Transformer Overcurrent Protection Table 450.3(B)

[Grendal]

According to NEC Table 450.3(B), for more than 9 amps for primary and secondary protection; the primary protection max is 250% and the secondary protection is sized at 125% max of the transformer rated current.

Is there any issue sizing both the primary and secondary at 125%? When doing renovation projects sometimes we try to reuse the existing conduits and wiring to save money. So the primary side might end up being only 125%.

Is there a rule of thumb that if you size the secondary at 125% the primary should be at least XXX%? Understand that there are different types of breakers and many breakers you can adjust the trip curves.

Any help would be appreciated.

 

[texie]

According to NEC Table 450.3(B), for more than 9 amps for primary and secondary protection; the primary protection max is 250% and the secondary protection is sized at 125% max of the transformer rated current.

Is there any issue sizing both the primary and secondary at 125%? When doing renovation projects sometimes we try to reuse the existing conduits and wiring to save money. So the primary side might end up being only 125%.

Is there a rule of thumb that if you size the secondary at 125% the primary should be at least XXX%? Understand that there are different types of breakers and many breakers you can adjust the trip curves.

Any help would be appreciated.

You can use 125% for the primary conductors and OCPD and this is the norm for most installations. The code does not require 250% on the primary, rather it is allowed when certain conditions require it.

 

[mayanees]

The reason you're allowed to go up to 250% on the primary ocpd is to accommodate transformer inrush, which is typically listed as 8-12 times the fla, but with more efficient transformers, can be much higher. The downside to the 250% number is that your cabling now has to be at that ampacity.
There is no "rule of thumb" for the sec'y breaker, just be sure to protect the load conductors.