15 ft. 10/2 Solid Romex SIMpull CU NM-B W/G Wire

I would say that it should be ok for the water heater, as long as it wasn't one of the 80 gallon models, and you don't have 20 people living in the house. On the exterior building, as long as you are not running a long ways to get to it, and you are not going to be running a bunch of power equipment at one time, you should be just fine. As an aside, I would put in LED lighting, because it uses less electricity, and would benefit you in the long run. DD

No, this wire cannot be used for an electric dryer. The typical electric dryer requires a 30 amp, 120/240 volt circuit and since the 1996 NEC consisting of 4 wires; 2 hots, 1 neutral and 1 grounding conductor. You need to use 10-3 NM-B cable.

No, NM-B cable is for indoor dry locations only. ALL outdoor locations, even in conduit, are classed as wet locations by the NEC. I would recommend you use individual #10 THWN conductors in the liquid tight flexible conduit between the disconnect and the condensing unit; 2 blacks and 1 green.

A package unit would either be installed on a rooftop or on a pad outside. This NM-B cable can only be installed indoors in dry locations. If this is a pad installation this cable can be used to feed to the disconnect mounted on the exterior wall of the building, but the feed between the disconnect and unit must be THHN/THWN-2 individual conductors installed in liquidtight flexible conduit. If this is a rooftop installation, this cable can be installed to a junction box under the roof, but from the junction box through the roof to the disconnect and into the unit must THHN/THWN-2 conductors in conduit.

That is really overkill. 15 amp plugs should be ran via 12-2 on a 20 amp breaker.

This is capable of carrying 220v (wired correctly at the breaker), though generally, you would require 10/3 for a spa connection. For a 4-prong installation you must use 10/3 wire. Don't forget to install a GFCI disconnect near the spa (required by code, and safety reasons) If you plan on possibly upgrading the spa in the future, you should put in the correct service (50 amp, 4 wire service - or 6/3 w ground) for most larger spas.

It depends on how much drop whatever you're powering can tolerate, and what voltage you're actually running through it. If it's 120V You'd be on the edge of needing to drop down to 8 gauge. If it's a 240V you'd be fine. I say this just as a general DIYer though not a licensed electrician.

For a 220V/240V line, you will normally need 3 conductors. 12/2 or 12/3 NM-B Cable is rated for 20 Amps and will work for this application as long as the circuit isn't very long where voltage drop is a concern. For circuit runs longer than about 50', it would be better to use 10/2 or 10/3. You can check this on our voltage drop calculator available at https://www.southwire.com/calculators

12-2 weighs 82 lbs/M. 12-3 weighs 112 lbs/M.

Yes, however the circuit breaker must be changed to a 20amp (max.) Your 10/2 circuit breaker (most likely 30amp if not smaller) cannot be larger than the maximum capacity of the smallest gauge wire in your circuit can safely handle...... So if you connect a 12 guage wire to that 10 gauge wire, the maximum ampacity that particular circuit could handle would actually drop to 20 amps versus the 30amp that your existing 10/2 wire (if left alone is capable of handling) is probably connected to now. In short The breaker MUST be changed.to a smaller ampacity in order to do so.