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Brain Tease: Service Conductors

jar546

jar546

CBO
Joined
Oct 16, 2009
Messages
12,938
Location
Not where I really want to be
You are called to inspect a detached residential garage that recently had a new service installed. The owner decided to run a new service rather than try to run a sub-feed from his already overtaxed main panel in the home.

A load calculation for the detached 2 car garage is not provided. For now, there are only receptacles, lights and a 5hp 220 vac compressor installed.

The overhead service is 2/2/4 aluminum SE.

Anything wrong with those sized conductors?
 
Are you serious? 25 views and not one response. I would like to ask the electrical heavy hitters to stay out of this one since you folks know the answer already.

Come on electrical inspectors, whats up?
 
I will respond and see how many people jump all over my responce. Since this is not a single phase dwelling service, the de-rating of Table 310.15(B)(6) can not be used. This now allows for not greater than an 75 amp rating on the conductor. 230.79(D) allows for a minimum size disconnecting means of 60 amps. Assuming that grounding has been properly addressed, I do not see a problem and looking forward to being proven wrong. Definitily has my interest up.
 
I see that no one has challenged this installation which has my interest up. Have I approved these incorrectly in the past? Assuming there are 10 receptacles @ 180 va each, that calculates to 15 amp draw, and having 6 light fixtures @ 180 va equals 9 amp draw. From what I have checked, a 5hp motor will draw approx 32 amps maximum @ 220v giving a total demand of 56 amps for this service. At 80% loading of the main breaker, this would require a 70 amp overcurrent device. Since 70 amps is a standard rating per 240.6, It appears that this would be acceptable under those circumstances. The question is what will they add later without the inspector having knowledge of the additional loads. This also assumes that all loads will be used at once under full load conditions.
 
It would be nice if the size of the service was specified.

In my location the service conductors are sized by the electric company.

I don't think you have a say in the matter.
 
Inspector 102 said:
I will respond and see how many people jump all over my responce. Since this is not a single phase dwelling service, the de-rating of Table 310.15(B)(6) can not be used. This now allows for not greater than an 75 amp rating on the conductor. 230.79(D) allows for a minimum size disconnecting means of 60 amps. Assuming that grounding has been properly addressed, I do not see a problem and looking forward to being proven wrong. Definitily has my interest up.
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Without a load calculation being provided, this would not be allowed. If they provided a load calc then they may upsize to the next highest OCPD

Debate?
 
I would respectfully disagree since your original post indicated that it is not a dwelling unit and therefore Table 310.15(B) is not applicable. That was the point of allowing the 70 Amp OCPD being used. I would have trouble going with load calculations on this application because of the ability to change equipment without alot of effort. When they add the table saw and the welder, who are they going to tell. Is all the equipment going to be used at once, probably not, but how do you allow for that. Protect the service conductors for the known rating, provide the proper OCPD and move on down the road. Document for future use if the service burns up after they switch out the main breaker.
 
E3301.2 Scope.

Chapters 33 through 42 shall cover the installation of electrical systems, equipment and components indoors and outdoors that are within the scope of this code, including services, power distribution systems, fixtures, appliances, devices and appurtenances. Services within the scope of this code shall be limited to 120/240-volt, 0- to 400-ampere, single-phase systems. These chapters specifically cover the equipment, fixtures, appliances, wiring methods and materials that are most commonly used in the construction or alteration of one- and two-family dwellings and accessory structures regulated by this code.

The overhead service is 2/2/4 aluminum SE. 2006 IRC TABLE E3503.1 says 100 amps

Do you require all residential garages to submit load calcs. I agree with inspector 102 "Protect the service conductors for the known rating, provide the proper OCPD and move on down the road."
 
I was not questioning the follow-up post other than the fact it did not say that in the original post. "If" can include an awful lot of circumstances. If it was considered a C1D1 location, would that change the responces? I said respectfully and meant that. this forum has assisted me many times over and I do not intend to offend anyone. Thanks for allowing me to be involved.
 
mtlogcabin - never checked the IRC for allowance but agree with your analysis that 100 Amp OCPD would be allowed on the detached garage. One could argue that the most restrictive would apply for this installation and I think there could be some discussion on IRC vs. NEC. As a side note, I have allowed the 100 amp on past occasion and that is why this thread had my interest. Was not sure what I was missing, and still could be confusing.
 
Under the IRC, the 2/2/4 al se to the detached garage would be good without a load calc unless there was some known equipment that may tip the scale.

Without a load calc under the NEC, a garage is not a dwelling unit and the relief given for dwelling service conductors would not apply.
 
I think there could be some discussion on IRC vs. NEC
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I don't think there is an IRC vs NEC. If you adopted the electrical portion of the IRC it applies if you did not then the NEC applies. I don't think you need to be jumping back and forth to see if one is more restrictive than the other.

We do not compare the plumbing and mechanical portions of the IRC with the IPC or IMC. We use the adopted code for the structures they are applicable to.

2009 IRC

This Electrical Part (Chapters 34 through 43) is produced and copyrighted by the National Fire Protection Association (NFPA) and is based on the 2008 National Electrical Code® (NEC®) (NFPA 70-2008), copyright 2007 National Fire Protection Association, all rights reserved. Use of the Electrical Part is pursuant to license with the NFPA.
 
* * * *

Inspector 102,



No offense perceived or taken!.....I too am here to learn on a hourly /

daily basis......This forum is an invaluable resource for me.



Jeff and others,

I hope and am requesting that more Commercial & Residential

electrical brain teasers [ and pictures ] be submitted, ...regularly!

I need all the help I can get!....These exercises really help me!

Thanks!

* * * *
 
Our state adopted the IRC for residential so the NEC does not apply unless there is something that is not covered such as a generator.

When we write up deficiencies, we have to list the IRC section, not the NEC.

Commercial is the NEC

There are clear differences between the two that were lost in translation probably by accident.
 
a garage is not a dwelling unit and the relief given for dwelling service conductors would not apply
Click to expand...
But it is an accessory structure regulated by the IRC and unless the plans indicate a need to use the NEC you should not go there.

E3301.2 Scope..... Electrical systems, equipment or components not specifically covered in these chapters shall comply with the applicable provisions of the NFPA 70.

Granted a compressor is not covered in the IRC so you probably could go to the NEC but would you have questioned it if it was a dryer outlet with no equipment plugged in?

Just curious.
 
I think there is some over analyzing here.

If a state does not have the IRC adopted but does have the NEC adopted, that could influence the decision/answer

Not all states are IRC and some states that are IRC did not adopt the electrical sections of the IRC so the shoe does not always fit
 
As Northstar indicated, these brain teasers are great resources to challenge or abilities. More is better. I also agree that if a jurisdiction has adopted the IRC for electrical that it will trump the NEC for the situation described in this thread. I have to remember that there a slight interpretation issues between the two. Under the NEC, i would not allow the 2/2/4, but under the IRC, yes. "If" this were a storage building for a commercial business, the rules would change. Bring on the next one, things are slow here and if I am digging through the code books, people think I am actually busy.
 
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