For a transformer less than 600V, is it permissable to size the overcurrent protection on the primary at LESS THAN the nameplate primary full-load current rating? Is it permissable to size the primary OCPD based instead on the secondary load current? For example, a 75kVA transformer serving a 45kVA load - can the primary OCPD be sized at 54A (FLA at 45kVA) x 1.25 or should it be 90A (FLA at 75kVA) x 1.25, or is even even less acceptable? If it is permissable to undersize the primary OCPD, where is it stated (or is it acceptable by not being addressed?)? If not acceptable, where is that stated in the NEC?
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an alternative browser.
You should upgrade or use an alternative browser.
Primary overcurrent protection for transformers less than 600V
- Thread starter corny
- Start date
- Status
- Not open for further replies.
If one were to provide an OCPD that is less than that which is allowable one would still be providing over current protection. If the 75kva xfmr were applied as a 45kva xfmr I would understand that the cabling would be for a 45kva transformer. And, as such one would protect the conductors even though you had 75kva availble. if the cables were sized for 75kva then why would the OCPD be sized for a 45kva application?
Alos, the danger of undersizing a pri. OCPD is the posibility of it nuisance tripping when the xfmr is energized.
It is interesting to note that often times questions are asked in such a way in order to get an answer that one wants to hear. Often times a 75kva 480v 3ph xfmr is installed that is to be feed by a panel with 100a breakers maximum frames. Since a 75kva is rated 90fla you are allowed to go 125% of that or 112.5a where you are allower to go up to the next higher rating or a 125a ocpd. When you have limited yourself to a 100a ocpd you have also limited yourself in the mag. trip pick-up which may nuisance trip when the transfomer is energized especially with energy efficient and K factor transformers.
It is also interesting to not that a 45kva tranformer is the next lower size with 54fla where a 70a ocpd can be used.
Alos, the danger of undersizing a pri. OCPD is the posibility of it nuisance tripping when the xfmr is energized.
It is interesting to note that often times questions are asked in such a way in order to get an answer that one wants to hear. Often times a 75kva 480v 3ph xfmr is installed that is to be feed by a panel with 100a breakers maximum frames. Since a 75kva is rated 90fla you are allowed to go 125% of that or 112.5a where you are allower to go up to the next higher rating or a 125a ocpd. When you have limited yourself to a 100a ocpd you have also limited yourself in the mag. trip pick-up which may nuisance trip when the transfomer is energized especially with energy efficient and K factor transformers.
It is also interesting to not that a 45kva tranformer is the next lower size with 54fla where a 70a ocpd can be used.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
You cannot use the primary side OCPD to protect the secondary side on a transformer with a multi-voltage output (i.e. 208Y/120 or even 120/240).
That said, article 450 deals with maximum sizes you are free to use a smaller size, but as templdl outlined, the transformer may not start.
That said, article 450 deals with maximum sizes you are free to use a smaller size, but as templdl outlined, the transformer may not start.
- Status
- Not open for further replies.