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# Wiring a Garage

## Wiring a Garage

#1
01-25-02, 07:18 AM
matthewgfox
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Wiring a Garage

Looking for insight on this project. I have 200amp service in my house and would like to put 80 or 100 in my garage. I'm not an electriction just and anal DIY. I built the garage myself with no help at all. Its 24x36 with a loft above, attached to my house with a 12x12 mud room. Anyway, a big question I have is the definition of a 200 amp panel. Does 200 amps mean that all the breakers must add up to 200 amps or is there a rule of thumb to figure in some appliances wont be used at the same time? Or is it simply the size (area) of the panel? If I pull 80 off it for my garage would that reduce its capicity to 120? And, does anybody know of any good books writen for DIY's on this topic? Alot of times you find book in Menards, Home Depot for \$20. My garage will have two door openers, and I am thinking about a few circuits below and above for just various 15amp power tools. No welders yet or anything bigger then 15amps. I will have an air compressor that I think Id like on its own circuit. I figure if I ever get a welder I can run it off my generator.

#2
01-25-02, 07:46 AM
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A 200-amp panel means that the maximum possible load is 200 amps. It means nothing more.

There is no relationship between the sum of the individual breakers and the size of the panel. The breakers may add up to more or less than the main breaker. Sometimes a lot more.

The way the calculation is done is more complicated than any of the speculations you have made. You must do a "demand load" calculation. This calculation has lots of alternative methods. It takes into account square footage and the numbers, types, and sizes of various appliances. There are different rules for different types of appliances. As you stated, the calculation does indeed consider the fact that everything won't be in use simultaneously.

In general, it's hard for any reasonably-sized house to overload a 200-amp panel, unless you have unusual appliances such as welders, kilns, or a lot of air conditioning, a pool, all-electric heat, a mother-in-law apartment, or two of things that most people only have one of (like two dryers or two kitchens).

From time to time, Wg and a few others offer to make a demand load calculation for somebody. But it requires you to answer a lot of questions and to do some research about how many watts your various appliance are. If you're willing to do the leg-work, let us know.

#3
01-25-02, 07:55 AM
matthewgfox
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Thanks for the info. I'm a Mechanical Engineer and have no problem with math of any sort. I'd hate to ask of someones elses time to crunch my numbers. Nice to know there are good folks out there though. I think what I need most is a good reference sorce (book). Last year I knew nothing about building but I got a book and built this garage with only myself and the boom on my ford tractor. Took awhile but I did it. You know of any internet site that is usefull? Or any easy to read book? Ive seen my share of books with too much information if you know what I mean.

#4
01-25-02, 01:02 PM
Guest
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I advise a 100amp feeder supplying a 20-circuit panel in the garage because you may finish the loft as a play-room or computor/entertainment center for the kids which may require electric heat----which remind us-consider installing TV/communication cables if you finish the loft. Good Luck.

#5
01-25-02, 04:26 PM
Bruce D.
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I've found these two books very helpful: "Advanced Home Wiring" from Black and Decker, and "Wiring a house" by Rex Cauldwell (Taunton Press). The later goes into more theory and detail than the B&D book. Both walk you through a demand load calculation. I'm pretty sure I bought both books at Home Depot.

-bd

#6
01-26-02, 07:49 AM
Wgoodrich
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What you are seeking is called diversity or intermittent load calculations for credit of equipment connected but not all running at the same time.

Your garage should be calculated with the thought of how many people are going to be running machinery installed in the garage at any one given time. If only one person working in the garage at any one given time the you would ignore all manually operated machines except the largest machine load you have in the garage. Then increase that largest manual load to 125% of its rating and add to that load .25 volt amps per square feet of hte garage for general lighting then add to that total any light that will pull more than 600 volt amps or watts. Then add to that any automatically ran machines such as air compressors, water pumps, water heaters, furnaces, or air conditioners. This will be what is called you demand load of your garage. Then the service supplying power to the garage must be at least equal to or larger than the answer to your demand load calculation of that garage.

This demand load calcualtion answer to your garage is what you must consider that you are adding to your house panel load no matter how much bigger you make the service to the garage that is larger than that demand load calculation.

I suspect that you will find the minimum feeder size of 4 awg for 100 amp copper or 2 awg aluminum for 100 amp feeder to your garage to be the most bought sizes and the cost factor of that 100 amp feeder would be very little more in cost than say a 30 or 60 amp feeder. Kind of makes sense to pay the minor extra money to provide for future possible loads sizing your panel at 100 in the garage.

If you want to see more explanation on wiring designs of inside your garage and options of sizing and designing your feeder from your house to your garage then try the following link that may be of a good source of information.

http://homewiring.tripod.com/detgarage.html

Once you get there look on the left and click on where it says "show entire article". Lots of pictures on that article also with lots of options supplying power to garage from main house.

Let us know how you come out and good luck

Wg

#7
01-26-02, 09:36 AM
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Wg, I have a question for you.

Every time I look at this calculation, I reread the part about the dryer. The code seems to require you to add 5,000 watts for the clothes dryer, even if you don't have a clothes dryer or even if your clothes dryer is gas.

Is this true? Does code require you to allow for at least one electric clothes dryer even if you don't have one? Perhaps on the assumption that somebody will add one?

#8
01-26-02, 11:17 AM
Wgoodrich
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John if you calculate the motor to be 120 volt you should find the full load current to be 7.2 amp approximately. If you calculate the 5000 volt amps subtracting the 7.2 amps you should get around 3360 volt amps left for the heating element that cycles on and off during the drying cycle. Depending on the model and make of the residential dryer you should find the volt amp load of the heating element to be at least the 3360 volt amp and may be higher.

The NEC 2002 Article 220.18 says that while calculating with the general method of calculation for a demand load you must use a minimum of 5,000 volt amps or the name plate rating whichever is greater.

While it is possible and may even be common that the name plate rating may be larger than the 5,000 volt amps I left that part out for simplification for the do it yourselfer to ease there attempt to understand how to calculate. To be precise in your calculation you must use the name plate rating if it is larger than the minimum rating of 5000 volt amps. I felt that the difference between 22 amps if 5000 volt amps was used compared to the full load of a 30 amp rating which would be an 8 amp difference probably wouldn't cause that much difference in there final answer of calculating their demand load to be worth making it more difficult to understand than it already was.

The NEC actually says in 220.18 that you must use at least 5000 va or the name plate rating if larger rated than the 5000 volt amp minimum.

As for a gas dryer the 1500 volt amp required to be added to all dwelling calculations for laundry found in article 220.16.B as required in step two of that demand load calculation format not only covers the load of a washer but is also accepted to cover the load of a gas dryer.

Also the 3000 volt amps required to be added as found in article 22.16.A to cover the two small appliance branch circuits for all dwelling calculations also covers any load of a gas cooking range.

If a person has a gas dryer the 5000 va for an electric dryer may be omitted from the demand load calculation. That 5 KW is added only if you either don't know what type of dryer you are using preparing for the worst scenerio or if you know you are using an electric dryer.

This last paragraph also applies to the electric range also. If you know you are going to use gas cooking equipment the requirements of 220.19 may also be omitted. If it is unknown whether the cooking equipment is going to always be gas or electric then you should add that requirment of 8 kw for an electric range to cover the worst scenerio.

If you are reading that article that way most likely others are reading it that way also. What is your opinion, should I add to the difficulty of understanding the demand load calculation by adding an explanation when to use the dryer 5 kw and 7 kw for an electric range or should I leave it where they would make the mistake on the safer side causing their calculations to be calling for a larger panel size? I thought it best to make it as simple as possible yet if you read it that way maybe I should add the detailed explaination. What do you think?

Wg

#9
01-26-02, 12:23 PM
Jxofaltrds
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Mathew,
How much space is left in your panel?
It might be cheaper and easier to run circuits for you needs.

#10
01-26-02, 12:35 PM
Guest
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You can use your utility bill for calculating the AVERAGE CONSTANT power load. You are billed for Kilowatt Hours consumed,usualy for a 30-day period. We have 30X24=720 hours in a 30-day period.If you bill was 720 KiloWatt-Hours your power consumption was equivalent to a constant load of 1000 watts (1 kilo-watt). KiloWatt-Hours / 720= constant load in Kilo-Watts. At the risk of appearing didactic allow me to point out that a KiloWatt-Hour meter is an integrating instrument. To "integrate" mathematicaly is to calculate the area under a curve.The graph of a fluctuating power load plotted against time is an iregular curve.The Kw-Hr meter "averages out" the curve which results in a straight line plotted against time.

#11
01-26-02, 01:26 PM
Wgoodrich
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Switchman, I understand that the method you are suggesting is an easier and common method of guaging your demand load of a certain dwelling. I have seen your suggested method often brought up as an alternative demand calculation.

What concerns me is if you use the months electic bill or even a years electric bill calculating from the average usage would your panel be prepared for loads such as 100 degree days in July or possibly when the whole family comes in while cooking on Thanksgiving day.

If this alternative calculation is used I can see where it would invite short sheeting a service size unless it is mentioned that a larger service should be added to be prepared for peak usage on certain days of the month.

I also have the same concerns on calculating by the optional method and the general method.

Maybe we should also add in warning those we suggest these calculations to that this is the minimum service size depicted and suggest an adjustment up in amp rating of a service might be prudent for their best interest.

I never seem to think of saying those things when explaining the general method or optional method of the NEC either. Maybe I should try to remember to issue that warning that the calculations are for minimum sizing only.

What you said is fine, just brought out my worry wort side, second guessing all of us in what we forget to say at times.

My worst crime is forgetting to tell people to shut off power before they do what I suggest in a wiring project. Guess that is one of my many shortcomings.

Just a thought

Wg

#12
01-28-02, 06:08 AM
matthewgfox
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Breakers to Use

Our main is about 70 feet from the garage.

Also does anybody have any insight on what not to go with for breakers? My house is a Thomas Betts (I had no say). I see most of the big supply stores (HD, Menards, Lowes) sell Square D or Seimans.

#13
01-28-02, 06:30 PM
Wgoodrich
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I don't recognize Thomas Betts. You breakers must be listed for use within the brand panel that you have.

Jxofaltrds, If I picked up your suggestion correctly you were thinking of installing more than one branch circuit to the detached garage. I believe that the NEC forbids more than one power source to a building unless it is all in one building. The NEC allows only one branch circuit 120/240 volt 20 amp max to serve a detached structure. If you want to install more than that you must install a feeder from a common service installing a distribution panel in that detached structure from a common service of two buildings or install a separately derived power source.

Curious

Wg

#14
01-29-02, 04:13 AM
Jxofaltrds
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WG you are right.

His first post said "I'm not an electriction just and anal DIY. I built the garage myself with no help at all. Its 24x36 with a loft above, attached to my house with a 12x12 mud room."

Some how the conversation got switched to detached.

#15
01-29-02, 12:19 PM
Wgoodrich
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Jxofaltrds, I would have suggested that we beat the guy with a wet noodle, that first introduced the detached garage idea when this garage is actually an attached garage. But I really think compassion is in my mind at this point considering the fact that person who messed up and suggested the detached garage idea was me. I vote to forgive the mistake instead of any torture etc.

With this garage being attached and only 70' away I am inclined to agree with your idea of several branch circuits from the main panel.

Guess I goofed,

Wg

#16
01-29-02, 04:07 PM
Jxofaltrds
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No! WG You are barred from giving us your expert opinion for 10 minutes.

WG you are showing your age when you talk about a wet noodle.

#17
01-30-02, 05:11 PM
Wgoodrich
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Oops I did it again, now everyone knows I am a bit over 20, or 30 , or, or, well whatever!

Wg