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#34846 - 02/25/04 10:00 PM Sub panel wire size  
Delton Ebanks  Offline
Junior Member
Joined: Feb 2004
Posts: 2
hampton,va
Adding a 100 amp panel in a garage 175 ft from house what would be size to use for that long a run?

Just a note
Also first one I have seen. Replaceing a old ceiling light fixture. Had wire nuts but both of the wire nut connections had a small glass jar around them with a special cap the wire nut could fit thru.


Work Gear for Electricians and the Trades

#34847 - 02/26/04 09:34 AM Re: Sub panel wire size  
earlydean  Offline
Member
Joined: Dec 2003
Posts: 751
Griswold, CT, USA
To limit the Voltage drop to 3% I use the formula: R = 15 x E / I x L
E = 240 volts
I = 100 amps (worst case)
L = 175 feet
R = 15 x 240 / 100 x 175 = .2057 ohms

I want a conductor with a maximum resistance of .2057 ohms per 1,000 feet. Looking at Table 9, Chapter 9, I see that for PVC conduit, I need #2 copper or 1/0 Aluminum. Checking with Table 310.16, I see that #3 copper or #1 Aluminum are the minimums, so I choose #2 copper or 1/0 Aluminum.

Earl


Earl

#34848 - 02/28/04 06:34 PM Re: Sub panel wire size  
triple  Offline
Member
Joined: Apr 2002
Posts: 184
Wisconsin
Earlydean, using the worst case scenario of 100A is VERY unrealistic. First of all (and probably most importantly) the panel should only be loaded to 80% of its max. Secondly, even 80 continuous amps being used in a residential garage is extremely rare. In other words, the actual percentage of home garages that have a continuous load of 80A would be quite small. To wire all garages based on findings from say 0.5% of the heaviest loaded would be overkill to say the least.

I like the fact that you consider voltage drop in your calcs since far too many electricians fail to do so. However, the drop needs to be based on realistic info gathered for each specific case and not just generalizing all situations as being equal.

Delton Ebanks, the minimum size feeder allowed for that application is #3 Cu or #1 Al (remember table 310.15(B)(6) cannot be used for garages even if they are for residential use). If this is an average residential garage with a few lights and a few 120V receptacles then voltage drop should not be much of an issue. If heavier usage is expected now or in the future then you need to estimate a calculated load with which to do some voltage drop calculations. If it were my project, I would probably size the wires above the minimum since the distance involved is somewhat excessive.

BTW earlydean, are you sure that table 9 in chapter 9 can be used in this case? The heading says that table is for three-phase and I doubt that the home falls in that category.

[This message has been edited by triple (edited 02-28-2004).]


#34849 - 02/28/04 07:56 PM Re: Sub panel wire size  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Good points Triple you would not want to dangerously oversize the conductor.
[Linked Image]

If it was my garage I would run the 2 AWG that earlydean suggests, the cost difference is minimal.


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#34850 - 02/28/04 09:05 PM Re: Sub panel wire size  
LK  Offline
Member
Joined: Jan 2003
Posts: 1,429
New Jersey
Go with the #2 the last three garages we did, that were detached from the house, they all needed the #2, One had a welder and heater, One had Electric heat and hot tub connected, and another had compressor and electric heater. The labor is about the same, so as iwire noted go with the #2.
These customers refered us, had we undersized the jobs, we would have lost, not only those customers but the leads also. We learned this early in the business, as we noticed the contractors using minimun standards were not getting refered as much.

[This message has been edited by LK (edited 02-28-2004).]


#34851 - 02/28/04 09:46 PM Re: Sub panel wire size  
triple  Offline
Member
Joined: Apr 2002
Posts: 184
Wisconsin
Iwire and LK, do you guys/gals perform load calcs on your projects or just try to oversize everything to be safe. I personally do at least a minimal amount of estimating on nearly every circuit I run. If you are experienced and knowledgeable, it takes almost no time at all. This reminds me of the time a carpenter was "ribbing me" about using 14-2 in the bedrooms of a home. He said he always used 12-2 when he worked in an area that didn’t require licensing and allowed anyone to call themselves an electrician. In that particular case, I put less than 8 receptacles on 15A circuit and loaded the lighting circuits to well under 80% of the max I thought they would ever see. Some people think they can try to oversize everything and always be safe. The truth is, if you don't really know all aspects of what you are doing, you are going to perform a number of non-code compliant actions. It doesn't take much of an imagination to think of the "electrical atrocities" that an average carpenter may perform. I would never pretend to be a homebuilder without the proper training for fear of the harm I may do.

1) Iwire and LK, how would you perform the voltage drop calculations?

2) If you read my first post more carefully you will realize I don't aim for the minimum. I was trying to explain to Delton how to make a complete analysis instead of a blind stab in the dark.

3) Can chapter nine, table nine be used here (second attempt for an answer)? I use two different methods for voltage drop but have never heard about using some three-phase table for a single-phase application.


#34852 - 02/28/04 10:13 PM Re: Sub panel wire size  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Personally I use a voltage drop calculator on my PC.

And I, like you will generally use 80% of the OCPD rating as the starting point unless I have reason to believe the load will exceed that.

Even if the load is intermittent it will effect voltage drop.

Nothing irritates me more than dimming lights when a motor kicks on, like say a compressor in a garage.

You sound very experienced, do you typically see the load on a panel decrease as time goes on, or do you find like I do that it only gets added to over time.

I spend a lot of time at electrical forums and it never ceases to amaze me how many people want to know the absolute minimum size conductor for what ever it is they are wiring.

Maybe that does not describe you, but your post following earlydean's sounded like you are saying it is foolish to go above the minimums because most time a garage will not have that much load.

No I do not always run wires larger then NEC required but I would say at least 75% of the time I do, by choice or by the prints, you will not find many engineered prints running the minimum wire sizes allowed.

JMO, Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#34853 - 02/28/04 10:29 PM Re: Sub panel wire size  
triple  Offline
Member
Joined: Apr 2002
Posts: 184
Wisconsin
Rereading LK's response made me think (boy does that hurt). While hot tubs and some large heaters can draw a good deal of current they are a couple of the few exceptions. Most home welders and compressors have relatively small continuous loads (for a 100A panel). Put an ammeter on the average home welder and you may be surprised. Add up the continuous load on those garages you wired LK and see if oversizing the wire was necessary. If your voltage drop calculations show that you are under 3% then what is the benefit of running a larger wire? Why not run a 4/0 Cu in case the customer wants to put in a 200A panel in the future and wants that feeder oversized also? There is no end to how big you can go but if you are not going to base it on "gather intelligence" then what is the point. A number 2 Cu feeder may be the right choice but I would NEVER guess. Without at least some type of load calc, how do you even know that a 100A panel is large enough for the garage load now (please someone answer this question)!

As far as keeping and growing your customer base, I help my company do that with well-thought-out application of electrical code for the job at hand. I also make every effort to communicate with the customer to know about future endeavors that they plan to have that will require electricity. As an example, rather than running a couple new circuits all the way to the other side of a large house, I have talked the customer into adding a subpanel there for future expansion.

I’m not “trying to start trouble”. I honestly want to know how others figure these things out. I find it hard to believe that I could ever be wrong… but if I am, let me know.


#34854 - 02/28/04 10:39 PM Re: Sub panel wire size  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Triple in your posts you made reference to continuous and non-continuous loads.

What have they got to do with voltage drop?

If that non-continuous load drags the voltage down for 1 minute or 179 minutes it still effects the power quality.


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#34855 - 02/28/04 11:16 PM Re: Sub panel wire size  
triple  Offline
Member
Joined: Apr 2002
Posts: 184
Wisconsin
Iwire, the ocp in that residential garage is not going to allow (code-wise) to be loaded beyond 80% so I still don't understand why using a calculation above that amount would ever make any sense. Also, if I were feeding a new garage and it appeared that the load seen would be anywhere near 80% of the 100A ocp then I would install a larger panel (min. 125 but probably a 150 or 200 depending on the customer's future plans and the cost of the related materials). I worked a number of years as a residential electrician and even the most used garages in that realm had a realistic continuous load of maybe 40A (now I am a commercial electrician and that small-load generalization does not apply).

The strange thing is iwire, I think you and I actually see eye-to-eye on this particular issue but go about arriving at the end result completely different. However, I don't believe the end justifies the means when the means have no basis. In other words, you and I may actually wire a given shed identically but I would have data to back-up my claims. Bigger is better as a stand-alone reasoning does not work for me. Also, I understand your frustration with those that "underwire". I live/work in Wisconsin and in 95% of this state anyone can call themselves an electrician. No license is required so you can probably imagine the stuff I run across. Due to the small load it may not be a big issue but residential garages here are almost always fed with #2 Al. While this is OK for the residence itself it is not allowed for the garage associated with it. It is tough for a "real" electrician to compete with these hacks and even harder to explain to an electrically uneducated customer why a larger wire is needed.


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