Estimated Load Calculations

[Collin]

I have been asked to give an evaluation of an existing power supply so that a client can know what amount of load can be added to it(it is a warehouse with some office space). For loads with known FLA, I do the proper calculation, however, for unknown loads I calculated "worst case" with each at 80% of the breaker amps. This obviously inflates my load beyond what it says the panels are rated for(for all but one panel). Without redoing a load calculation for all the lighting, receptacles, heaters, etcetera, does the NEC allow me to make assumptions about these loads?

I have read into NEC 220 a little and have seen the values of 180VA per receptacle and that Warehouse lighting is 1/4VA per Sq. Ft., but these are 20A receptacles and lighting circuits just labeled, "receptacles" and "warehouse lights," no idea how many are on each circuit.

General information: Some lighting is on 277V(480/277V) and the majority is on 120(208/120V), all receptacles are 120V, warehouse is 51,200 sq. ft., office is 2,500 sq. ft.
My spreadsheet calcs: I'm multiplying and dividing by 1000 for KVA since it looks nicer in the spreadsheet
3p KVA = 1.73*VLL*IL/1000
2p KVA = VLL*IL/1000
1p KVA = VLN*IL/1000
Add up all the KVA and find the amps = (KVA*1000)/(VLL*1.73)

 

[augie47]

Have you checked with the local power co. to see if they can give you a demand number ?

 

[Collin]

Have you checked with the local power co. to see if they can give you a demand number ?

No, that is a good idea though. I've asked the client for the transformer nameplate(the pad mount was locked by the utility when I was there) and they haven't got back with me yet. I might ask them that when they get back with me about the main transformer.

 

[PD1972]

As noted in post #2, I believe the path you would want to take is ask the utility for demand information. Refer to 220.87; that should tell you what you need to know and next steps forward.

It seems like the owner is asking you how much additional load he may add to the building. A calculation evaluating FLAs of existing loads similar to the process outlined in most of article 220 and appendix D will yield a higher number that may not be indicative of how the building is actually being used.

Additionally, I don't think owners necessarily need to know the size of the utility transformer as that is typically owned and sized by the utility company. The owner's responsibility would be to notify the utility if they are adding significant load, after which the utility would determine if they need to upsize their transformer.

 

[JoeStillman]

For existing buildings, I always try to get copies of 12 months of electric bills. Most utilities have demand charges as part of the rate structure and the peak demand in kW is metered every month. Only one utility I worked with didn't bill demand charges, but they had peak kW records on file.

The utility-owned padmount size is determined by the power company, not the customer, and is usually smaller than any NEC calculation would dictate.

 

[Collin]

As noted in post #2, I believe the path you would want to take is ask the utility for demand information. Refer to 220.87; that should tell you what you need to know and next steps forward.

Thank you for the code reference, that's really helpful. It seems that getting the utility information is really the only acceptable path forward.

It seems like the owner is asking you how much additional load he may add to the building. A calculation evaluating FLAs of existing loads similar to the process outlined in most of article 220 and appendix D will yield a higher number that may not be indicative of how the building is actually being used.

Once again need utility information or setup ammeter for 30 days(client will not want to do that).

Additionally, I don't think owners necessarily need to know the size of the utility transformer as that is typically owned and sized by the utility company. The owner's responsibility would be to notify the utility if they are adding significant load, after which the utility would determine if they need to upsize their transformer.

They do not need to know, I need to so that I know if they can add loads to the transformer.

It sounds like the best strategy is not to calculate the current load at all and instead do the following:

1. Calculate the difference between SPARED load and ADDED load(they are adding more than they are sparing)
2. Hand calculated additional load to the utility
3. Let the utility decide if they can supply it with the THEIR current equipment or upgrade the TX
4. I would then only specify what the client needs in terms of equipment downstream of the transformer

Just some additional info for perspective:
The client has a 1600A bus switchgear which is fed from (3 parallel set of) 4W (full neutral) 500KCMIL 480V 3PH.
So, 3 X 380A(500kcmil @75 C) = 1140A usable bus. I've calculated around 1200A using 80% of breaker amps for unknown loads and FLA to KVA calcs for known.

 

[Collin]

For existing buildings, I always try to get copies of 12 months of electric bills. Most utilities have demand charges as part of the rate structure and the peak demand in kW is metered every month. Only one utility I worked with didn't bill demand charges, but they had peak kW records on file.

Thank you for the info. Sounds like I just need to get the contact information for the utility! I've been trying to go through the client, that was probably a bad idea. I will have to start with that information moving forward.

What is typically your next step? If you are trying to add XX amount of load to a panelboard and have the peak kW, how would you run that down to a panel? You can subtract all the known loads, but would then have to divide the remaining kW among all the panels somehow to determine the total load on one panel. Simply put:
MAIN SWGR(kW from utility)=>5 or 6 different (amp)sized panels with known motor FLA and unknown lighting and receptacle loads.
For each panel, you have the schedule and breaker sizes. There's no way to run it down to this level is there?

In my case, we will probably be running new wires to new panels and only reuse breakers of the same size or smaller, but I'm still curious if you can do what I described above.

The utility-owned padmount size is determined by the power company, not the customer, and is usually smaller than any NEC calculation would dictate.

I just thought the utility might supply the information on the one-line, however, I can see why they do not.
I've heard lots of stories about utilities "running transformers so hot you could fry an egg on 'em" and more of that sort.