Help on Commercial Load Calcs

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BMacky

Senior Member
Location
Foster City, CA
Hello to any of you out there that like to do load calcs. :happysad:

At the last minute I received word that my project needs load calcs, so I figured I'd hire an engineer and get it done quickly for submittal with plans. Well, not so simple around here. I guess everyone's pretty busy. I figured I'd take a stab at it myself, and though at first it seems pretty easy, I am wondering if any of you pros out there can help wrangle me through this one, perhaps help me out on a few points.

Project Data:

Office Space: 8,491 Sq ft
Existing Service: 120V, 3-phase service
# of outlets: 130
Other Equipment: copiers (6 is a fair number for the number/size of office suites in the building)
A/C units: (6) 3-ton units. I guess some other data is needed on the units. I know they are 3-phase sytstems and are rated for 40A services.

Anything else?

I started the calculations and was a little puzzled by some of the numbers I came up with. Wasn't sure what to do considering there is a 3-phase system for the A/C, and how that factors in. There's an awful lot of references and clarity becomes more and more difficult as you refer to each reference upon reference.

Any pros out there who do this stuff eyes shut want to throw a tip or two my way? I need to learn how to do this anyway but it seems like a very simple project and I know someone out there will look at this and be able to ask the right questions and guide me through it.

I'd really appreciate help from anyone that likes these sort of things and has the time to tackle it.

Thanks!

Bob
 

Smart $

Esteemed Member
Location
Ohio
I just happen to be working on a spreadsheet for load calc's, but far from an eyes-shut performer... that's the whole reason for the spreadsheet ;) ...and to tell you the truth (not that I'd outright lie about it), I don't even have the need to do calc's professionally.

But I'll tell you up front that helping you will be a lot less painful if you just answer the questions on details that I/we have rather than ask a lot of why it's done that way, and so forth.

In getting started, the first thing we have to cover is the service. It would be a rarity to have a 120V 3? service. Need to know whether 240/120V 3? 4W (slang aka 240V high leg delta), or 208Y/120V 3? 4W.

Next, Number of main panels, service disconnects? Any subpanels? Any sub-subpanels? Need number of, their designations, and fed by info. Ratings not important at this point.

Space regarding general lighting... in ft? — 1) halls, corridors, closets, stairways, 2) storage spaces not in the previous, and likely not but 3) assembly halls and auditorium? Not necessary, but these will reduce the load. Also need to break down into noncontinuous and continuous categories for each space, including main office space. Also need to further break these (and actually all loads) into what panels they are fed by, if more than just one main panel. Also supply connected load (i.e. unit load) information to determine whether connected load is greater than minimum required load.

Signs and outline lighting info, if any. Commercial requires a minimum of one 20A outlet at each entrance accessible to pedestrians.

Show windows? Need unit load, if any, and horizontal footage.

Track lighting? Unit load and footage.

When you say copiers, I assume dedicated circuits not included in number of receptacles (which I assume is what you meant by "outlets").

Any other specific utilization equipment loads?

Any cooking appliances, perhaps in a break room?

Need nameplate data for AC units. MCA at supply voltage to be exact. (BTW, that's 40A circuits, not "services".) Any fixed electric space heating? Need load data on that, too.

Motors or motor-operated and combination loads?

The above is essentially going through each section of Article 220 Part II as it applies... Part III release date TBD :D
 

BMacky

Senior Member
Location
Foster City, CA
Commercial Load Calc Help

Commercial Load Calc Help

Hello Smart$,

Thanks for the reply. Here is some supportive data, as much as I can muster at present. I have the HVAC contractor on alert for the nameplate data:

In response to your query:

It?s a single story office building for primarily day-use offices. No heavy equipment. Lighting and receptacles mainly.

Number of main panels, service disconnects? Any subpanels?

- We are design/build on this one. It?s an empty shell right now. Existing service is 200A 240V hi-leg delta; on exterior of building with fused knife sw disconnect. It goes!

Space regarding general lighting... in ft? ? 1) halls, corridors, closets, stairways, 2) storage spaces not in the previous, and likely not but 3) assembly halls and auditorium?

All in Sq Ft
- Main corridor: 1,044
- Janitor storage: 25
- (2) Bathrooms totaling: 352
- Electrical Equipment closet: 55

Not necessary, but these will reduce the load.

Also need to break down into noncontinuous and continuous categories for each space, including main office space.

- Examples? These are going to be offices with computers, monitors, etc. Occupancy during the week mainly, hours are prob like 7AM-7PM. There will be server rooms with equipment running continuously, for sure, but just servers, computers.

Also need to further break these (and actually all loads) into what panels they are fed by, if more than just one main panel.

- We are design/build and it?s up to us to break up the loads. I?d like to have each of the six suites have their own sub panels for lighting and general circuits, but not sure if that?s feasible with the space we are dealing with. If there was a requirement to monitor individual suite?s usage, that would be necessary.

Also supply connected load (i.e. unit load) information to determine whether connected load is greater than minimum required load.

- Not sure of the terminolgy here. What?s an example of unit load?

Signs and outline lighting info, if any.
- Exit signs/emergency lighting will be LED units, approx 10 lighted exit signs and
though not sure of our local Code requirements yet, but can guess we?ll have roughly 40 emergency lighting units.

Commercial requires a minimum of one 20A outlet at each entrance accessible to pedestrians.
- Will add (1) outlet in main corridor front entrance, accessible to pedestrians

Show windows? Need unit load, if any, and horizontal footage.
- None

Track lighting? Unit load and footage.
- None

When you say copiers, I assume dedicated circuits not included in number of receptacles (which I assume is what you meant by "outlets").
- Yes, were not included in receptacle count.

Any other specific utilization equipment loads?
- All I can come up with is computer equipment, like servers, maybe exhaust fans (for sure two fans, one per bathroom, which will be on sensors), and battery back-up (UPS) units on charge.

Any cooking appliances, perhaps in a break room?
- You can bet there will be a break ?area? in each of the six suites, so yes, I would figure a microwave and/or small capacity refrigerator in each suite. No kitchen areas with ranges.

Need nameplate data for AC units. MCA at supply voltage to be exact. (BTW, that's 40A circuits, not "services".) Any fixed electric space heating? Need load data on that, too.
- Gas heating. All the HVAC contractor mentioned was he needed a 40A circuit for each unit, (6) total. I have queried him on make/model so I can get the full nameplate data, like FLA for the motors which I suppose you?ll need.

Motors or motor-operated and combination loads?
- None at present

Thank you for the assistance on this.

Bob
 
Bob,

First tip, hire an engineer. You will need to engage one at some point and it is a lot easier to do it from the get go. Plus, you can get him to do these load calcs instead of trying to learn everything in a day or two. Second tip, I do commercial office buildings anywhere from 12 - 18 w/sf which puts you around a 400A service. I would try to make this work but would go to 600A if you have a heavy computer/electronics load component. You are looking at a 400 or 600A service entrance rated main distribution panel and then some smaller subpanels that will be fed out of the main panel. Good Luck!
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
Bob,

First tip, hire an engineer. You will need to engage one at some point and it is a lot easier to do it from the get go. Plus, you can get him to do these load calcs instead of trying to learn everything in a day or two. Second tip, I do commercial office buildings anywhere from 12 - 18 w/sf which puts you around a 400A service. I would try to make this work but would go to 600A if you have a heavy computer/electronics load component. You are looking at a 400 or 600A service entrance rated main distribution panel and then some smaller subpanels that will be fed out of the main panel. Good Luck!

Why does he need to hire an engineer? In Florida my EC license allows me to design services up to 800 amps 3 phase commercial.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
IMO, bite the bullet and do the best you can. Lighting based on table 220.12. @125% Heating/AC based on name plate rating. Receptacles at 180VA each. Only add for dedicated loads, such as kitchen break equipment, water cooler, dedicated copiers, not little desktop printers. Then put in a fluff factor. For example if you put a duplex on each of three walls in every office, the 180va is fine. If you are only putting in one, the I would use 360 per outlet. Look at how your local Engineers do it. They are not as accurate as one might think. They just make sure they have enough information to "fool" the building department and enough overhead to cover the load plus expansion. Frankly the calculations need to serve two purposes. Satisfy the building department and ensure adequate power for expected loads. When I first started doing it, I got really down and dirty. Same thing I did with my estimating. Now I just make sure I have enough.
 

Smart $

Esteemed Member
Location
Ohio
Hello...

Thank you for the assistance on this.

Bob
Here's what I've gleaned (coupled with some assumptions) from your postings...

Service
Noncontinuous
General Lighting Load:
Hall, corridor, closet, stairway - 80 ft? @ 0.5 VA/ft?
40 VA
General Use Receptacles - 80 @ 180VA
14,400 VA
Computer and peripheral receptacles - 50 @ 450 VA
22,500 VA
Copier outlets - 6 @ 1,500 VA
9,000 VA
AC RTU - 6 @ 240V 3? 30A
74,825 VA
25% largest motor
3,118 VA
Exhaust fans - 2 @ 1,500VA
3,000 VA
Subtotal
126,882 VA
Continuous
General Lighting Load:
Office - 7,817 ft? @ 3.5 VA/ft?
27,360 VA
Hall, corridor, closet, stairway - 1,044 ft? @ 0.5 VA/ft?
522 VA
Sign and outline lighting - 1 outlet @ 1,200 VA
1,200 VA
Subtotal
29,082 VA
Total
155,964 VA
 

Smart $

Esteemed Member
Location
Ohio
Here's what I've gleaned...
Another aspect you'll have to consider is that the 120V equipment can only be connected A-N or C-N on a 240V hi-leg supply... which may require larger Line conductors and OCP from unbalanced loading.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
Another aspect you'll have to consider is that the 120V equipment can only be connected A-N or C-N on a 240V hi-leg supply... which may require larger Line conductors and OCP from unbalanced loading.
I got the impression he was replacing the high leg service.
 

Smart $

Esteemed Member
Location
Ohio
I got the impression he was replacing the high leg service.
Well I took it as the fused knife switch will be demo'd, not the entire service.

Besides that don't matter until doing service and feeder ocp and conductor calc's. I just took a guess on the 3? AC units anyway.
 

BMacky

Senior Member
Location
Foster City, CA
Bob,

First tip, hire an engineer. You will need to engage one at some point and it is a lot easier to do it from the get go. Plus, you can get him to do these load calcs instead of trying to learn everything in a day or two.

Hello Crazy,

Ya, tried that. Cannot get one to respond or even through an architect get a recommendation! Left messages, and instead of waiting around moved on to try to figure it out for myself. That's when I came here because there always seem to be enough varied viewpoints to sort out what's popular, Code-worthy and eventually correct. I appreciate your input. Thanks!
 

BMacky

Senior Member
Location
Foster City, CA
Commercial Load Calcs

Commercial Load Calcs

First of all, to all who responded I like the feedback. For starters, the AHJ doesn't really care about who does the load calcs. The architect that did a job across the street from this project said her contractor took care of the work, and so far the local AHJ hasn't said I cannot do it. BUT, I did go the engineer route first for several reasons, namely accuracy and time. However, they seem to live under a rock around these parts. I called one firm and they said they design power plants, sorry. Huh? I would think there would be some load calculating done around that shop! Anyhow...

I will be installing a completely new service along side the old and demoing the old when the utility has switched over to the new. It will remain 240V/3-phase.

Smart$, the comment regarding larger line conductors and unbalanced loading, I see your point because yes, I had planned to run a few single phase sub-panels for the recepts and lighting. Can you reference the proper table for doing so please?

So after the VA sum, dividing by 240V gets me over 600A. Is that the correct way to come to the amperage with a three phase system. We're out of my league here. I know Ohm's law but with a three-phase service is there a special consideration, factor, etc?

Also, throwing this in last minute, what about provisions for solar? Let's say there is a 15-20KW system installed. I believe the sizing of the main service would have to be beefed up to ensure the additional source current. I don't have any raw numbers at the moment but figure about 100 panels at 200W each.

Thanks again everyone and Smart$, the HVAC contractor has not given me the nameplate data for the MCA. Will foreward that shortly.

Bob
 
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GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Also, throwing this in last minute, what about provisions for solar? Let's say there is a 15-20KW system installed. I believe the sizing of the main service would have to be beefed up to ensure the additional source current. I don't have any raw numbers at the moment but figure about 100 panels at 200W each.

20 kW of panels is a pretty big system. Definitely commercial. Usually you do not have to beef up the service itself, but depending on how you wire it (line side tap or backfed breaker) you may need to upsize the panel and run it with a smaller main breaker. Main breaker amps plus backfeed amps cannot exceed 125% of the bus rating of the panel.
Reason you do not have to upsize the service is that solar amps subtract from POCO amps, not add.
 

Smart $

Esteemed Member
Location
Ohio
...

Smart$, the comment regarding larger line conductors and unbalanced loading, I see your point because yes, I had planned to run a few single phase sub-panels for the recepts and lighting. Can you reference the proper table for doing so please?
Proper table? None in the NEC. Other than requirements, there only other inference is Annex D Example D1(b)... but that's for 120/240 ?.

That said, still too early in calc' stages to be concerned with that. You can't balance loads until loads are nailed down. Another is what voltage some loads will operate at. For example, you have not stated whether lighting will be 120V or 240V. Server equipment can used separately derived power systems that can be 240V even though they supply 120V equipment loads (and these aren't even in the preliminary calc' I posted earlier).

So after the VA sum, dividing by 240V gets me over 600A. Is that the correct way to come to the amperage with a three phase system. We're out of my league here. I know Ohm's law but with a three-phase service is there a special consideration, factor, etc?
Still have to add 25% for continuous loads.. then divide that VA amount by 240V and sqrt(3) to get amps. But demand factors have yet to be applied. Receptacle loads demand factor is 50% over 10kVA

Also, throwing this in last minute, what about provisions for solar? Let's say there is a 15-20KW system installed. I believe the sizing of the main service would have to be beefed up to ensure the additional source current. I don't have any raw numbers at the moment but figure about 100 panels at 200W each.
No beefing up of service conductors required. May have to beef up distribution and feeders. And we're getting into the situation I mentioned back in my first post. Asking more questions than providing details... ;) I understand speculating when details are not known (I do it myself all the time on personal projects)... but someone has to be making these decisions and nailing down what the project actually entails.
 
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BMacky

Senior Member
Location
Foster City, CA
Commercial Load Calcs

Commercial Load Calcs

Still have to add 25% for continuous loads.. then divide that VA amount by 240V and sqrt(3) to get amps. But demand factors have yet to be applied. Receptacle loads demand factor is 50% over 10kVA

Hello Smart $, and again, thanks for walking me through this. And what do you mean by "sqrt(3)"? Not sure what you are doing there.

I received A/C unit info: MinCA is 28A, Max 40A.

I'm starting to lose you a bit with all the added factoring. I realize you pointed out to "do" rather than "ask why", so trying to bite my tongue throughout. I am reviewing the info and trying to make sense of as much as possible.

I am mainly interested in getting a ballpark calcualtion so I know what direction to take things in the early phase, like how much space I will need for distribution, switchgear location which will probably be exterior to the building, etc.

By your calculations, where is this leading? 600A service? 800A?

Thanks.
 

Smart $

Esteemed Member
Location
Ohio
Hello Smart $, and again, thanks for walking me through this. And what do you mean by "sqrt(3)"? Not sure what you are doing there.
sqrt(3) is how its written in Excel formulas, meaning √3, which evaluates to approximately 1.732. Sorry, been working in Excel a bit lately and got lazy in pulling up Character Map for the symbol.

Anyway, for 3? power... P = V ? A ? √3

Just an FYI, for Watts.... W = V ? A ? √3 ? pf ...where pf is power factor.... but not needed for most load calc's.

I received A/C unit info: MinCA is 28A, Max 40A.
Yeah, that's about what I figured when you said mechanical needed 40A circuits.

I'm starting to lose you a bit with all the added factoring. I realize you pointed out to "do" rather than "ask why", so trying to bite my tongue throughout. I am reviewing the info and trying to make sense of as much as possible.
Biting your tongie isn't required... :happyno:

...but it sounded like you wanted quick figures.

Besides, biting one's tongue is rather painful in my experience :happyyes:


I am mainly interested in getting a ballpark calcualtion so I know what direction to take things in the early phase, like how much space I will need for distribution, switchgear location which will probably be exterior to the building, etc.

By your calculations, where is this leading? 600A service? 800A?
Ballpark estimating puts you around 350A with loading that I listed earlier. A 400A service would give some cushion if loads are balanced.

As I see it now, you need to determine whether general lighting will be 120V or 240, and determine if there will be any additional loads. You mentioned servers earlier... rack, room (really need the electrical info, not space or size). Electric water heater or not? IIRC, you said gas heat, so I've been assuming gas water heater. Hard to guess from here...
 
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Fulthrotl

~Autocorrect is My Worst Enema.~
Hello to any of you out there that like to do load calcs. :happysad:

At the last minute I received word that my project needs load calcs, so I figured I'd hire an engineer and get it done quickly for submittal with plans. Well, not so simple around here. I guess everyone's pretty busy. I figured I'd take a stab at it myself, and though at first it seems pretty easy, I am wondering if any of you pros out there can help wrangle me through this one, perhaps help me out on a few points.


Bob

well, you are in calif... i have an electrical engineer that i've known for 20 years that might be interested in a quickie.
i know his rate is in the $150~$175 per hour range, and i don't know how slammed he is at the moment.... if you
are interested in just making the pain go away, and have drawings in either autocad, or revit, i can ask if he's interested.

if that interests you, send me a private message here with your contact information, and i'll pass it along. if he's interested,
he'll contact you directly, and you can work the details out quietly among yourselves....

the fact that you'll get an engineers wet ink signature, along with the title 24 calcs tends to make plan check a bit smoother.
 

BMacky

Senior Member
Location
Foster City, CA
As I see it now, you need to determine whether general lighting will be 120V or 240, and determine if there will be any additional loads. You mentioned servers earlier... rack, room (really need the electrical info, not space or size). Electric water heater or not? IIRC, you said gas heat, so I've been assuming gas water heater. Hard to guess from here...

Hello Smart $, sorry I forgot to add it in my last.

This building is for rental office spaces, six separate suites, so the thought is if they fill them individually, they will each have (maybe) water coolers, small refers, microwave, coffee makers. Break area stuff. Aside from that, 120V-based server equipment, lighting will be 120V as well. Gas heaters for water as you had thought.

Thanks for the clarification on the formula. Makes sense now.
 

Smart $

Esteemed Member
Location
Ohio
Hello Smart $, sorry I forgot to add it in my last.

This building is for rental office spaces, six separate suites, so the thought is if they fill them individually, they will each have (maybe) water coolers, small refers, microwave, coffee makers. Break area stuff. Aside from that, 120V-based server equipment, lighting will be 120V as well. Gas heaters for water as you had thought.

Thanks for the clarification on the formula. Makes sense now.
Yeah, that's part of the problem when setting up a service for tenant space when tenants' requirements aren't known in advance. For making decision on the unknown loads, you may want to surf for info on typical office electrical loads. I googled and found plenty of info... but not in terms of doing an NEC proscribed load calc'...

Anyway, using my previously posted numbers, I recalculated for unbalance with everything but the AC units being 120V.... pushed the line current on A and C up to about 470A, and that's not counting the other loads you mentioned.

Design question: are the tenant spaces going to be separately metered? If so, AC units divided up one per meter? Reasong I'm asking is consideration of one 1? service and one 3? service (and let the POCO worry about how their going to power it)... but if the spaces are separately metered and AC divvied up to those meters, that'd compound equipment and labor cost.


Oh... and BTW, you do realize that I'm only going to be showing you how to do the load calc... heck, I may even do the calc' itself with supplied numbers... but I will not be making the decisions on what the loads actually are.
 
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BMacky

Senior Member
Location
Foster City, CA
Design question: are the tenant spaces going to be separately metered? If so, AC units divided up one per meter? Reasong I'm asking is consideration of one 1? service and one 3? service (and let the POCO worry about how their going to power it)... but if the spaces are separately metered and AC divied up to those meters, that'd compound equipment and labor cost.
Oh... and BTW, you do realize that I'm only going to be showing you how to do the load calc... heck, I may even do the calc' itself with supplied numbers... but I will not be making the decisions on what the loads actually are.

We'll consider the spaces are NOT metered separately, but at the meeting we discussed running each suite through separate lighting and receptacle panels to have the option of monitoring usage at some later point.

I appreciate whatever you hapen to come up with, since the excercise itself has shown me there are many factors to consider. I'll need to do a little brushing up on some of the terminology used to get a better idea of how everything comes together.

You need not worry about lack of knowing the additional loads. That's true for me as well. We have no idea what the tenants will be doing. The whole building could be rented by one company as well, and they could turn a whole suite into a break area with all sorts of equipment. We just need to be prudent with our foundation and leave room for expansion if feasible.

I was originally thinking a 600A service, based on a similar building in the complex. They have an 800A service, but with 8 single-phase AC units on a 400A service that is pulled off of the main.
 
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