I have a box on my house for a A/C which I believe it is 50 amp. The A/C box has 2 black and a ground lug. I want to hook a hot tub to that box but it has 4 connection, 2 blue, white, ground lug. How do I hook this up. Does the white wire on the hot tub go to the ground on the A/C box or do I have to run a line from the circuit breaker box. Thanks for your help.

It is best to run a new line from box as you are likely to overload circuit.. however you should also wire hot tub on a GFCI circuit breaker to comply with code and also for your safety. Outdoor hot tub box is sold with this breaker already installed,

Thanks... so can I run a neutral wire to the circuit breaker box or do I have to run all new wires and new box? Thank you for your reply.

Thanks...so can I run a neutral wire to the circuit breaker box or do I have to run all new wires and new box? Thank you for your reply.If it's in conduit you can simply pull in a new white wire. If this s cable then you cannot do this and a new proper cable must be run.

This assumes you are not using the A/C any more. If you are still using it then I'm not sure why you even considered using this circuit. A new must be run.

Mark,
You need to run a new circuit that will supply "only" this hot tub...
Next, what do the installation instructions call for? What voltage?
What amperes?