Jim Bujtor
Member
- Location
- Palo Alto, California, USA
Hi, I have a two-part question about the interpretation of this small paragraph: "The service disconnecting means shall have a rating not less than the calculated load to be carried....".
We have an existing panel where the future projected load is going to be much lower than the building was originally designed for. The first question is: if you have a main breaker in the main service entrance panel that has the ability of accommodating multiple trip settings, say it is a 3000 amp frame breaker and you set the "trip setting" to 50% or 1500 amps, did you change the "rating" of the service disconnecting means? If the answer to the first question is "no, the rating did not change", then we know we can change the trip setting without affecting the rating and the second question is, "what should the trip setting be set to: the engineering calculated maximum load or the connected load?" (the maximum load is typically much lower than the connected load calculation).
I believe there is a difference between a 'rating' and a 'setting', but my local building official does not agree yet. I believe that the intent of the NEC and that section also is so that you don't have a panel, busswork, and breaker frame size that is smaller than your calculated maximum load. That makes sense. Yet, if you find that your highest load is only going to be half of the 'rating' of the existing panel, it would be prudent to set the trip to be lower, for life safety and equipment damage precaution in sync with the overall intent of the NEC to begin with!?! To me, lowering a trip setting in this situation is prudent and still maintains the overall "rating" of the system.
I believe lowering the trip setting to match near what your calculated maximum load demand is safer and better protection for personnel and equipment, than leaving the trip setting higher. Basically, in the event of an overload or fault condition, the breaker would trip sooner with the lower trip setting, which would be a safer installation. For those of us who have seen melted main breakers, bussway, etc., it is obvious that limiting the overcurrent trip setting is more prudent and more safe.?
We have an existing panel where the future projected load is going to be much lower than the building was originally designed for. The first question is: if you have a main breaker in the main service entrance panel that has the ability of accommodating multiple trip settings, say it is a 3000 amp frame breaker and you set the "trip setting" to 50% or 1500 amps, did you change the "rating" of the service disconnecting means? If the answer to the first question is "no, the rating did not change", then we know we can change the trip setting without affecting the rating and the second question is, "what should the trip setting be set to: the engineering calculated maximum load or the connected load?" (the maximum load is typically much lower than the connected load calculation).
I believe there is a difference between a 'rating' and a 'setting', but my local building official does not agree yet. I believe that the intent of the NEC and that section also is so that you don't have a panel, busswork, and breaker frame size that is smaller than your calculated maximum load. That makes sense. Yet, if you find that your highest load is only going to be half of the 'rating' of the existing panel, it would be prudent to set the trip to be lower, for life safety and equipment damage precaution in sync with the overall intent of the NEC to begin with!?! To me, lowering a trip setting in this situation is prudent and still maintains the overall "rating" of the system.
I believe lowering the trip setting to match near what your calculated maximum load demand is safer and better protection for personnel and equipment, than leaving the trip setting higher. Basically, in the event of an overload or fault condition, the breaker would trip sooner with the lower trip setting, which would be a safer installation. For those of us who have seen melted main breakers, bussway, etc., it is obvious that limiting the overcurrent trip setting is more prudent and more safe.?