What size NM cable do I need for a 50 amp range?

6/3 NM with ground. NEC 334.80 caps NM at the 60 degree C column, where 8 AWG is only 40 amperes and 6 AWG is 55 amperes.

Can a range use a three wire circuit?

Only on existing installations that meet the conditions in the NEC 250.140 exception. New range circuits must include a separate equipment grounding conductor, which means four wires.

Is 8 AWG good for 50 amps?

In conduit with 75 degree C rated conductors such as THHN, yes, 8 AWG copper is 50 amperes. Inside NM cable it is limited to 40 amperes by NEC 334.80.

Can 8/2 NM cable go on a 50 amp breaker for an electric range?

No, on two separate counts. The ampacity is wrong and the conductor count is wrong.

The ampacity problem

Section 334.80 states that the ampacity of Type NM cable shall be determined in accordance with 310.14, and that the allowable ampacity shall not exceed that of a 60 degree C rated conductor. The 90 degree C rating of the conductors inside the jacket may be used for derating calculations only. Look at Table 310.16 in the 60 degree C copper column and 8 AWG is 40 amperes. That is the number that applies to 8/2 or 8/3 NM cable regardless of what the THHN inside is rated for.

Section 240.4 requires conductors to be protected at their ampacity, and Section 240.4(B) permits the next higher standard device only where the conductor ampacity does not correspond to a standard rating, the circuit does not supply receptacles, and the next higher rating does not exceed 800 amperes. 40 amperes is a standard rating under 240.6(A), so the round up allowance does not apply. 8 AWG NM tops out at a 40 ampere breaker.

The four wire problem

Section 250.140 permits frames of ranges and clothes dryers to be connected to the grounded circuit conductor only for existing branch circuit installations that meet all the conditions listed in the exception. For new installations, the branch circuit must include an equipment grounding conductor in addition to the ungrounded conductors and the grounded conductor. That is a four wire circuit. Type 8/2 NM contains two insulated conductors plus a bare equipment grounding conductor, which gives you two hots and a ground with no neutral, or a hot, a neutral and a ground. Neither arrangement supplies a modern range, which needs 120 volts for the clock, controls and lights along with 240 volts for the elements.

What to actually run

Section 210.19(A)(3) requires branch circuit conductors supplying ranges to have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served, and it sets a minimum branch circuit rating of 40 amperes for a range rated 8 3/4 kW or more. Most residential ranges land on a 40 or 50 ampere circuit depending on the nameplate.

For a 40 ampere circuit, run 8/3 NM with ground. For a 50 ampere circuit, run 6/3 NM with ground, since 6 AWG in the 60 degree C copper column is 55 amperes. If you are in conduit with individual THHN conductors at 75 degrees C, 8 AWG copper is 50 amperes and works on a 50 ampere breaker, because 334.80 does not apply to conductors in raceway. That difference between cable and raceway is exactly where this question usually starts.

Field takeaway

8/2 NM never works for a range. For 50 amperes with NM cable, run 6/3 with ground. For 50 amperes in conduit, 8 AWG THHN copper at the 75 degree C column is fine, with a separate neutral and equipment grounding conductor.