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Posted By: Delton Ebanks Sub panel wire size - 02/26/04 02:00 AM
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.
Posted By: earlydean Re: Sub panel wire size - 02/26/04 01:34 PM
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
Posted By: triple Re: Sub panel wire size - 02/28/04 10:34 PM
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).]
Posted By: iwire Re: Sub panel wire size - 02/28/04 11:56 PM
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.
Posted By: LK Re: Sub panel wire size - 02/29/04 01:05 AM
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).]
Posted By: triple Re: Sub panel wire size - 02/29/04 01:46 AM
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.
Posted By: iwire Re: Sub panel wire size - 02/29/04 02:13 AM
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
Posted By: triple Re: Sub panel wire size - 02/29/04 02:29 AM
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.
Posted By: iwire Re: Sub panel wire size - 02/29/04 02:39 AM
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.
Posted By: triple Re: Sub panel wire size - 02/29/04 03:16 AM
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.
Posted By: triple Re: Sub panel wire size - 02/29/04 04:20 AM
Forgive me, our posts are overlapping so I am a little behind.

It is doubtful that the starting load of a number of residential garage motors will be acting in unison. In a commercial application you could have workers numbering anywhere from 2 to 5 to infinity but in the typical home shop you have one or two. Again, you need to do a load calc to have any idea what you will need and #2 Cu may be a better choice than #3 for some garages (especially ones 175' away!). One thing you need to realize is that some motors will blink the lights no matter what. Unless you have wired right to the ragged edge, using a #2 over a #3 may not help. The home panel may be overloaded or underwired. I have been called out to homes (wired by others) that experience a slight blinking of the lights when the a/c kicked in. All connections can be tight, relay contacts can be good, the load on the panel can be ESTREMELY small, the wire feeding the panel and the a/c can be large in diameter and short in distance but the problem persists. In those cases, a new a/c or a larger lateral from the power company MAY have solved the problem. The point is, oversizing a subpanel feeder is not a cure-all. A larger wire provided by the electrician does not somehow act as a capacitor for an extra burst of energy when needed (I realize you know this). Voltage drop in the subpanel feed is just one of many possible causes and may not factor in at all.

You have to draw a line somewhere and use some math skills rather than making a shot in the dark and hoping for the best. If you talk your customer into paying for a wire that is 1 or 2 or 3 sizes larger than another contractor quoted, and his lights blink anyway when the job is done, what is he going to think about your competence then. Without data to back-up your claims then you have nothing to fall back on. Like I said, from my experience, some loads will blink the lights no matter what actions you take (do a search here on a/c loads for verification).

...all those words and I don't think I have answered your question yet iwire...

Of course you need to add all the continuous and noncontinous loads together (here I am using the code definition of those words). The starting load of a motor is not a continuous or a noncontinuous load for use in calculations. For motors you need to use the full load current found in the codebook. No pc program can take into account: the condition of the motors involved, the load already on the main panel, how well that main panel was wired, etc..

I notice that the more I write, the more I ramble and the harder I am to understand. Thus I will sum up all my words in one sentence.

You can not assume a proper size panel without load calculations much less sizing the wires that feed it.
Posted By: iwire Re: Sub panel wire size - 02/29/04 11:32 AM
Triple, I think you are right, we are closer in opinion on this then I first assumed. [Linked Image]

Here is the info we where given.

Quote
Adding a 100 amp panel in a garage 175 ft from house what would be size to use for that long a run?

Sure it would be good to know if the panel is feeding a 100 watt lamp and a garage door opener, or a full machine shop, we don't.

Given that info, I will stand by 2 AWG, yes this could still be overkill or undersized, we do not know, but IMO based on the info given is what I would do.

Quote
A larger wire provided by the electrician does not somehow act as a capacitor for an extra burst of energy when needed

Very true, on the flip side an undersized wire creates a restriction to the current available from the utility service. [Linked Image]

Quote
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.


Of course the panel can be loaded to 100%...non-contiuous.

If you do the calculations, the continuous loads on a 100 amp panel can be 80 amps, non-continuous loads can increase the load to 100 amps.

If this was not the case there would be no reason to figure non-continuous loads separately from continuous loads. [Linked Image]

Anyway it looks like we both want to provide a good job. [Linked Image]

Again, Just My Opinion, as voltage drop is not really a NEC issue that is all any of this discussion is. [Linked Image]

Bob



[This message has been edited by iwire (edited 02-29-2004).]
Posted By: Active 1 Re: Sub panel wire size - 02/29/04 06:22 PM
December we did the same job. 4+ car garage fed off pool house about 200'. We used #2. Why? Because we get hit all the time with oh by the way we are going to have this. Car hoists, electric floor heat, finish the 2nd floor, tennis / basketball court lighting, furnace, A/C, electric gate, ???.

We only had a few breakers but we still used a 20 space panel. Why? Because we are ready to do more work for them. If the customer spends more than my home cost on a garage then they desirve more that NEC minimums. There is also the other side of the spectrem where a person is running an auto shop out of his back yard.

The added costs for us was little. Does that make us a bunch of hacks for not calculating the draw of only what is on the print? Are we bad for making the EMT and boxes too big for the wire used also?

Tom
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