You're in conduit, use individual wires - they are cheaper, they pull easier, and the conduit fill on cables is terrible.
Typically we shoot for less than 3% voltage drop at rated current. Less drop is OK.
I'm fairly sure you need 6Ga wire minimum for a 60 Amp feed - given a short 30 foot run, this is also probably perfectly adequate. I'm getting 1.8% drop for 60 amperes at 240V on 30 feet (one way) of 6 Ga. Edit - Copper!
The limiting temperature rating if using THW, THHN, etc wire is generally the temperature rating of the connectors on the service equipment (breaker or panel) which is usually 75 C - even if using 90C wire, you have to follow the 75C section of the table due to the connections.
Copper .vs. Aluminum.
Copper (Cu) costs more, Aluminum (Al) less.
Copper has better conductivity - effect being, smaller wire to carry the same current. In this case, 6 gauge copper .vs. 4 gauge aluminum.
Copper oxides are conductive. Aluminum oxide is an insulator, forms quickly, and sticks very tightly to the wire. Aluminum connections need special procedures and materials to make a solid connection that won't get loose over time and overheat. These include things like brushing the wires with a stainless steel brush and an aluminum wire connection compound that coats the wire to prevent contact with air. The connections themselves must be rated specifically for use with aluminum, but most large ones will be (generally marked Cu-Al meaning they work with both, where unmarked connectors are assumed to be copper wire only.)
Aluminum connections are further complicated by cold flow, but that gets long and complicated to get into, and is supposed to be addressed by using the proper connector types. Essentially the connection gets hot, the wire swells, the swollen wire deforms, the wire shrinks, the connection gets looser, so next time it gets hotter, repeat until fire.
Best Answer
When determining feeder conductor size, you'll want to consider the "lowest temperature rating of any connected termination, conductor, or device" as per National Electrical Code (NEC) Article 110.14(C).
While the cable/wire may be rated at 90°C, you'll likely find that the terminals are rated at 75°C, or not labeled at all. 110.14(C)(1)(a) tells us, that since we're working with 100 amperes or less. We should use the 60°C column of Table 310.15(B)(16) to determine the conductor size, unless the equipment is listed and labeled for a higher temperature.
Since the cable will run from a breaker in the main service panel, to either a breaker or lugs in a subpanel. We have to consider the temperature rating of...
We'll then use the lowest value, or 60°C if any of the above are not labeled. Once we know the size of the overcurrent devices, and the lowest temperature rating, we can use Table 310.15(B)(16) to determine the conductor size and material we'll need.
This will give us the current carrying conductor size required for our feeder.
But wait...
If you're working in a one-, two-, or multi-family dwelling unit, is Article 310.15(B)(7) applicable?
NO.
Notice the codes says
After reading the definition of these terms, it's clear that this does not apply to the wire between the main panel and a subpanel.
tl;dr
Conductors and all terminals rated at or above 75°C.
Use 3 AWG copper or 1 AWG aluminium for the current carrying conductors.
Conductors rated at or above 75°C, terminals rated at 60°C or unlabeled.
Use 1 AWG copper or 1/0 AWG aluminium for the current carrying conductors.
Conductors and terminals rated at 60°C.
Use 1 AWG copper or 1/0 AWG aluminium for the current carrying conductors.
Conductors rated at 60°C, terminals rated higher than 60°C
Use 1 AWG copper or 1/0 AWG aluminium for the current carrying conductors.