LUIS FERNANDEZ
Member
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.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 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
Primary and secondary scheme allows primary up to 250%, limits secondary protection to 125%. [Table 450.3(B)]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.
Like I said, I don't the rules that apply in his location.Primary and secondary scheme allows primary up to 250%, limits secondary protection to 125%. [Table 450.3(B)]
He doesn't list his location, so I based my answer on NEC requirements... being this is basically an NEC Code forum....I don't the rules that apply in his location.
...
I based mine on engineering judgement...............He doesn't list his location, so I based my answer on NEC requirements... being this is basically an NEC Code forum.
...and your recommendation is also compliant with the NEC.I based mine on engineering judgement...............
At 250% of rated current, what exactly is being protected?
At 250% of rated current, what exactly is being protected?
Exactly. The secondary protection handles overcurrent. Primary at 250% is just short-circuit and ground-fault protection.I assume it protects a shorted primary from shutting down the rest of the supply side electrical system.
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.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.