220.87 Determining Existing Loads - Engineer's opinion?

[anbm]

In order to determine existing load in an existing building that we do some rennovation, assume we do not have demand data electrical consumption load for one year, we can use the exception for 30 days metering record - and use that metering data for our calculation. Does anyone actual run into any inspectors that actual want to see 30 days metering result, sometimes 30 days metering task will costly and kill the project budget. Thanks!

220.87 Determining Existing Loads. The calculation of a feeder or service load for existing installations shall be permitted to use actual maximum demand to determine the existing load under all of the following conditions:

(1) The maximum demand data is available for a 1-year period.

Exception: If the maximum demand data for a 1-year period is not available, the calculated load shall be permitted to be based on the maximum demand (measure of average power demand over a 15-minute period) continuously recorded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording. The recording shall reflect the maximum demand of the feeder or service by being taken when the building or space is occupied and shall include by measurement or calculation the larger of the heating or cooling equipment load, and other loads that may be periodic in nature due to seasonal or similar conditions.

(2) The maximum demand at 125 percent plus the new load does not exceed the ampacity of the feeder or rating of the service.

(3) The feeder has over-current protection in accordance with 240.4, and the service has overload protection in accordance with 230.90.

 

[charlie b]

I have done it many times. If I add load to an existing facility, I am going to be called upon to demonstrate that the new load will not overload the panel to which it is connected, or any panel upstream. A 30 day metering process is how I usually achieve this demonstration. Some facilities have their own electricians install meters, and sometimes I have to arrange for a testing company to set up the meters. This is always included in the proposed cost of the project.

In fact, the Washington State Department of Labor and Industries (the AHJ for most of my projects) has an on-line form they want us to use. We show the highest demand over the 30 day period, do a judgment call on whether to increase that value based on such things as whether the weather was mild during that period and would require higher load for HVAC at other times of the year, add 25% to that value, then add in the new load, and show that the panels affected still have capacity left.

 

[hmspe]

Where I work having to have a full 30 day measurement is rather common.

 

[Pitt123]

I have done it many times. If I add load to an existing facility, I am going to be called upon to demonstrate that the new load will not overload the panel to which it is connected, or any panel upstream. A 30 day metering process is how I usually achieve this demonstration. Some facilities have their own electricians install meters, and sometimes I have to arrange for a testing company to set up the meters. This is always included in the proposed cost of the project.

In fact, the Washington State Department of Labor and Industries (the AHJ for most of my projects) has an on-line form they want us to use. We show the highest demand over the 30 day period, do a judgment call on whether to increase that value based on such things as whether the weather was mild during that period and would require higher load for HVAC at other times of the year, add 25% to that value, then add in the new load, and show that the panels affected still have capacity left.

So even though with adding the new load the calculated load (old + new) may exceed panel capacity, however if you demonstrate that actual existing load doesnt come close to existing calculated load then you would have this additional capacity. Assuming that the existing measured load + additional calculated load does not exceed panel ratings?

In the design phase for the origonal loads obviously there is no metered data that can be used for a brand new installation but can diversity factors from existing similar applications be used to decrease load calcs beyond the diversity factors that are given in the NEC?

 

[IslandSparky]

You are either calculating NEC connected load (usually 3x demand load), or using existing load plus 25%. As engineers, we have actually bought equipment to meter 5 panels mostly for this reason. ElectroRent is much cheaper though.

We didn't do this as often 5-6 years ago, but now it is pretty much standard practice for us.
Our biggest issue is usually schedule. We generally install the meters, check peak after 7 days, and use that number for plan check. We will note that 30-day numbers are pending, and correct things if the final results are materially different.

 

[IslandSparky]

Sorry, second part of your question on design demand factors... It is one of the weaknesses of the NEC, but there is very little room for engineering discretion on demand factors. We have times where we use them, but you need to be VERY careful.

We had a service garage with 60 2hp lifts. We assumed about 25% demand in that case, as it was physically impractical for more than 5 to be raised at the same time. The NEC doesn't really have provisions for this, but a quick talk with the plan checker will usually allay any concerns.

Many of the assumptions in the NEC are based on lightly loaded equipment. I squirm a bit just taking 25% on existing loads if the metered data reflect fairly continuous loads, and generally use an additional 10% safety factor if my new load will be fairly constant and continuous.

 

[hmspe]

So even though with adding the new load the calculated load (old + new) may exceed panel capacity, however if you demonstrate that actual existing load doesnt come close to existing calculated load then you would have this additional capacity. Assuming that the existing measured load + additional calculated load does not exceed panel ratings?

In the design phase for the origonal loads obviously there is no metered data that can be used for a brand new installation but can diversity factors from existing similar applications be used to decrease load calcs beyond the diversity factors that are given in the NEC?

On paragraph 1, the NEC allows using calculated load, or actual load from either the power company's demand data or a 30 day peak measurement plus calculated new loads. The NEC requires a 1.25 multiplier on the existing measured load, and you will need to convert KW to KVA if you use power company data. I assume a 0.8 power factor for this.

On paragraph 2, I do not take any diversity factors that are not specifically called out in the NEC, partly because it is hard to get plans reviewers around here to accept it, and partly because if real world isn't what I guessed (maybe the mechanics have a party at the garage and decide to make a video of the 20 lifts doing "the wave"....) I could be held liable for the costs to fix any problems.

I agree that what plans reviewers want to see has changed over the years. I remember a time when a single clamp-on reading was acceptable here. What I see now is a requirement for a full 30 day period, and the times when I have sent a 7 day reading because the client demanded it the reviewer has redlined it and required a full 30 day test.

On jobs that just can't wait 30 days the alternative is to do a normal load calculation on each panel. That is easiest if there are good as-built drawings available and if the breakers are labeled well, but it generally doesn't take too long to trace circuits from a panel even if there are no markings.