chino

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I have a single phase 10 kva xmer 480/120/240 Im using 40a for the primary overcurrent protection and 50a for the secondary, am I cover
 

Besoeker

Senior Member
Location
UK
I have a single phase 10 kva xmer 480/120/240 Im using 40a for the primary overcurrent protection and 50a for the secondary, am I cover
I don't know what the rules are in your location but 10kVA at 480V is about 21A.
So, it seems to me that a 40A primary OCPD just might be a bit high.
 

Smart $

Esteemed Member
Location
Ohio
I have a single phase 10 kva xmer 480/120/240 Im using 40a for the primary overcurrent protection and 50a for the secondary, am I cover
Yes.

I don't know what the rules are in your location but 10kVA at 480V is about 21A.
So, it seems to me that a 40A primary OCPD just might be a bit high.
Primary and secondary scheme allows primary up to 250%, limits secondary protection to 125%. [Table 450.3(B)]
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
So, the assumption is that if there is a problem within the transformer, or on the cable between the transformer and the breaker, damage has already occurred, and thus no need to "protect " it from damage.
 

iwire

Moderator
Staff member
Location
Massachusetts
So, the assumption is that if there is a problem within the transformer, or on the cable between the transformer and the breaker, damage has already occurred, and thus no need to "protect " it from damage.

The transformer is protected from overload by the secondary side breaker.

What other mode of failure could happen to the transformer that a smaller primary breaker could protect it from?
 

Smart $

Esteemed Member
Location
Ohio
So, the assumption is that if there is a problem within the transformer, or on the cable between the transformer and the breaker, damage has already occurred, and thus no need to "protect " it from damage.
The cable/conductors must be protected by their OCPD. In this case it works in reverse in that their OCPD is sized to the transformer primary, thus conductor ampacity must match the OCPD.

If there is a problem with the transformer windings, leads, or terminals, the transformer is sacrificial at that point.
 
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