I recently was asked to install a single phase 50 KVA 480/240 volt transformer. The 600 amp 480/277 panel board has a max breaker limit of 125 amps. Primary amps of 50 kva = 104 x 1.25 = 130 amps required primary OCPD. Dropping down to 1.20 comes in. At 124.8 amps. Is there any way to make this work? With the equipment in place
Transformer primary OCPD
- Thread starter J-Bar
- Start date
- Status
- Location
- Connecticut
- Occupation
- Engineer
You are not required to provide a primary breaker sized at 125% of the rated current of the transformer.
You are required to provide an primary OCPD that is large enough for the load per 215.2. If 125A is sufficient for the load, I don't see any problem.
You are required to provide an primary OCPD that is large enough for the load per 215.2. If 125A is sufficient for the load, I don't see any problem.
215.2 is very interesting. I'm just trying to understand how it applies to transformers. the continuous load after the brief saturation inrush is 104 amps. the OCPD protects the conductors. So per 215.2.(b) next highest breaker size and #2
- Location
- Connecticut
- Occupation
- Engineer
How did you arrive at a load of 104A?
- Location
- Connecticut
- Occupation
- Engineer
Seems unlikely that the actual load would be the exact transformer rating. To apply 215.2, you would not only need to know what is the load, but how much of the load is continuous and how much of the load is non-continuous.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The transformer is not the load. Although the load may be equal to the full load amps of the transformer.
The 125% rating, that you calculated as 130A, is the maximum size of a transformer primary side protection, not a required size.
The 125% rating, that you calculated as 130A, is the maximum size of a transformer primary side protection, not a required size.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
What is your estimated loading on this panel? I would assume it is substantially less than 200A, because you may be allowing for future growth, and that there is a 200A main breaker in the panel.
- Location
- Connecticut
- Occupation
- Engineer
In addition to what Jim said, if your load is limited to 200A on the secondary by the panel OCPD, it will be limited on the primary side to 100A.
I think I would put the 125A on the primary to allow for inrush.
I understand where you are coming from now. In the past Iv'e always just sized them at full load. under these particular circumstances I guess I'm going to have to pay attention. Just wondering what the AHJ is going to expect. yes the panel has a 200 amp main. I guess if I had to limit load I could add fused disco ahead of the secondary main.
I just realized that
Last edited by a moderator:
- Status