sub feed panel grounding

[gino93]

we are going to add another building 20' x 40' next to another about 5 feet away in a residential dwelling. the existing building has a 100 amp main lug panel . the building has 2 8 ft electric heaters. the guy uses the buildings for small get away offices. now the existing office is fed by # 4awg copper 2 hots 1 neutral and a ground. can i feed another panel off of the original panel? i was planning on coming off with a subfeed panel 100 amp main lug and put a 2 pole 60amp breaker in the existing panel. also someone told me that you can get away with not running an equipment ground with the feeder conductors and just install a ground rod? i always run a ground which is better practice or is both better?
thanks for all the input.

 

[david]

Re: sub feed panel grounding

I?m not sure what you mean by the first building has a 100 amp main lug panel.

I was going to give a short answer. Any time you add a load to an existing electrical system you need to consider the load already on the system. You are not giving enough information to know how to calculate the total load of your existing dwelling and the additional building.

I will assume that the calculated load for your service is less than would require a service upgrade. Yes you can feed an out building from a 100 amp dwelling service. As far as is it better to feed this building with a 3-wire or a 4-wire feed. Most electricians believe a 4- wire feed is better. Yes you need a grounding electrode system at the 2nd building. It may end up being two ground rods, but you have to determine if there are other grounding electrodes available at the 2nd building that would be required to make up the grounding electrode system. By the way you need the grounding electrode system even if you run a 4-wire feed to the second building.