Split 110v off a 230v branch circuit?

[jeffeverde]

More likely an electrician who had the wire on his truck, or assumed that ALL 220/240 circuits needed a neutral, the same as electric ranges and dryers. Few would ever think ahead for that purpose, since a subpanel at an A/C unit would be a decidedly rare installation. Not all 6/3 wires have grounds, so until you specified that yours did it was not obvious. WHERE is that third wire connected now, since it should have no purpose in life, or need to exist.

Sorry, I forgot to add after "planning ahead"

The NM-B terminates in a junction box, then jumpers for the hot pair run through a mud ring and bushing into the back of the disconnect box mounted outside. The neutral was capped off with a wire nut in the junction box.

[hj]

In that case your original plan for a sub panel is viable.

[jar546]

I've got a 230v/50A circuit supplying the A/C compressor and the compressor only requries 35A, so overloading the 50A circuit isn't an issue.

Ahh,...yes, yes it is an issue.

What is the motor FLA, FLC, HP & Service Factor

[codeone]

I've got a 230v/50A circuit supplying the A/C compressor and the compressor only requries 35A, so overloading the 50A circuit isn't an issue. The A/C circuit terminates in a fused disconnect box, and then a whip into the unit.

What is the minimum circuit ampacity for you a/c?
What is the max breaker size for your a/c?

These facts are needed to do the calculation you need.

[codeone]

I've got a 230v/50A circuit supplying the A/C compressor and the compressor only requries 35A, so overloading the 50A circuit isn't an issue. The A/C circuit terminates in a fused disconnect box, and then a whip into the unit.

The issue is if your minimum circuit ampacity is 35A you haved to multiply 35 x 125% to come up with your calculated load for the circuit. This is for start up current.

35 x 125% = 43.75 or 44A
50A - 44A = 6A
Dont have as much extra as you thought!

Now if the 35A is you max breaker size you might be ok.
Give us some more info please.