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I have a house that was built in the 1950s. It has over-head wiring coming from the transformer. The load box is connected to the service panel through the concrete wall and only three heavy duty wires are coming from it. Two black hot wires 120 each and one neutral. There is a pipe coming from the bottom of the load box outside that goes down into the ground. I am assuming that this is the grounding wire, but we know what they say about assuming. In the service panel the neutral wire is connected to a neutral bus at the top of the panel. The two black hot lines are connected to a 100 amp main break as there are no incoming terminals. All of the white neutral wires from the inside wiring are connected the neutral bus; seems appropriate. All of the aluminum and copper grounding wires go to a separate grounding bar; again seems correct. I cannot understand how this grounding bar is actually grounded as it is not connected to anything except inside wiring as far as i can see!?!?
First, is the neutral bar connected to the grounds or to the metal box in any way, or is it isolated and mounted on a plastic or other insulating material? Some of the older houses in that era did not use a grounded system and would connect a ground wire from various boxes throughout the house to the closest grounded object they could find. It sounds though, like you have grounds run to your panel. Do they appear to be original or added wiring?
If there is no grounding installed, and this is a main, not a sub-panel (it sounds like a main from your description but see the difference on my website under services), then you can add the two ground rods and connect them to your ground bar using a copper wire, #8 minimum for your 100A service. If it is a main, the neutral and ground bars need to be connected somehow, The best way to make sure would be another #8 between them.
If you had some good pictures it would help me.
This is a fantastic site! I own a large residential electrical company in Virginia and intend on using some of this information to help train my guys. My only suggestion- There is nothing mentioned about drilling out. Always make sure your bit is sharp, drill the least amount of holes as possible, map your circuits as you drill, and know what you can and cannot drill.
Great comments on the drilling Chris. There's definitely some good information I could add on the subject. This site is a work in progress (so far, about a 6 year work in progress, lol), but I try to make time to add and adjust things as I can, when my regular day job permits. It is great to hear positive comments from other skilled tradesmen!
Thanks for the question Aaron. If you want an actual accurate load calculation for your service, you can put all of your information into my service size calculator here: http://wireyourownhouse.com/tools/housecalc.html . Just at a glance though, if you are using all gas appliances, and with the cabin being that small, I'd say a 100 amp service would be plenty. Your calculated load will most likely come out to be a fair amount less than that, but the NEC does require a minimum service of 100 amps to a dwelling unit. As for whether to use 12-2 or 12-3, it depends on what you are using it for. 12-2 has two conductors (think hot and neutral) and a ground. That means that 12-2 can carry one 20A circuit, which is fine for a single circuit run of outlets, lights, or whatever. 12-3 has three conductors (think two hots and one neutral), which means it can, in most circumstances, carry two 20A circuits (one on the black, one on the red, and they share a neutral).