Garage Sub-Panel questions

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  #1  
Old 10-24-06, 10:25 PM
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Garage Sub-Panel questions

I am just building a separate free-standing 1597 sq ft garage (motorhome, car boat, small wood shop). The power company discouraged me from running a service directly to it (results in a second meter/account and a higher rate as non-residential). So I am left to consider running off of the house's main panel and running about 70 ft of wire in buried conduit to the new garage, with a sub-panel there. The house is 2100 sq ft and I think the existing 200 amps is plenty (we're conservative) and there should be room to add the garage's electric tasks to it (alternately, the power co said I could change the box to a 400Amp service [as the feed wires will support it], but that seems way too much and too much $$$). The existing panel is a Siemens G3040MB1200CU and it has all 30 slots filled (with standard sized breakers).

The questions: 1. Is there a formula for me to determine how much circuit breaker "capacity" there is in this "200Amp" box? It can't just be physical space, but I can't seem to find a formula anywhere. If allowed, I'd convert 4 existing breakers (2 on each leg) into half-height breakers to make room for a full-height double 30 amp (220) breaker for the shop/garage so that I can provide myself with 220 in the building and be able to run the garage door openers, lights and 110 outlets.
2. Same question for the sub-panel in the garage. Having a 30/30 220 breaker, how many circuits can I put in the box?...recognizing, of course, that taking over 30 amps from one side or the other at the same time will pop one or both breakers (panel and/or sub-panel).
3. Is 6/3WG the correct size to use? Or would 8/3WG work? Would that let me put in more than my intended 60A total...say a double 50A breaker?
Thanks for any help. I thought I had this all worked out with a new 200A service, but from the way the power co guy talked, this will be a lot less expensive than the second service I'd planned.
Brad
 
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  #2  
Old 10-25-06, 04:44 AM
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You want to poerform a "demand load calculation". Google that term and you will find calculators all over the place, or buy a good book or two on home wiring and find it.

If you are building a garage that large, consider nothing less than 60 amps to feed it. While it is true that you can bury wires in conduit, you might be better off with direct burial cable. You can find aluminum cable for direct burial that will support up to 100 amps.
 
  #3  
Old 10-25-06, 11:02 AM
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Why would buried aluminum be better than conduit?

The calculators all seem to reflect dwelling space. I agree on the 60A minimum. There will be no heaters or major appliances. This is a 3-bay garage. The motorhome, when plugged in with dead batteries (not likely), will draw up to 1200w to charge. In the second bay is a boat and car, and if all batteries were dead, I'd use 2 chargers taking a total of 2000W to charge them at the highest rate (not likely). When using a 750W garage door opener (1/2HP), only one of the three will be active at once. The total of all lights in the place (10 fixtures x 2 bulbs x 40w) = 800W and not likely to be all left on at the same time.

So, worst case scenario, all dead batteries in everything and all lights on and all 3 doors going up at once gives me a draw of 6250W (VA) or just under 57A. Very, very unlikely.

The more likely constant load when I'm out there will be the two light fixtures in the shop (160w) and the motorhome (1200w to be safe). If I have 60A out there (110VX60A=6600W), that will leave me 5140W or about 45A to use on woodworking tools, radio, TV... which is more than plenty for me.

I still have my original questions about capacity and wire size. I'd rather have 100A out there as more is better, but not if there is a huge cost difference from a 60A service which seems to be fine, from my calculations. I want to put in lots of 15A outlet locations for convenience, but really only 260 sq ft of the shop is flexible space and the rest is dedicated to simply storage and battery chargers are the only real planned usage.

I have a 320 page book on order and a couple of other smaller guides.
Thanks,
Brad
 
  #4  
Old 10-25-06, 03:03 PM
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You really aught to buy a copy of IRC 2006 (or whatever code your locality has adopted)- it includes the service calculation you need, as well as all the other codes you'll need to build your garage- structural, electrical, plumbing, hvac, etc. It's definitely a worthwhile investment for a project like this.
 
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Old 10-25-06, 04:09 PM
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the only answer is to do a demand load calculation.

there is no short cut. you cannot add breakers and such.

If you do not know how to do one, hire it done by a licenced electricain.
 
  #6  
Old 10-25-06, 05:20 PM
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Originally Posted by jwhite
the only answer is to do a demand load calculation.

there is no short cut. you cannot add breakers and such.

If you do not know how to do one, hire it done by a licenced electricain.
I have no idea what you are trying to say in that I "cannot add breakers."
Also, I thought I had defined my load calculation sufficiently. If I missed something, please tell me what I missed.

And regarding your last comment, I am surprised to hear from a moderator on the DO IT YOURSELF forum a statement not of help, but only a suggestion that I NOT Do it myself. Electric work isn't rocket science...I just need a few pointed answers so that if I do decide to hire a licensed electrician for some or all of it, I'll have a good idea as to what to expect.

The last "licensed electrician" who did work for me couldn't get a 3-way light circuit to work after a half hour of trying. I had to re-wire it after he left. The one before that was generally drunk by noon. The one who built this house couldn't figure out how to wire the 4-way so that all 3 switches worked in the garage. Out of self-defense, I came here looking for some advice and answers. Someone being a "licensed electrician"...or even a "licensed master electrician" doesn't necessarily guarantee me a quality solution.
 
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Old 10-25-06, 06:02 PM
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I would interpret Jwhites comment in the context of guidance, to help you get an actual correct load calc. Not as you should not do the job. The first calc can be very confusing.

Contractor dillema: Bad luck? Perhaps you need a better interveiw process.

Ive been at it a while and I don't know That many bad Lic. people.
 
  #8  
Old 10-25-06, 08:42 PM
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I suggest direct burial aluminum cable because it will be much cheaper than conduit and individual (copper) conductors.
 
  #9  
Old 10-25-06, 08:59 PM
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Thanks for the reason for the aluminum. Less expensive is better if sufficient.

I have completed a Demand Load Calculation for the garage and it comes out to 6176VA...or 51A. Surprisingly close to my real-life estimates above of 6250VA and 6600VA as worst case, so a 60A supply appears sufficient. One question down and another half dozen to go....
Brad
 
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Old 10-26-06, 08:58 AM
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I like to recommend mobile home feeder cable for a situation like yours. It is a four conductor heavy-duty direct burial aluminum cable available in 100A (#2-2-4-6) and 200A (#4/0-4/0-2/0-4) sizes. It will be about 3 times cheaper than copper in conduit.

In your situation I would recommend the 100A aluminum cable directly buried, even if you only feed it with a 60A breaker for now. It should be under $2/ft for this cable and will allow you substantial future power in the garage if you need it someday.
 
  #11  
Old 10-26-06, 09:01 AM
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> it comes out to 6176VA...or 51A.

That's actually 25A. The service to the garage will be 240V, so you divide 6176 by 240 to get 25A. Still, I think a 60A service minimum is appropriate for a building that size.
 
  #12  
Old 10-26-06, 09:18 AM
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Also, if your main panel is full, I suggest moving some lighter loads to a new sub panel inside the house. This will create space in the main panel for your subpanel feeder breakers.
 
  #13  
Old 10-26-06, 09:53 AM
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Great help!
How does one tell if their box is electrically "full"? I have all 30 full-size slots filled, but they are all full height breakers so it is physically "full." Theoretically, I could change most/all of them to half-heights breakers and gain lots of room to add my 30A/30A 220 breaker. But there must be some max? If I have 600A worth of breakers in a 200A service, surely that isn't legal. What is? The power co guy said he thought the max I could put in was 80% of 200A....a total of only 160A adding up all the breakers. I know that is not correct, but I doubt 600A is correct either (more good advice from a Pacific Power & Light professional). That was one of my first questions. Is there a formula (like his 80% formula)?
 
  #14  
Old 10-26-06, 10:27 AM
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Brad
The point these guys were trying to make is that adding the breaker rating does not give you the right information. Your having 600 amps in breakers is perfectly legal if the calculated load is less than 200 amps. Understand that there is diversity in the load such that load comes on at different times.
Quote " Electric work isn't rocket science...I just need a few pointed answers so that if I do decide to hire a licensed electrician for some or all of it, I'll have a good idea as to what to expect". It may not be that complex but most laymen do not understand the complexity in the calculations. If you don't get it right, you may have smoke coming from you home were it should
not come from. The NEC has rules for making the calculations and it does depend on the square footage of your house.
You may load the panel up to 125 % of continuous load + 100% of the non-continuous load. Thats the formula. Why don't you list sq footage, and the load in the house. Range, WH, Dryer,
A/C(name plate amps), Heat(electric), DW.
If you want to try it yourself go here for a calculator
http://www.electricalknowledge.com/SFDLoadCalc.asp. No
guarantees you will get it right.
 
  #15  
Old 10-26-06, 01:17 PM
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You also need to find out if your panel supports the use of twin breakers. Not all panels support them and some only certain numbers and positions. So you might be able to twin-up some circuits, to get enough room to add a 220 breaker, but the panel will still be full the next time you add something. By adding the sub now, you give yourself room for expansion. An easy gut check load calc on your house is what are the large electric loads (range, AC, resistance heat, dryer, hot tub, pool heater, WH, etc.) If you have more than a couple of these (especially heat and hot tub), than a full blown calculation may be in order. Otherwise, for a 2100sq ft house, you should have plenty of capacity available to add your garage.
 
  #16  
Old 10-26-06, 01:54 PM
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There is a difference between your load off-hand calculation and the NEC-required load calculation. There is specific code on what must be added on the load calculation, and it's not just for the garage- you need to compute your entire panel to make sure you're not overloading it by adding this circuit. You'll need to run the calculation twice- once for the garage, and then again for the house, with the garage load added in.

The sum of the breakers, surprisingly, doesn't really matter. It's what those breakers serve that must be computed.

Use IRC Table E3604.3(1) to calculate the load on your house. NEC Article 220 elaborates on this. I believe the calculator wareagle linked to is an automated version of this table, but it's hard to tell. Either way, I highly recommend you buy a copy of IRC*, it's $70 well invested as it will save you at LEAST that much in failed inspections and rework!

* check with your locality and make sure IRC 2006 is the right code book; there are a few oddball states out there.
 
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Old 10-26-06, 03:27 PM
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"And regarding your last comment, I am surprised to hear from a moderator on the DO IT YOURSELF forum a statement not of help, but only a suggestion that I NOT Do it myself. Electric work isn't rocket science..."

I apologize if you were offended by my comment. Sometimes part of my job, as a moderator is to sort out users who are taking on more than thay can handle, and suggesting that they get professional advice. If the way I worded that offends you I apologize.

The bottom line is that when mistakes are made doing electrical work people get killed. Un like plumbing where mistakes cause mostly property damage only.

My statement about counting breakers is because you cannot say that "I have six 20 amp breakers so that means I have 120 amps load" It doesnt work that way at all. you will not even get close.

some loads need to be multiplied by more than 100 percent others can be multiplied by less than 100 percent, and then you can take reductions on feeders and services. My suggestion was that you get help from a pro for this part of your project.

one disadvantage we have here is that we cannot walk the job site. we can only read what you are posting. a professional electrician who went to your home, would be able to give much more accurate advice.
 
  #18  
Old 10-26-06, 04:03 PM
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Thank you. I appreciate your comments. I am slowly gaining what I need to know. I'm trying to minimize the work I need to pay an electrician for...like the grunt work. I'm retired and have far more time than money. I need to know what to get dug...whether a conduit trench and lay a 2" conduit...or put in direct burial wires...and at what size. The road they go across is shared, so I'l need to get it open and closed quickly, likely way before I am ready to actually connect the wiring. Hopefully I can open it up and get the depth inspected before a rough electrical inspection.

I feel totally comfortable both rough wiring and finishing the garage and consider that just time-consuming but not requiring an expensive electrician. Connecting the wires via an external connector to my existing box, with the right sized breakers for both the house and the garage sub-panel is what I'm trying to learn and may have to pay someone to do.

I got a better picture with Grover's comments that I need to do the calculation for both the house (with garage circuits) and the garage by itself...altho I still can't find the "rules" about the non-dwelling space. I.e., no laundry circuit, etc. My first stab at that (couple of guesses on range/heating), combining the sq footage of both the house and garage...3700 sq ft and adding the 3 additional garage door openers...got me a total load of 165A according to http://www.electricalknowledge.com/SFDLoadCalc.asp
Not sure if the old garage needs to be added to the house sq footage for the calculation. I'll head to the library to read the code once I get a better handle on what I'll be reading.

Still, whether I need to change the panel to a 400A service (doubtful, but the PPL guy says the delivery wires are sufficient) or not, the same questions are valid: the size wire I need to get 60 or 100 amps to the garage.

Glad to learn that total breaker amperage has nothing to do with box capacity or the demand load. That tells me I can run as many breakers as I want in the garage to isolate circuits/outlets (I.e. 2 duplex plugs only on each circuit....instead of trying to put 10 on a circuit and having to kill all plugs to work on one.)

The garage was drawn by an architect, engineered, and will be built by a contractor. I learned on the last garage that was completely built for me that I need to be more aware of everything. I want to do all I can, but will not attempt what I'm not sure of.
Brad
 
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Old 10-26-06, 04:21 PM
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As coincidence would have it, I had scanned in both relavant tables some time ago. I think this will help you! (Ignore the highlighting, it was for another project)

Depth of underground cable runs:
http://img54.imageshack.us/img54/8692/irc2003e37031xw2.jpg

Service calculations:
http://img161.imageshack.us/img161/720/e360431rc5.jpg
Non-applicable parts to a garage wouldn't apply. The 3w/sqft wouldn't really apply either, as receptacles aren't required in a garage. Its not bad to still calculate it that way, though. Edit: it's hard to read, but in the first paragraph, that's "1,500 VA" for the kitchen/dining/etc. The floor area shall be measured by the outside dimensions of the building, but should not include garages, porches, or any unfinished spaces. In your case, you might still want to count the shop and garage as floor space, though. Garage door openers are fixed appliances and should be added separately.
 
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Old 10-26-06, 04:41 PM
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Bradnjan,
I think if you research your area and friends, you should be able to find a contractor who is (good and proffesional) willing to work with you on your concerns. I hate to dig, so if you can "getR' done, great. Most good people will work with their clients. Additionaly, you can get partial inspections in most areas. So dig up the road, lay the conduit, seal both ends and call in the inspection.Sometimes you don't need to dig, you can blow in a sleave.

Or run the cable last, You just can't run the cable and leave it exposed to the elliments, conduit is ok.
 
  #21  
Old 10-26-06, 05:01 PM
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Thanks guys. Got the jpgs...and The contractor will be pouring footings in about 2 weeks and then a bunch of backfill (the garage is on a 7% slope and 2 levels), so that seems to be the time to get a trench open. I have not talked with him about any electric work. Maybe he's my answer for what I don't know how to do. I will do a correct (by plate) review of my appliances/heating so I have that all done.

Not that long ago, I added a 20A breaker and ran a 12/2WG into the open attic area for both lights (2) and a duplex plugin (1 so far). Am I to understand that this really has no impact on the Demand Load Calculation? ...or did I miss another subtle point. I say this since the calculations seem to just use square footage for non-fixed items/appliances...like my lights and plugin. Or, do I need to now include the attic as "floorspace" since there is now electric there?

The previous owner/contractor had actually spliced into the garage light circuit (with the 3 switches) to put outlets and a chandelier (taped up wires from a cut extension cord fed the light) up in the attic. The garage lights had to be on to have any power up there. I removed the splice, put the wires back together in a junction box and then added the new 20A circuit.
 
  #22  
Old 10-26-06, 05:01 PM
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For a run like that, it would be easiest to lay the cable and conduit at the same time. It's a ***** trying to pull UF through conduit- way easier to just string pieces one at a time, and fasten them with the cable already installed. UF is OK exposed to the elements, too.

You only need to drop to 24" under the road- the rest can be shallower, per the table I linked to.
 
  #23  
Old 10-26-06, 05:12 PM
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Cable or wire, any cable or wire, must be installed in conduit AFTER it is glued together and the glue has dried.

Do NOT under any circumstances attempt to glue conduit with the cable already installed. The glue will destroy the insulation of the cable or wire and you will have a mess.
 
  #24  
Old 10-26-06, 05:43 PM
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Edit: my bad. This method only works with EMT/IMC/RMC, so if he's using PVC, he'd have to string a rope or snake as a proxy instead. I'd still recommend stringing it like beads during install as opposed to trying to fish a 70' run later.
 

Last edited by grover; 10-26-06 at 06:02 PM.
  #25  
Old 10-26-06, 05:51 PM
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No grover, it's not being paranoid. It's following the NEC and doing it properly.

How do you think the PVC stays together? The glue melts the plastic. It has the same effect on the plastic insulation.
 
  #26  
Old 10-30-06, 10:52 AM
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You have a great opportunity to improve the grounding of both the house and the garage. First make sure that the concrete contractor stubs up a piece of galvanized or stainless steel rebar out of the footer at the location of the garages building disconnecting means. This is only useful if the footer trenches are not lined with plastic sheeting to save on concrete. The normal tie wires that are used with the reinforcing steel is sufficient to make it behave as a single grounding electrode. If the footer trenches are to be lined with plastic then have a bare number two American Wire Gage copper conductor installed as a ground ring encircling the entire structure at a depth of at least thirty inches prior to back filling.

In the bottom of the trench for the feeder you can run a bare number two American Wire Gage conductor from the house to the garage in addition to the feeder conductors. You use it to bond the two Grounding Electrode Systems into one. If for any reason you are going to use driven rods as part of the Grounding Electrode System drive them through the bottom of the trench to improve their depth and use the bare number two as the Grounding Electrode Conductor to connect them to the grounded conductor of the service and the Equipment Grounding Conductor of the feeder at each building.

As for the wiring method to use for the feeder to the garage the combination of PVC conduit and aluminum conductors offers the advantages of the flexibility to change the feeders ampacity in the future with the lower cost of aluminum feeder conductors. If the conduit is likely to be repulled then you will want to run the trench for it deep enough so that the height of an upright metal ninety will remain at least eighteen inches below the ground. The code specifically allows the use or IMC or rigid nineties in plastic conduit runs if they are at least eighteen inches under ground. The reason for using the metal nineties is that the pull rope will not cut through the inside of the ninety as it can on plastic nineties during hard or repeated pulls of the conduit.
 
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