Determining Existing Loads
- Thread starter cppoly
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Its purpose is to allow you to add load to an existing system without first making an accurate and complete inventory of every single load that is presently connected to the system. You can, of course, do that inventory, and so therefore using 220.87 is not a requirement. But nobody will agree to pay for it. Some jurisdictions will allow you to add a small amount of load without going through the 220.87 process. What constitutes "small" will vary from place to place.
Sometimes if the power company has a demand meter installed, they will have that data.
Other times an engineer may specify a demand meter be installed on a new service or a new panel if they think there may be a need for added loads later.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I have done so many times. But there are things that you can do while waiting for the results of the 30 day load study. I have approached the AHJ with a request for a preliminary review based on just the first week's worth of data. I then gave the AHJ the final results at the end of the 30 days. You can use other available information sources as the starting point for the design, and either confirm or adjust your design when you know the final results.
Thanks. Why does the NEC allow 30 days? If testing is done during spring/fall, the AC or heat may not be on or at low cycle compared to summer/winter. And if this low amperage is now used as the basis of design, couldn't this potentially be bad if AC and heat loads aren't incorporated?
If heating and cooling loads are present, one is supposed to account for this possible additional load, although the NEC doesn't specifically say how.
I assume you could just assume the heat or AC was off during the monitoring, and add it as another separate load.
Yea I agree. It just seems like an omission on the NEC here because you could follow 220.87 by using the 30 day rule and still come up massively short on your amperages.
I would feel more comfortable if the 30 day metering was done in the hottest month of the year.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Washington State has a form that they want you to fill out, when you use the 220.87 process. It has you record the peak KW, assign a power factor, calculate the associated peak KVA, multiply by 125%, then multiply by three separate factors that you get to choose. One accounts for the possibility that the readings would have been higher, if you had taken them at a different time of year. One accounts for the possibility that the readings would have been higher, if the building had more occupants in residence during the readings. The third you get to assign based on any other related circumstances.
I guess I'm lucky where I live. The utilities in my area track maximum demand
for all commercial accounts. I always ask the owner to supply the past two years
of utility bills and pick the highest demand. If the owner gives me an authorization
letter, the utility will supply the information directly to me. I'll then show this
information on the plans, although most jurisdictions don't require it.
for all commercial accounts. I always ask the owner to supply the past two years
of utility bills and pick the highest demand. If the owner gives me an authorization
letter, the utility will supply the information directly to me. I'll then show this
information on the plans, although most jurisdictions don't require it.
- Status