Sub Panel Installation Questions

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  #1  
Old 08-27-01, 10:43 AM
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My house that I currently live in has a 200 amp panel inside. My detached garage has a 100 amp sub panel in it for all of the outlets, lights, etc. in the garage. My question is this...currently there are 5 20 amp breakers and 1 15 amp breaker in the sub panel. I would like to run some electric service to my barn and am wondering this...Can I put a 60 amp breaker into my sub panel for 60 amp service in the barn? I don't know what the rules are for maximum number of breakers or amps out of the sub panel. I know that my 200 amp service inside has a lot more than 200 amps total of breakers in it. Also, does anyone know how deep I must bury the wire below the ground and what gauge should be used (#6?)? Thanks.
 
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  #2  
Old 08-27-01, 11:20 AM
Wgoodrich
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Your total amperage added up of the branch circuit breakers in you panels whether a nonservice rated panel [subpanel] or a service rated panel [main panel] would have little to do with the actual load being applied to you main or subpanel. You need to use the general method of demand load calculation, the optional method of demand load calculation, or the demand load records of a long peroid of time printed out from records by your serving Utility Company to determine the true minimum service size of you main service rated panel. The subpanel would have to be calculated in demand load separate from the main service rated panel. This subpanel demand load calculation is normally done concerning connected load only. You again may adjust diversity loads for intermittent use concerning that non service rated panel [subpanel]. If you subpanel is installed right you have a breaker in your main panel that protects the feeders serving that subpanel from overheating. If you subpanel is being used normally and you have never had problems of that breaker serving that subpanel overheating or tripping then you probably would be ok to add the 60 amp load rated panel in your garage whether attached or detached. You might want to do a demand load calculation checking if you main service rated panel is adequate for your dwelling. Below is a link where you can find a format to calculate that demand load calculation telling you the minimum main service size in amps. This should give you an good feel as to how you are doing concerning you main service panel size. The subpanel, I would possibly take an amp reading with an amp probe tester telling you the normall load being applied to your subpanel. If you have not experienced problems with this subpanel kicking the breaker in the main panel protecting the feeders to that sub panel, I would not worry about adding that extra garage load to it.

Also keep in mind as to how many people are going to use the equipment in that garage and how much load is going to be automatically controlled that may run at the same time. Even though you are installing a 60 amp panel with 100 amps worth of breakers in that panel in that garage, you most likely won't be pulling near the 60 amps unless you have a welder, range, etc. that pulls heavy by itself. Be realistic as to what the actual load is being applied in that garage at one given time. You could have a 1000 amps worth of different motors in that garage but you most likely will only use one motor at a time plus any autocontrolled equipment such as air compressors, water heaters, etc.

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

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

http://homewiring.tripod.com/DwlingDmdCalc99.html

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

Hope this helps

Wg
 
  #3  
Old 08-27-01, 11:48 AM
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Thanks for the quick response...

I haven't ever had any problem with the sub panel tripping the breaker in the main panel in the house. The sub panel also has its own breaker, which gives duplicate protection I guess. The house is only five years old, so I am sure the service is adequate. My concern was adding the breaker to the sub panel in the garage to put another sub panel in my barn. Would you suggest a 60 amp panel? I am only going to use a few small items in the barn and doubt I will be plugging in anything that really draws a lot of current. Also, what size wire should I use and how deep should it be buried? Thanks again for all of your help and your quick response!
 
  #4  
Old 08-27-01, 12:14 PM
Wgoodrich
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The feeder size would depend on what your actual load applied at any given time in that barn as a maximum. I would suggest you read the link called wiring a detached garage. The same rules would apply to a barn unless this barn in an agricultural barn with livestock and no machinery stored in this barn. Then GFI receptacles would not be required but corrosion resistant wiring style would apply as per article 547 of the NEC.

If this barn is an accessory building to a dwelling that is non agricultural use, GFI protection would be required for any receptacle found on the first floor with exception to large appliances. If this building is a detached garage designed to store or repair vehicles then GFI protection would again be required for any receptacle on the first floor.

The feeder size would depend on the exact loads you are going to use at any given time. You are going to have to be precise as to the loads in that barn before we could provide our opinion as to minimum feeder size. That feeder size would range from 15 amp 120 volts to 200 amps or more. We need you exact loads and how many people are going to work in that building at one given time.

I advise you to perform the calculations. Being sure due to faith and guessing loads probably will lead you to install wiring that is either way too big or way too small to do the job you need accurately. Hit the links I provided, read all available then come back in with accurate info while being informed yourself through those links we provided.

I highly advise you not to design on faith or guessing. You may be spending money you possibly may be wasting due to not being able to do the job you want on finished product.

Good Luck

Wg
 
  #5  
Old 08-28-01, 10:16 AM
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Location: Fayetteville, NY, USA
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Wg,

If NDA4 is providing overcurrent protection in the garage panel of 60 amperes, why are we having him perform calculations? I'd tell him to run #4 copper wire, whether using table 310-15(b)(6) which goes down to only 100 amps, or table 310-16 which gives you 70 amps for type UF.

NDA4, if completely on your private property and going across a yard or driveway, parking area, etc, 18" deep is the minimum burial requirement for 1 and 2 family dwellings, for voltages of 0-600 volts, and if used for dwelling-related purposes only.

For a couple simple lighting and convenience receptacle general purpose circuits a 30 amp panel may well be enough out there. For only 150 feet run #10 AWG wire will satisfy minimum requiremnts for a 30 amp panel. I'd probably run #8. But finding a main breaker type panel with more than two breaker spaces will be difficult. If protected upstream by a 30 amp breaker in the garage panel an MLO (Main Lugs Only) panel with 4 spaces shouldn't be hard to find though.

Hope this helps.

Juice
 
  #6  
Old 08-28-01, 08:15 PM
Wgoodrich
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Juice, I think you may be right. Just being cautious. I have had many tell me I don't have much in the barn and found loads well over what you might expect. I noticed he talked of a garage then a barn. I was pushing for more info as to what actual equipment he planned now and in the future before I stuck my neck out and said run this wire. A demand load of a garage or barn is simple. Just tell us what you largest motor load is such as Air compressors, fans, 1/2" drills, planers, etc. Then tell us the full load current of all other equipment you plan to use in that barn. Then tell us if more than one person is going to work in that barn. You could then come up with adding 25% to the largest load, add any automatically run loads and add .25 va per square feet and you have the demand load of an accessory building. Right now we don't even know the actual use of this barn. Livestock, pets, mechanic work, etc.

Juice you have no idea how many people that I have inspected that said what he said; The house is only five years old, so I am sure the service is adequate. I have seen new homes not last a week before the main panel kicked due to over load. Just had my doubts on what he knew and what he was taking for granted to be right. He said he had 200 amps in his house yet 100 amps to a garage and now adding a 60 amp sub in this barn from the 100 amp sub in the garage. Is there any left in the house? What are the loads. What are you going to do if he comes back in and says
I blew my main breaker in my house. I have had that done type of guess in my advice and ate it later, just being reluctant to provide a guess without more info as to what he actually has.
I was just reluctant to stick out my guess as to advise him to install such and such wire size.

Worry wart I guess,

Wg

 
  #7  
Old 08-29-01, 05:50 AM
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Location: Fayetteville, NY, USA
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Wg,

I fully understand your caution. You are taking a conservative and comprehensive approach, which I entirely respect. I, on the other hand, got a bit jumpy and shot from the hip. I took NDA4 at his word that there would be next to nothing in the barn electrically.

The demand calculation is truly a simple, basic procedure that actually should be done. You read it off straight and included examples to connect with lay persons. I thought that perhaps your first reply, although correct, might have been a little intimidating to a guy who just wanted to snap in a little panel, but the second time around I feel that most folks could grasp and might even punch a few numbers into their calculator and bother to "look before they leap". You are, as always, offering these folks prudent and responsible advice, and they would be wise to follow it I think.

Juice
 
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