According to the table for lighting loads in the NEC, I have to allow 2 volt-amps per square foot for a commercial building. Doesn't volt-amp mean the same thing as Watt? If so does that mean that a 400 watt bult covers 400 square feet? If not, how do you calculate lighting? Thanks!

Use the info from the NEC IF you do not have a lighting layout. IF you have a lighting layout, you use the info from the luminaires.

John

Hotline1,
I guess I do not understand what you are saying. Please explain. Thanks.

NEC wants you to multiple Volts x Amps to get VAs. For lighting, Volts and Watts are pretty much identical, but since NEC is talking VA, you don't need to worry about power factor at all, it makes your job much easier.

This thread might help: https://www.electrical-contractor.net/ubb/Forum7/HTML/000551.html

That is really the calculation for the general lighting load including receptacles and it is a monimum number. Occupancy
can and usually will raise what you actually need. Lighting is really calculated by lumens per square foot and each industry has different recomendations.

[This message has been edited by gfretwell (edited 02-19-2007).]

I don’t mean to be rude, and part of me would love to wax on enthusiastically about what this means and how to apply it, but if you are asking this question then I sincerely hope you are not actually doing electrical work.

Perhaps you should consider taking a code study class, it would explain this and all the basics behind it, which you really should have under your belt to be an electrical contractor in the first place.

Again this is not intended to be mean or rude, and is meant in complete sincerity.

This is a forum where electricians, contractors, apprentices, and others, all with different levels of experience and knowledge post here to get or give advice and pick the brains of others. Everyone has their strengths and weaknesses, and in my opinion, shouldn't be shot down for asking an honest question. Just my 2 cents.

I agree Sixer. Lighting can be as simple or as complicated as the occupancy requires. The table in 220 can be misleading. That is not a lighting plan, it is just the minimum calculated load you can use for service computations. I'm sure we could go on for days about what light you need in various situations.

Thank you for your responses. I have worked with electrical contractors who just placed lights all over the place and if the inspector said anything, they would move them or add more lights. But, I want to actually know how to do it right. When a commercial building calls for 2 volt-amps per square foot does that mean for example that I take the voltage of the fixture (120 volts) times the amperage (.50) which gives me 60VA. Then if I divide that by 2VA (per square foot), that gives me an area of roughly 30 square feet that this fixture will cover. This is probably way off, someone help me please! : )

Again this is with complete sincerity; if you are asking if VA is the same as W, then you really should be looking into a code study class instead of asking people you don’t really know on the internet to explain the difference between a load calculation and whether a 400W lamp is adequate for 400sf based on reading T220.12.

I am not shooting anyone down, but there are some fundamentals here that can not be adequately taught on a message board, and the original poster is asking questions that would be thoroughly covered in a code study class, or even a structured apprenticeship through a local union.

This is what I feel is offering the correct advice for this question.

K, I think you're using the table wrong. That table has absolutely nothing to do with actual lighting you ought to install.

The table is in the part of the code that discussed "load calculations." This same area also defines panels as one of two types (power or lighting). By "lighting load" they are referring to the basic minimum electrical service that ought to be supplied to an area. The different occupancies listed are more of a guide, than actual code distinctions; a wise designer will likely break up an area into several different 'occupancies' for these calculations - even if there are no walls!

This "lighting load" is but a starting point. The same section of the code includes methods to calculate loads for receptacles and specific appliances or areas. These figures, added to the 'per sq. ft.' figures, gives you the minimum service size.

This is completely different from calculating how much light you need, and where that light should be located. For example, you'll want a lot more light over a work bench or aisle, than over the tops of the pallet racks. In those calculations, you will typically be using lumens, or candlepower .... not watts.

As for watts vs. volt-amps, for all intents and purposes they are the same thing. The difference only becomes relevant in discussions of power quality. Simply put, loads with large impedances (motors, transformers) or lots of electronics (ballasts, frequency drives) change the waveform from a nice, smooth sine wave to something else ... fooling you meter into thinking they are using less power than they really are.

Load calculations are a basic part of traditional electrical apprentice training programs. They are not, generally speaking, ever mentioned in the training of an industrial electrician. The industrial guy's training is focused on the equipment, rather than the building. It is certain that the industrial guy deals a great deal with motors and controls - but he can still go his entire career without ever seeing a GFCI or pulling a meter.

renosteinke, thanks for the great explanation! That is what I thought this forum was for. I misunderstood the table. I understand now that it is for load calculations and not for instucting on how much light to provide. Yes, you are correct. I have been an industrial electrican for 12 years now and have not had to deal a whole lot with lighting. I have read the code book through and study it when I have questions about different situations. I have mostly been doing small jobs on the side for several years. Over the last year, my jobs have been getting a little bit bigger as the area that I live in is experiencing heavy economic developement. I have wired several houses and have helped wire several commercial buildings in the last few years. But, like I said, no one that I have worked with has actually showed me how to determine how much light to provide. They just seem to put up enought light that either the inspector likes it or he does not. Again, thanks for giving a positive response. I'll try to read the book and understand a little better next time before I post the question.

Don't worry about it. This is supposed to be the place where you can ask "dumb" questions, among friends. This trade is so varied from job to job that there are no dumb questions, only questions about parts of the job we don't see every day.
I have said some dumb things here and been corrected. It was just part of the learning experience. In the volatile job market we are in it is wise to know a little about a lot of things you never thought you would ever need to know. Knowlege is power.
You also have energy laws and new products coming on the scene that make a lot of old "rules of thumb" obsolete. That is particularly true of lighting. That 2va/sqft may actually be very generous in California.
The next generation of LED (or other solid state lights) may make office space capable of being lighted with a fraction of a watt per foot.

Guys:
I was out of town for a few days, hence I could not respond promptly.
John