100 amp service wire gauge

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  #1  
Old 05-30-04, 07:45 AM
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100 amp service wire gauge

I have to run underground cable about 75 ft from my meter base to my shop. I have a 320 amp meter base and a 100 amp breaker which I need to mount on my house and then the 75 ft of cable to the breaker box in the shop. I'm guessing 2-2-2 aluminum wire. Any suggestions?
 
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  #2  
Old 05-30-04, 08:53 AM
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I have a dilemma. In many areas, the use of 2-2-2 aluminum for a 100-amp feeder is common. On the other hand, my interpretation of the NEC is that this is insufficient. I suggest you call your building department and ask if it is okay in your area. Be sure to let us know what they say.
 
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Old 05-30-04, 09:22 PM
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Around here (middle Ga.) 2-2-2 is called "trailer feed" by the guys at the electric supply places because it's commonly used for single wide trailers. I'm going to call the inspector in the AM and see what they say. What gauge would you use if it were your shop?
 
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Old 05-30-04, 11:43 PM
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I assume your shop must have a large welder or a couple of kilns. Otherwise, it's difficult to imagine requiring 100 amps (24KW) of power in a residential shop. That's a ton of power.
 
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Old 05-31-04, 05:45 AM
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Using 2ga Aluminium for a 100A feeder is permitted by table 310.15(b)(6). The catch is that this table applies only if the load is the 'main power feed for a residence', not to be used for any other application. This permission to run the conductors on feeders with higher amp rating than normal depends upon the loading characteristics of a residence.

2ga Aluminium normally has a 75C ampacity of 90A, and if you were to run 100A continuously through it, it would overheat. But when you have a _home_, and you perform the appropriate demand load calculation, if you get a number of (say) 98A, you can be darn certain that you will _almost_ never actually reach the 98A value. On the other hand, if you have an industrial appliance that calculates out as needing 98A, then you will frequently actually need that 98A.

I believe that it is confusing to say that 2ga Al is 'good for 100A'. Rather say that 'for residential main feed applications, because of the load characteristics, the NEC permits 90A conductors to be used when the demand load calculation indicates that a 100A breaker must be used'. This is similar to the protection of motor circuit conductors with breakers that have higher rating than the circuit conductors, and makes it clear that you can only use these feeders for the specifically identified application.

For your application, if you actually want to have a 100A feeder to your subpanel, and be able to safely run 100A of load, you would need to use 1ga Al conductors with 75C rated wire and terminations, or 1/0 ga Al conductors with 60C rated wire and terminations. Workshop loads have different characteristics from ordinary home loads.

You also may wish to consider using Copper conductors, which will be thinner for the same ampacity, more expensive for the same ampacity, but also probably more reliable and longer lived.

Note John's point; 100A is a vast amount of power for a workshop in most cases...but can be required if you have large electric heating loads.

-Jon
 
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Old 05-31-04, 02:54 PM
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All I run is a 110v MIG, a 220v compressor and I have a 220v overhead heater that I'm going to install. Of course there are lights, bench grinder, fans, a small drill press and the occasional hand tool. I know that the amps are important but I don't have the info right now.
100 amps just seemed like a good round number and it was the cheapest option, the 60 amp stuff costs a bit more.
It's just a 20 x 24 shop but if I ever want to put a big window AC in there I'd like to just let 'er
rip

Thanks for all the help. The inspection office is closed today so I'll have to check back later.
 
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Old 05-31-04, 03:17 PM
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the 60 amp stuff costs a bit more
What "stuff"?
 
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Old 05-31-04, 03:21 PM
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I'd get that info together. Depending on the size of the equipment you may want to run bigger cable. running 75' underground is not something you want to do again. Pay for the right cable now or do it again later?
 
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Old 06-01-04, 04:26 PM
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I spoke with the inspector and he said that 2-2-2 aluminum would be fine.
John, I was referring to the breaker box and disconnect. The 60 breaker box was about $20 more and didn't come with any breakers. The 60 amp disconnect was also a bit more. I figured it's because they aren't used as much as a 100a.
My GE loadcenter with a main and 5 breakers was $55 and the outdoor box (Square D) with a 100a breaker was $42.
 
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Old 06-01-04, 06:49 PM
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Just FYI, it's okay to use a 100-amp panel and a 100-amp disconnect on a 60-amp feeder. This need not dictate the choice of feeder size. But it sounds like you're comfortable with 100 amps, and I'm not going to try to talk you out of it. Good luck!
 
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Old 06-01-04, 09:24 PM
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I think I've got it. Now I just have to get the wire, call to have the underground wires marked,
rent a ditch-witch etc...
I'm going to have to change out my meter base from the current 200a which only has lugs for one load to a 320a (Freebie from the power company) which has room for two outputs. One will go to the disconnect for the house and the other will go to the shop.
I haven't opened the 200a meter base but the 320 is a lot taller and I'm not sure if the power company's feed is going to be long enough if I mount the new base at the same height. Are they going to be responsible for hooking up their side and lengthening the cable?
I'm just trying to plan ahead because once I have the power disconnected I want everything ready so I can work fast and get it back on quickly.
 
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Old 06-03-04, 09:26 AM
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Your power company probably has a web sight were you can look at drawings of their service standards.
--
Tom Horne
 
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