Addition of Load Center / Outdoor Electrical Box

I am building an addition on to my house.
I am ready to install the electrical and have decided on a Load Center.
I also wish to Install an Outdoor Box to power a Green House, Lights and Outlets around my 5 acre place.
Both will be powered from a single 220 V breaker from an existing panel.
My question is:
How do I determine the size of the Feed from my existing Panel.
Where can I locate the current NEC's?
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Residential lighting and general purpose recepts are figured at 3 watts/sq.ft. (size of greenhouse.) I would use a single 15 or 20 amp circuit for the greenhouse lights and a second one for the general purpose recepts. As for whatever outlets you are anticipating, figure one more 15 or 20 amp circuits to cover these. No limit in residential as to how many you can have on a circuit, and a greenhouse is not required to have recepts or lights in the first place; so placement and quality is up to you. (Everything will be on a GFCI breaker).
All total a 30 amp feeder should be more than ample. Figure on 10/3 with ground (or 8/3 with ground if going more than 100 ft.) from the house to the panel. That's for general loads, if you are anticipating a special load such as a welder or whatnot, that will have to be figured in separately.
Article 220 of the NEC covers the methods used for Branch circuit, feeder and service calculations. Article 225 covers installations of outside branch circuits and feeders.
You can pick up a copy of the 2005 NEC on eBay (best prices.) Don't forget to pull a permit for this work prior to buying anything (just in case there's a local ordinance you may have to comply with.)

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