de-rating a cable ???

I need to install a 100amp sub-panel in a shed I am building. The length of the wire run is approx. 230 feet and the sub-panel
I have purchased is rated for 100amps max and does not have a provision for a 'main' breaker. I will install a 100amp breaker in the house main panel.
I used the calculator at the electrician.com site and think I can get by with #1 al awg wire. The local stores around here claim they do not stock #1 al because no one uses it. I have no problem with moving up to 1/0 al for the 100amp service to the sub-panel.
The problem with the 1/0 al is that the wire will not physicaly fit into the 100amp breaker. I was told by the electrical supply store that I could de-rate the cable by clipping one or two strands off of the end until the cable would fit into the breaker and that this method was routine and would not be an issue with the building inspector. I have not discussed this matter with the building inspector yet as I did not wish to sound like a total idiot in the event the info I have been provided by the supply store is incorrect.
I cannot see how de-rating the cable accomplishes anything other that allowing the cable to fit into the breaker. My goal was to use a suitable size cable to minimize the loss due to the length of the run. It would seem to me that if I have to reduce the surface area of the cable at the breaker I might as well have used #1 al to start with.
Any suggestions would be appreicated on how to make the 230 foot run with an acceptable loss at the sub-panel end.
At this time the best that I can come up with is to do what the electrical supply store suggests or:
Install a 125amp breaker in the main house panel Run 230 feet of 1/0 al wire Ditch the 100amp panel I purchased Purchase a sub-panel rated for 100 or 125 amps with the provision for a 'main' circuit breaker Install a 100amp circuit breaker in the sub-panel as the main disconnect for the sub-panel
Thank you for your time.
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Run #2 copper instead. You could ask your inspector if he will allow #2 aluminum for a 100A building per NEC 310.15(B)(6) (typically needs to be dwelling/residential use and not a big tool shop or farm building). If so, use #2 Al pigtails to the breaker. Do you really need 100A -- what kind of equipment are you running and did you do a load calculation per NEC guidelines? Will the items that care most about voltage drop be 120V or 240V?
Your subpanel doesn't need a main breaker unless you have more than 6 breakers installed.
Technically, I would be OK with cutting one strand off the 1/0 to make it fit. A 75C #1 al wire is good for 100A, you just have larger wire to reduce voltage drop. As long as its about a #1 in the breaker hole, it should be fine. The breaker should accomodate a #1 wire, and most allow for at least 1 size upgrade. What brand of breaker won't take a 1/0 for a 100A breaker? Perhaps you have a bending space issue in that panel, so that is one way to enfore that.
I don't think the inspector will like it if he notices that you "cut it to make it fit" though.
--
Mark
Kent, WA
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Try smaller copper pigtails that crimp onto the aluminum wire and rent a compression tool.
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Reducing the diameter at the breaker (by cutting 'strands' or smaller pigtails), will leave a larger diameter cable for the run. The current will re-distribute to the 'cut' strands within a very short distance. So the voltage drop will be less than if you use a #1 the whole length.
Two things to consider when sizing, the heating of the insulation caused by I^2R and the voltage drop (IR). For most residential wiring, the former is dominant because of the short runs within a home. But when you start going longer distances, the voltage drop can start to become important. Operating a cable within its thermal capacity can still result in an unacceptable voltage drop if the length is really long.
daestrom
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First of all the derating is based on load. if you considered the load with your calcs then that is good. Second is that you can not cut strands from a cable for a multitude of reasons. The building inspector might not even see it but if the electrical inspector is aware you will fail. Why don't you step up to copper wire? RAK

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Clipping strands, HOLY SHIT!!!! Someone is smoking dope in your local supply house. Go copper!
I need to install a 100amp sub-panel in a shed I am building. The length of the wire run is approx. 230 feet and the sub-panel I have purchased is rated for 100amps max and does not have a provision for a 'main' breaker. I will install a 100amp breaker in the house main panel.
I used the calculator at the electrician.com site and think I can get by with #1 al awg wire. The local stores around here claim they do not stock #1 al because no one uses it. I have no problem with moving up to 1/0 al for the 100amp service to the sub-panel.
The problem with the 1/0 al is that the wire will not physicaly fit into the 100amp breaker. I was told by the electrical supply store that I could de-rate the cable by clipping one or two strands off of the end until the cable would fit into the breaker and that this method was routine and would not be an issue with the building inspector. I have not discussed this matter with the building inspector yet as I did not wish to sound like a total idiot in the event the info I have been provided by the supply store is incorrect.
I cannot see how de-rating the cable accomplishes anything other that allowing the cable to fit into the breaker. My goal was to use a suitable size cable to minimize the loss due to the length of the run. It would seem to me that if I have to reduce the surface area of the cable at the breaker I might as well have used #1 al to start with.
Any suggestions would be appreicated on how to make the 230 foot run with an acceptable loss at the sub-panel end.
At this time the best that I can come up with is to do what the electrical supply store suggests or:
Install a 125amp breaker in the main house panel Run 230 feet of 1/0 al wire Ditch the 100amp panel I purchased Purchase a sub-panel rated for 100 or 125 amps with the provision for a 'main' circuit breaker Install a 100amp circuit breaker in the sub-panel as the main disconnect for the sub-panel
Thank you for your time.
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Thank you for your response and your time. I need to check some things before I can answer some of your questions.
'Cutting it to fit' was the suggestion of the electrical supply store and even with their assurance the building inspector would not have a problem does not mean it is the right way to do the job.
I have discussed the 'shed' project with the building inspector and even though we are in a rural farm area the shed is considered residential use and he will allow the use of #2 aluminum. He left it up to me to determine whether or not it would be suitable for the loads I expect to place on it. A licensed electrician will inspect the job after I complete the work as a favor to me prior to the inspection by the building inspector. The electrician expects me to do the leg work since he is providing the favor which is of course more than fair.
Did the load calculations which is how I arrived at the 100amp service and ran the voltage drop equations up to the max of 100amp and determined the #1 al awg would be the absolute minimum needed.
Did not consider the #2 copper due to expense but will re-consider.
Understand the sub-panel does not need a main breaker which was why I purchased a 100amp / 6 breaker panel. My thought on replacing the sub I purchased for a 125 amp panel with a main breaker was to allow the 1/0 wire size to run from the house to the sub without having to shave the wire to fit into a 100 amp breaker at the house. The main panel at the house could then have a 125 amp breaker and the sub could receive the 100 amp breaker as it's main since the 1/0 wire would connect directly to the line terminals of the replacement sub panel. 100amp breaker is by Square D, bending space is not an issue
Some answers are: 'Shed' still under construction and the square footage will be approx. 1,150 SF
The 'shed' items which will need power will be: (not all used at once)
Arc welder - 50amp breaker 240V
Outlets will power: Small fridge 2 1/2 Cubic Feet Various fluid pumps - probably not more that 1/2 hp 120v Light - at least 8 dual bulb flouresent Power tools - saws, drills, etc. Computer Dedicated breaker(s) for (1) Window AC 15,000 - 18,000 btu 120v (only need to cool one half of shed) 30 amp breaker installed to feed additional service to a second sub panel in a garden shed. No equipment except 2 flouresent lights and a couple of outlets.
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