malachi constant
Senior Member
- Location
- Minneapolis
Hi all,
Is there a consensus on if emergency distribution equipment can be "undersized" with respect to NEC calcs? I've done this in the past based on the following logical steps (consider this example as a new building, very diverse loads, with whole building backed up by generator):
Step 1. Calculate NEC size of main (normal) service gear.
Step 2. Select generator based on comparable history with similar buildings. In our experience if you match generator to NEC calcs (which are conservative) you can end up with an oversized/underloaded generator at risk of wet stacking.
Step 3. Select generator output breaker(s). Generator manufacturers often have a maximum output breaker size that they allow on a given unit, which is usually below what our normal service size is.
Step 4. Size emergency distribution equipment to generator output breaker
So in this example we end up with a calculated NEC service load that is a hair over 1200A, a normal service size of 1600A, and a generator output breaker size of 1200A. The generator output breaker is below the calculated NEC service load. In my experience the actual peak load will be in the 600-800A range, and because of load diversity (plus soft start motors and relative lack of large motors on the system) the generator will have no trouble starting.
In this example we have the generator output breaker feeding a 1200A emergency panel which has separate vertical sections each feeding a smaller ATS. The em system utilizes selective load management so if the generator struggled to maintain load the ATS's would drop the least critical (i.e. "optional/NEC 702") loads first.
I feel this meets the intent of NEC 700.4 and related sections. My biggest concern is the emergency service panel is technically undersized (1200A) compared to the NEC calcs (slightly over 1200A). But if I increase the em gear to 1600A, this size generator does not come with a 1600A output breaker as a standard option. I could increase the em gear to 1600A but what's the point if it is fed off a 1200A breaker?
I am NOT looking for an iron-clad / bullet-proof code justification for this - I think there is leeway in 700.4 ("or by another approved method") for the engineer to use their discretion, with the inspector's approval/blessing. I've been doing this for years - I think the code changed in the mid-2000s to add "Article 220" language in Article 700, at the time I asked an inspector his thoughts and he was very deferential...and since then I've never thought to ask any other inspectors. The vast majority of inspectors I've encountered would IMO likely defer to the engineer because of the inherent tension between the conservatism of the NEC calcs and the need to right-size (not oversize) generators. Just want to get some second opinions - not to hold up this thread up to a future inspector as "proof" but more to ease my conscience that I am not crazy to take this approach. FWIW in speaking with generator manufacturers they think I am taking the correct/normal approach.
Any thoughts are appreciated!
Is there a consensus on if emergency distribution equipment can be "undersized" with respect to NEC calcs? I've done this in the past based on the following logical steps (consider this example as a new building, very diverse loads, with whole building backed up by generator):
Step 1. Calculate NEC size of main (normal) service gear.
Step 2. Select generator based on comparable history with similar buildings. In our experience if you match generator to NEC calcs (which are conservative) you can end up with an oversized/underloaded generator at risk of wet stacking.
Step 3. Select generator output breaker(s). Generator manufacturers often have a maximum output breaker size that they allow on a given unit, which is usually below what our normal service size is.
Step 4. Size emergency distribution equipment to generator output breaker
So in this example we end up with a calculated NEC service load that is a hair over 1200A, a normal service size of 1600A, and a generator output breaker size of 1200A. The generator output breaker is below the calculated NEC service load. In my experience the actual peak load will be in the 600-800A range, and because of load diversity (plus soft start motors and relative lack of large motors on the system) the generator will have no trouble starting.
In this example we have the generator output breaker feeding a 1200A emergency panel which has separate vertical sections each feeding a smaller ATS. The em system utilizes selective load management so if the generator struggled to maintain load the ATS's would drop the least critical (i.e. "optional/NEC 702") loads first.
I feel this meets the intent of NEC 700.4 and related sections. My biggest concern is the emergency service panel is technically undersized (1200A) compared to the NEC calcs (slightly over 1200A). But if I increase the em gear to 1600A, this size generator does not come with a 1600A output breaker as a standard option. I could increase the em gear to 1600A but what's the point if it is fed off a 1200A breaker?
I am NOT looking for an iron-clad / bullet-proof code justification for this - I think there is leeway in 700.4 ("or by another approved method") for the engineer to use their discretion, with the inspector's approval/blessing. I've been doing this for years - I think the code changed in the mid-2000s to add "Article 220" language in Article 700, at the time I asked an inspector his thoughts and he was very deferential...and since then I've never thought to ask any other inspectors. The vast majority of inspectors I've encountered would IMO likely defer to the engineer because of the inherent tension between the conservatism of the NEC calcs and the need to right-size (not oversize) generators. Just want to get some second opinions - not to hold up this thread up to a future inspector as "proof" but more to ease my conscience that I am not crazy to take this approach. FWIW in speaking with generator manufacturers they think I am taking the correct/normal approach.
Any thoughts are appreciated!