[2017 NEC] Calculating the removed load

[InkRF]

Howdy all,

I'm working on an addition to an existing building where an entire chunk of the building is being removed and another chunk (very technical wording, I know) is being added. I'm trying to do a service calc by doing [12 month peak demand * 125%] - [load removed] + [load added]. Are there any specific sections of the NEC that discuss how calculate the load removed from a system in particular, or is it a matter of demanding the connected loads based off their respective demand factors. For a sense of scale, the connected load to be removed is 68,693 VA.

Thanks!

 

[wwhitney]

While I can't provide an exact NEC reference disallowing the idea, my first reaction is that you can't or shouldn't do that. The value produced by Article 220 Part III or 220.82-220.86 are fairly conservative calculations; measured load is almost certain to be lower, and very likely lower than 80% of the calculation. While the value determined by 220.87 is a real world observation, and is conservative by exactly the 125% factor.

So if you take a 220.87 value based on observation, and you subtract a calculated intentional overestimate, that subtraction becomes non-conservative. You are subtracting too high a value.

If you have a year's worth of 15 minute demand data for both the whole service and the loads to be removed, then obviously you can pairwise subtract those, then apply 220.87 to get a value for the remaining load. Otherwise, I'd say you can't both use 220.87 and take credit for removing some existing load.

The only alternative would be to discount the calculated value for the removed load by some value, but how do you pick that value? 50%? Maybe that's still too high, hard to know.

Cheers, Wayne

 

[InkRF]

Thanks for the input Wayne, I agree with that 100%, that above method is kinda using the far extreme values to calculate an unrealistic load calc. I'll probably end up "derating" the calculated load removed value by a factor, or find an alternative route. Thanks