Last edited by Matt Marsh 10-02-2014 at 2:37 PM. If you run a #6 bare copper GEC, you can run it exposed on the surface unless it is subject to severe physical damage (250.64(B). Although the minimum size grounding electrode conductor is typically #8 copper for a 100 amp service (it's actually determined by feeder size, not amperage), the NEC requires that conductors smaller than #6 be physically protected (usually conduit). It is almost always easier and more cost effective to satisfy the NEC requirement by supplementing one ground rod with an additional one (250.53(a)(2). To save on wire cost you could use 2 aluminum and still have 70a breaker at the main. Since this is detached you will want a ground rod also. The measurement must be performed with a dedicated ground resistance meter, something that very few contractors have. At 125’ 4 awg copper would provide 1.79 voltage drop using 4 gauge wire would allow for 70 amp breaker in your main panel. Given the 42.4 mm2 thickness, the 1 AWG copper wire has a current-carrying ampacity of 130 amps at 75☌. 1 gauge wire diameter is 7.348 mm and it has a 42.4 mm2 (1 AWG to mm2) cross-section or area. 1 gauge wire is the first ‘non-negative or 0’ wire gauge. The only way that you can get by with a single one, is if you can prove by measurement that the ground resistance is 25 ohms or less. 1 AWG wire amps (ampacity at 75☌): 130 amps. If you need wire sizes for shorter service entry cables up to 200A or for both short and long branch circuit wires from 15 to 50A. If you are using rod type electrodes, you really need to drive two of them, at a minimum of 6' apart. This page gives the necessary increase in electrical service entry wire size for long runs, for example between the electric meter and the main electrical panel. The NEC specifies that the cable has a cross-section of 250,000 circular millimeters. The common size of aluminum wire is 250 MCM, which can be written as 250 kcmil or 250 MCM. From NEC 250. The National Electrical Code (NEC) and Canadian Electrical Code (CEC) use the circular mil to define large wire sizes. The only exception is if the separate building is fed with just a single branch circuit (2014 NEC 250.32a). Your 1AWG Al is good for a 100 AMP feeder (actually 110). Jason, A separate building always requires a grounding electrode system in addition to the equipment grounding conductor that you mention.
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