I am installing a small 120v sub panel in my basement. The builder pre-wired from the main service panel a #10 four wire (red, white, black & ground) to my basement. I purchased a small Square D panel and two 30 amp breakers. The box I purchased has no ground bar, only a neutral bar. I know that the white wire goes to the neutral bar. Where do the other wires go?:confused:

The service panel I purchased is a Square D Homeline 70 A with two spaces HOM24L70SPC. I bought a ground bar kit to attach.

Black goes to a main lug. Red goes to the other main lug. Neutral needs to be isolated from ground. You need a ground bar kit.

What's the make and model of the "sub-panel"?

There is a chance you have a main panel. Does the panel have a main breaker? It shouldn't.

#10 is only rated for 30 A if it's a single cable. It should be fed from the main panel from a 30 A breaker. 40 AMP if single conductors in conduit. See reference later. Typo on my part.

Edit: Changed the current ratings. Thanks labman.

If the 4 wires are bundled together in an orange jacket, it is NM-B and only rated for 30 amps. We have a great electrician here posting under tkrusell. I think you would be better off ignoring the first answer and waiting for one you can trust.

labman:

My reference: Okonite Ampacity Tables: NFPA 70, National Electrical Code, 600 Volt Ampacity Tables

Where is yours?

If the builder ran conduit and pulled THNN, then 40 amps may be correct. Unless the OP lives in Chicago, likely it is the 30 amp NM-B. I have seen a lot of terrible answers here where somebody knowing little of the subject did a search and found a web page that didn't apply.

Kiss, note the disclaimer Okonite has for this wire ampacity chart:

*Ampacities are maximum allowed by the National Electrical Code.

The provided chart is a bit misleading, as it does not reflect the entire wire ampacity Table 310.16 as shown in the NEC.

Table 310.16 has a disclaimer directing to Section 240.4(D), that reduces the maximum of #14 to 15 amps, #12 to 20 amps, and #10 to 30 amps, for all but for specific installations and insulations.

Then one must consider Section 334.80, which only allows any NM-B cable, AKA Romex, to the 60 Deg C column of NEC table 310.16.

Your statement regarding #10 wire: "40 AMP if single conductors in conduit." is incorrect for this particular installation.

40 Amp would only be allowed on a #10 wire if the installation falls under those listed 240.4(E) or (G), AND if the equipment has terminals rated for 90 Deg C.

For normal, routine, general purpose wiring, #10 can only be rated 30 amps, no matter if Romex cable or pulled THHN wires is used.