120v/240v 3 phase load calculation

Status
Not open for further replies.
I have been talked into creating a load calculation for a restaurant and I am stumped. The service voltage is 120/240v 3 phase (high leg). I talked to an engineer earlier and he told me that I have to calculate the total connected load and the design load and which ever is greatest is the one I need to use. Is this correct? It seems like I should be able to use the demand factors regardless.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
I agree with the engineer. The article 220 load calculation process will tell you the minimum amount of load you need the service to be able to handle. What good will it do the owner, to tell them that they need a minimum of 120 amps, when they are already using 150 amps on a 200 amp service?

I would go through the article 220 process, including any demand factors that apply to this facility, just to verify that the existing service is not lower than the NEC required minimum. Then I would find out how much power is really being used. You can do that either by making a list of all connected loads and obtaining the nameplate ratings of everything on the list, or by taking 30 day load measurements at strategic locations. When you are done, what the owner will have is the knowledge of whether there is capacity to add more equipment.
 

Twoskinsoneman

Senior Member
Location
West Virginia, USA NEC: 2020
Occupation
Facility Senior Electrician
I have been talked into creating a load calculation for a restaurant and I am stumped. The service voltage is 120/240v 3 phase (high leg). I talked to an engineer earlier and he told me that I have to calculate the total connected load and the design load and which ever is greatest is the one I need to use. Is this correct? It seems like I should be able to use the demand factors regardless.

This calculation is for new construction. The service is existing but the building has been totally gutted. Sorry for the confusion.

Looks like your made in the shade :)
 

dkidd

Senior Member
Location
here
Occupation
PE
You really need a full equipment list first. The 120 volt equipment can only be connected to A or C phase, which sometimes means that the service ampacity will need to be bigger than assuming a balanced load. And yes, you should use the allowed demand factors.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Are you sure he's not referring specifically to the lighting load? For instance, if the restaurant was 5000sf, T220.12 would give a minimum lighting load of 10,000VA. However, if the connected lighting load was actually 13,000VA, you would need to use 13,000VA for lighting in your calculation.

He may also be referring to the optional method in 220.88, where you have to add the total connected load, including BOTH heating and cooling before applying the demand factors. Using the method in Art. 200 - Part III, you would be able to use the larger of the heating or cooling loads if the loads are noncoincident.
 
So basically the a/c equipment and ventilation equipment are calculated at 100% plus 25% of the largest load. The lighting load is calculated at 100% plus 125% continuos. The receptacle load is at 7500 kva total so no demand factor there. And the kitchen equipment is calculated at 65% due to there being a mass amount of it. Is this pretty much it?
 

Npstewart

Senior Member
I typically call the power company and ask for a peak KW. If it is a demand meter then it wont be a problem. You can take the existing peak KW and multiply it by 1.25 aka 125% from there you can add what you need to. Obviously this only works if a tenant is ADDING some equipment. If it is a completely different occupancy then this wouldn't do you much good.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
Obviously this only works if a tenant is ADDING some equipment. If it is a completely different occupancy then this wouldn't do you much good.

This is the main point I was thinking about. If your gutting the building then you obviously don't have any loads to measure now and obtaining a recent peak demand will be of no help unless the owner plans only to remodel the space and put back mostly the same loads. Even so, I'm still guessing that with a gutted building, you will have different lighting, HVAC, controls, etc. that will require you to recalculate.
 
The building is gutted and has an existing 800 amp service. We are about to put a restaurant in the existing space. We are trying to avoid upgrading the service due to obvious cost issues. The problem is it is a 120/240v 3 phase service. I have all of the loads but am having trouble converting them. I guess my question is do I keep my 3 phase loads separate when converting to kva from my single phase loads?
 

skeshesh

Senior Member
Location
Los Angeles, Ca
As far as dealing with the high leg delta system, as mentioned briefly dkidd, you need to be careful as the unbalance caused by the 1-phase loads may call for a larger transformer than you would need in case of a balanced load.
As far as part of your original question, I'm still a bit confused: what do you mean by connected load vs. design load? Reading Charlie's response, I also started thinking about some sort of measured or documented load but it seems that it will be a new setup altogether so please clarify.
 
Last edited:
Yes sir. This is a new set up. We are trying to use an existing service. There is a riser and a service gutter on the building but that is it. Everything else has been gutted.
 
Connected load being full load on each phase before any demand factors. These loads are not existing. They are all of the equipment and lighting loads I have received from the GC that are going to be installed in the new restaurant. Design load is the full load after using the demand factors.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
Connected load being full load on each phase before any demand factors. These loads are not existing. They are all of the equipment and lighting loads I have received from the GC that are going to be installed in the new restaurant. Design load is the full load after using the demand factors.

Thanks for the clarification, but that still makes little sense: if the engineer you spoke to claims you should use the larger of the two values, then obviously the larger value will always be the calculated load before applying demand factors. No, I'd say apply demand factors but use discretion as well - this means calculate the load load using demand factors, then see what the most heavily loaded leg of your transformer would be to size the service; if the result falls very slightly under your 800A service it's up to you to use your technical judgment and experience as well as questioning the owner to find out if there's a significant chance of undersizing your service. I'm thinking you're right on that border or else why worry about the size of service...
 
You got it. We are at 801 amps on one phase before applying any demand factors. Now again that is if every light and every piece of equipment is running at the same time.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Have you contacted the utility yet? If not, you may want to do so, just to confirm the availability of that existing service. I don't know your location, but some utilities use every opportunity to get rid of the high leg delta systems. All you hard work may be a mute point, if the utility dictates that you have to switch service anyway.
 
Status
Not open for further replies.
Top