Article 445 doesn't say anything on how to size circuit breakers on a generator. Same rules apply I take it? Or, can you take the approach that all loads on a generator are considered continuous and size the breaker at 125% of all loads and take no demand factor?
Circuit breaker sizing for generator
- Thread starter cppoly
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1st, I think we need to determine what purpose the "breaker on the generator" serves. Art 445 does require the generator to be protected against overload and, if a circuit breaker is used for that purpose, it would be sized based on the generator output.
(The majority I see have been sized by the manufacturer).
Any breaker beyond that point would be sized by as you say "same rules apply".... to protect the conductor and to protect the connected utilization equipment.
Keep in mind Art 445 for the most part addresses the generator itself, so determining loads and breakers is back to Art 240 and 220.
To me, the tricky part can be sizing the generator itself, both from determining loads to taking such factors as "inrush" currents into account.
(The majority I see have been sized by the manufacturer).
Any breaker beyond that point would be sized by as you say "same rules apply".... to protect the conductor and to protect the connected utilization equipment.
Keep in mind Art 445 for the most part addresses the generator itself, so determining loads and breakers is back to Art 240 and 220.
To me, the tricky part can be sizing the generator itself, both from determining loads to taking such factors as "inrush" currents into account.
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Ok that helps. The generator overload protection would be sized based on the rated output of the generator.
Any idea why the conductor amapacity must be 115% of the generator output based on section 445.13? If the load is continuous I thought it needs to be 125%.
Any idea why the conductor amapacity must be 115% of the generator output based on section 445.13? If the load is continuous I thought it needs to be 125%.
This is the way that I addressed this application issue:
Generators are unlike utility power in that a Generators output is basically limited. As such the characteristics of the breakers trip curve must be compared to the limited available output of the generator. A generator must be able to provide enough short circuit current to trip a breaker instantaneously as well as provide enough over current for a long enough period of time to trip a breaker thermally. I believe that you will find that the generator may not come close to tripping common UL489 breaker magnetically and that the generators field may collapse should it attempt to provide enough overload current to trip a breaker on overload.
Some breaker manufacturers have addressed this issue with a breaker produced for the generator use with custom thermal and magnetic trip features.
Generators are unlike utility power in that a Generators output is basically limited. As such the characteristics of the breakers trip curve must be compared to the limited available output of the generator. A generator must be able to provide enough short circuit current to trip a breaker instantaneously as well as provide enough over current for a long enough period of time to trip a breaker thermally. I believe that you will find that the generator may not come close to tripping common UL489 breaker magnetically and that the generators field may collapse should it attempt to provide enough overload current to trip a breaker on overload.
Some breaker manufacturers have addressed this issue with a breaker produced for the generator use with custom thermal and magnetic trip features.
ceb58
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You will be hard pressed to find your normal run-of-the-mill generators with out built in OCP. That said, if the generator has built in OCP the 445.13 will not come in to play. As for sizing mfg's like Generac are now using a 90 amp breaker on their 20 Kw LP/ nat. gas units. They were using a 100 amp but I think they were called on the carpet for 240.4(B) But when you do the math the unit is only capable of 83 continuous amps. But with any generator running it at its full continuous rating is nothing but a melt down waiting to happen.
Remember that simply placing a OCPD on a generator be just physiological at best. It may actually be considered as a disconnect switch if the actual thermal magnetic characteristics doesn't coordinate with the output capabilities of the generator to trip the device.
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