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.
Base Conductor Size
Start out by using Table 310.15(B)(16), and applying any required corrections, to determine what size conductors you'll need. For your situation, we'll assume we can use the 75°C column, that you want to use copper conductors, and there's no other corrections required. So in your case, if you want to install a 50 ampere panel, you'll need at least 8 AWG copper conductors. If you want a 60 ampere panel, you'll need 6 AWG copper conductors.
Voltage Drop
Once you have the base conductor size selected, you'll want to calculate the voltage drop across that size conductors for the length of the feeders. The first step here will be to use Table 8 from chapter 9 of the NEC, to determine the resistance of the conductors you've selected.
In your case, 8 AWG stranded copper wire has a resistance of 0.778 ohms per 1000 ft. 6 AWG stranded copper wire has a resistance of 0.491 ohms per 1000 ft.
Next you'll use the following formula, to calculate the voltage drop across the feeders.
V = L * 2 * R * A
Where:
- V = Voltage Drop
- L = Distance along the wire from one breaker to the next.
- R = Resistance per foot of wire.
- A = Current running through the conductor.
For a 50 ampere circuit, 130 ft. long, using 8 AWG stranded copper conductors, the calculation looks like this...
V = 130' * 2 * 0.000778 * 50 A
V = 260 * 0.000778 * 50 A
V = 0.20228 * 50 A
V = 10.114 V
10.114 V is 4.2% of 240 V. The NEC recommends having a voltage drop less than 3%. To achieve this, you're going to have to use larger conductors.
6 AWG stranded copper conductors have a resistance of 0.000491 ohms per foot, which means the voltage drop would only be 6.383 volts or 2.7%.
For a 60 ampere circuit 130' long, 6 AWG stranded copper conductors would have a voltage drop of 7.6596 volts or 3.2%. While 4 AWG stranded copper would be 4.8048 volts, or 2%.
Conductor Type
Once you know what size conductors you need, you'll have to determine what type of insulation the conductors should have. Since you're burying the conduit, you'll need a wire rated for wet locations. The popular choice in this situation, would be to use THWN wires.
Wire Size
Now that you know what size conductors, and what type of wires you'll use. Then next step is to determine the physical size of the wires, and how much space they'll take up in conduit. For this, you can use Table 5 from chapter 9 of the NEC. There you'll find that 6 AWG THWN wires have an area of 0.0507 square inches, while 4 AWG THWN wires have and area of 0.0824 square inches.
Conduit Fill
Using the size of one wire, you can figure out the area required for all four wires.
0.0507 * 4 = 0.2028 in.sq.
0.0824 * 4 = 0.3296 in.sq.
Use Table 1 from chapter 9 of the NEC, to determine the allowable conduit fill percent. Since you'll have more than 2 conductors, you can fill the conduit to 40%.
Conduit Type
If you know what type of conduit you're using, you can use Table 4 from chapter 9 of the NEC to look up the area fill values for various sizes of conduit.
Conduit Size
Since you've decided to use Schedule 80 PVC, you'll simply find that table in Table 4. Then look down the 40% fill column, until you find an area large enough for all your wires.
In your case four 6 AWG THWN conductors, will require 1" Schedule 80 PVC. While four 4 AWG THWN conductors, will require 1 1/4" Schedule 80 PVC.
Conduit Size Alt.
If you don't feel like calculating wire/conduit area, and all the wires are the same size, you could use Table C.9 from Annex C of the NEC to look up the conduit size required. There you'll find that you can fit five 6 AWG THWN wires throug 1" Schedule 80 PVC, and that you can fit six 4 AWG THWN wires though 1 1/4" Schedule 80 PVC.
tl;dr
- For 130' long 50 ampere feeder, use four 6 AWG stranded copper THWN conductors though 1" Schedule 80 PVC.
- For 130' long 60 ampere feeder, use four 4 AWG stranded copper THWN conductors through 1 1/4" Schedule 80 PVC.
NOTES:
- This answer contains some of the tables used in this answer.
- If you don't feel like doing any maths, you can surely find a calculator online to do all the work for you.
Best Answer
If you're spending the money to add a panel anyway, you might as well reduce the changes of having to expand it later. Unless you're installing the second panel really far from the main, you're likely not going to spend much more to put in the 100 amp panel over a 60 amp panel. Oversizing the panel will not hurt anything but your wallet, so why not do it?
The installation will be exactly the same either way, the only differences will be the size of the breaker in the main panel, the main breaker in the new panel, and the size of the conductors between the panels.
You can read this answer for more detail, but likely you'll need four 3 AWG copper conductors to feed a 100 ampere panel. Of course the size will change based on the length of the run, and other factors. So make sure you verify the size once you know exactly how long the wires will be, and how they'll be run.
Did a quick search for wire prices, and it looks like it's about $0.20 per foot difference between a 6 AWG stranded copper wire and a 4 AWG stranded copper wire. Then another $0.20 per foot difference between 4 AWG stranded copper wire and 3 AWG stranded copper wire. So you'd be looking at an $0.80 per foot difference between 6 and 4 AWG feeders, and an $0.80 per foot difference between 4 and 3 AWG feeders.