or below their ampacity, this is a complaint installation as secondary protection of the transformer itself is not required.
But the primary is not properly protected at not more than 125% (112A), so secondary protection is required.
 

infinity

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New Jersey
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So the question is even if code compliant is this a good design? This 75 Kva transformer has a secondary ampacity of about 208 amps so why install a panelboard with 400 amp bus, a 300 amp main OCPD, and 300 amp secondary conductors? And also there are no motor loads associated with the panel.

Our typical design is 125 amp primary, 225 amp secondary with a 225 amp bus panelboard with main CB.
 

Tulsa Electrician

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Location
Tulsa
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Electrician
I have seen this server times.
The first time I seen it I ask the engineer this question.
His response was it was due to connected load. Then he went on to add due to demand It will not be an issue. When I put the numbers to it it made perfect since. It was nice of him to take time to ansawer my question and further educate me.
So I would say it come down to design loads.
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
To me your "typical design" is better simply because with the proposed numbers the transformer could really be taxed over 125%
 

xformer

Senior Member
Location
Dallas, Tx
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Master Electrician
So the question is even if code compliant is this a good design? This 75 Kva transformer has a secondary ampacity of about 208 amps so why install a panelboard with 400 amp bus, a 300 amp main OCPD, and 300 amp secondary conductors? And also there are no motor loads associated with the panel.

Our typical design is 125 amp primary, 225 amp secondary with a 225 amp bus panelboard with main CB.
IMHO.... Future expansion possibilities.
 

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
Shouldn’t secondary be 250ampere OCPD? since secondary current comes out to be 208amps x 125%=260.

Or is there an exception allowing to round up to the next standard size OCPD?

This is without getting my book out and using the Mike Holt electrical toolbox app.
Why 125%. Continuous load? What about 1.73 PF?
 

topgone

Senior Member
I'd max out the primary OCPD to 225A (250%). I will be using the primary protection to take care of the short-circuit protection while I'll be setting the secondary protection at 125% (250A) to care for the overload protection of the transformer. Transformers will have inrush and I don't want to go come back to the breaker panel to do the resetting on circuit breaker trips!
 

kwired

Electron manager
Location
NE Nebraska
I'd max out the primary OCPD to 225A (250%). I will be using the primary protection to take care of the short-circuit protection while I'll be setting the secondary protection at 125% (250A) to care for the overload protection of the transformer. Transformers will have inrush and I don't want to go come back to the breaker panel to do the resetting on circuit breaker trips!
Whether or not your primary device has magnetic trip adjustment may be a factor there as well. Over 100, maybe 125 amps, that seems to be more common in the ones I come across anyway.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Sorry to bring back the old thread but do we still comply if we only used Primary Protection "ONLY" on this application ?
The secondary conductors still need to be protected per article 240, so you can't completely eliminate the OCP on the secondary, if that is what you are asking.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
So the question is even if code compliant is this a good design? This 75 Kva transformer has a secondary ampacity of about 208 amps so why install a panelboard with 400 amp bus, a 300 amp main OCPD, and 300 amp secondary conductors? And also there are no motor loads associated with the panel.

Our typical design is 125 amp primary, 225 amp secondary with a 225 amp bus panelboard with main CB.
If there are some larger branch breakers, the larger main breaker might be necessary for coordination.

just a guess.....
 
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