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 (http://www.okonite.com/engineering/nec-ampacity-tables.html)

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. (http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=70)

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.