WA_Sparky
Electrical Engineer
- Location
- Vancouver, WA, Clark
- Occupation
- Electrical Engineer
Per NFPA 70-450 (3) A.
Secondary protection 1000 Volts or less. Breakers or Fuse rating for secondary side of transformer (unsupervised locations). Breaker/fuse rating = No more than 125% of max current from transformer.
Say we have a project with 500A demand @208Y/120V 3Ø.
We estimate the appropriate size of transformer to be installed is 225KVA (624A @208 3Ø).
We size our main disconnect for 600A.
Utility then comes in and puts in 150KVA transformer (416A @ 208 3Ø). Ive heard from many utility guys talk about their rules of thumb are to load transformers up to 150%.
Now in theory, our design is a code violation because 416A*1.25= 520A max secondary protection<600A disconnect.
Due the chance of utility changing transformer sizes after CD's/Permit sets are issued:
WA
Secondary protection 1000 Volts or less. Breakers or Fuse rating for secondary side of transformer (unsupervised locations). Breaker/fuse rating = No more than 125% of max current from transformer.
- Is this applicable to secondary side of utility transformer, building main disconnect?
Say we have a project with 500A demand @208Y/120V 3Ø.
We estimate the appropriate size of transformer to be installed is 225KVA (624A @208 3Ø).
We size our main disconnect for 600A.
Utility then comes in and puts in 150KVA transformer (416A @ 208 3Ø). Ive heard from many utility guys talk about their rules of thumb are to load transformers up to 150%.
Now in theory, our design is a code violation because 416A*1.25= 520A max secondary protection<600A disconnect.
Due the chance of utility changing transformer sizes after CD's/Permit sets are issued:
- Is it standard practice to spec adjustable electronic trip breakers on utility xfmr secondary so protection can be adjusted to meet 125% requirement?
- Do fuses within utility transformer count for secondary protection, therefore this code article doesnt apply to service entrance situations?
- For changes that occur after project is issued, is it utilities responsibility to provide necessary protection or is EC/EE responsible for coordinating with utility during construction and eat any cost associated with for oversized equipment?
WA
