Calculating sub panel capacity

It's all about Volt Amperes.

NEC 2008 gives us an easy way to do things in residential.

220.82 Dwelling Unit.

(A) Feeder and Service Load. This section applies to a dwelling unit having the total connected load served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors with an ampacity of 100 or greater. It shall be permissible to calculate the feeder and service loads in accordance with this section instead of the method specified in Part III of this article. The calculated load shall be the result of adding the loads from 220.82(B) and (C). Feeder and service-entrance conductors whose calculated load is determined by this optional calculation shall be permitted to have the neutral load determined by 220.61.

(B) General Loads. The general calculated load shall be not less than 100 percent of the first 10 kVA plus 40 percent of the remainder of the following loads:

(1) 33 volt-amperes/m2 or 3 volt-amperes/ft2 for general lighting and general-use receptacles. The floor area for each floor shall be calculated from the outside dimensions of the dwelling unit. The calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use.

(2) 1500 volt-amperes for each 2-wire, 20-ampere small appliance branch circuit and each laundry branch circuit covered in 210.11(C)(1) and (C)(2).

(3) The nameplate rating of the following:

a. All appliances that are fastened in place, permanently connected, or located to be on a specific circuit

b. Ranges, wall-mounted ovens, counter-mounted cooking units

c. Clothes dryers that are not connected to the laundry branch circuit specified in item (2) d. Water heaters

(4) The nameplate ampere or kVA rating of all permanently connected motors not included in item (3).

So we can use 220.82 (B)(2) to figure for the dust collection, freezer, and an additional circuit for receptacles.

1500VA * 3 = 4500VA / 120V = 37.5 Amperes

You'll then have to use the values from the nameplate on the table saw to figure for that (A Volt-Ampere value should be listed on the nameplate, use that number for more accurate calculations). You could also use this method for the dust collection system and freezer since they are both "permanently connected, or located to be on a specific circuit".

3360VA / 240V = 14 Amperes

Now we'll add them up.

37.5A + 14A = 51.5A

So This is what our subpanel will look like.

• 60A double pole breaker in the main panel.
• 6 AWG feeder cable for a run up to 75 ft., 4 AWG feeder cable for a run up to 150 ft.
• 60A main breaker in the subpanel.
• 20A double pole breaker for table saw.
• 20A single pole breaker for dust collector.
• 20A single pole breaker for freezer.
• 20A single pole breaker for convenience receptacles.

Notes:

Don't forget to balance your loads between the two legs in the subpanel.

Yes, you can. The systematic computer programmer in me even thinks it's a good idea, allowing a "star" topology for electrical distribution.

1. You will need 4 wires, as others have mentioned.

2. Keep the ground and neutral separate in the subpanels. The only place they should be bonded is in the main panel.

3. You will need ground rods at the new location. Yes, it's redundant, but it's still required.

4. The wire feeding the new subpanel must be protected from overcurrent by an appropriately-sized breaker. It can be at either end - in the old subpanel or in the new subpanel - or at both ends. It's up to you, but if only using one, I slightly prefer the upstream location, so that I can easily kill power to the feeder wire. If using both, 1 must be sized for the conductor, and the other can be that size or larger -- convenient if you buy a panel with a 100A breaker preinstalled.

5. While there are wires rated for direct burial, it's good that you're planning to use conduit. Oversize the conduit for ease of pulling wires, and minimize bends. While you have the trench open, run a second conduit for future expansion (ethernet? gas? macerator?). May as well run a water line, too, so you can have a sink.

6. Most electric kitchen stoves are installed on 240V/50A circuits. If you're installing something like that, then I suggest a 60A subpanel. That gives you plenty of headroom to install lighting, a small point-of-use electric hot water heater, and some convenience receptacles. Those will come in handy when doing maintenance in the area.

7. Aluminum wire is usually much cheaper than copper, but harder to work with. It's stiffer. You must use 1 size larger than with copper. The ends must be protected from corrosion with anitoxidant goop. The lugs must be torqued correctly. Some people think you should retorque the lugs on some schedule. Last time I bought wire, I wanted 8ga copper, but 2ga aluminum was 1/2 the price, so going with Al was worth it.