Reading over transformers 450.3(B) the code wording shows "maximum rating or setting of OCP" .. For a single phase 50kva transformer 480pri- 120/240 sec...following the code for primary and secondary OCP would be 260.5 (after adding 250%) on the primary and 260.4 (after adding 125%) on secondary. Following article 240.6 next size up is 300amp circuit breaker...i recently came across two existing installations using 50kva single phase with a 150amp circuit breaker on the primary and 200amp on 1 secondary and another 250amp on the other secondary...since code says "maximum" how would you know when you can downsize or are these existing installations incorrect?
Transformer OCP
- Thread starter Kob
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- Massachusetts
Maximum permitted means only that, you are free to use as small of a OCPD as you want.
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
I believe if you calculate your load and it is much less then the trany then you don't have to use the max.
I always look at as the secondary protection provide the overload protection for the transformer. The primary OCPD protects the upstream distribution system should the transformer fail. As such you are looking at what the instantaneous trip rating of the pri OCPD will be even though it is a thermalmagnetic trip device. The overload protection already is being provided by a correctly sized sec. OCPD.
With the pri. OCPD you don't want it to nuisance trip when the transformer is first energized. You casn downsize it if you want but remember that the instantaneous trip of the pri OCPD is 10x the rating and maybe up to +20%. You can expect the inrush of a 150degC 50kva xfmr to be 11-13x the FLA. Since the pri. FLA is 104.2a @ 480v look for the inrush to be 11-13x above that. With energy efficient, 115 an 80degC it could end up to be an additional 20%.
The 250% should keep you well oiut of the nuisance trip area.
Does that make any sense?
petersonra
Senior Member
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- Northern illinois
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- Semi-retired engineer
I don't believe that you are ever required to use the max as far as requirements for transformer OCP go. You could put a 1 A CB on a 500 KVA xfmr and it would meet the transformer OCP requirements found in the code. It might not be a real practical thing to do though.
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- Placerville, CA, USA
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I get the impression that in some areas outside North America it is common to use soft start mechanisms (i.e. thermistor or switched impedance) for transformers.
JoeStillman
Senior Member
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- West Chester, PA
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- Electrical PE
I usually design the primary OCP based on the transformer's inrush rating and try to set it as low as practically possible. That still means it's typically around 150-250% in many cases. The major reason for doing this is because of arc flash. Usually the highest incident energy anywhere will be on the downstream side of the transformer and the only way to reduce this short of a "virtual breaker" arrangement with bushing CT's or installing fuses in the transformer compartment itself is keeping the primary side OCP as low as practically possible. OCP on the secondary side is generally always driven by downstream protection requirements.
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