I just bought an old house and I am running new 10-3 grounded cable to update for a 4 prong dryer receptacle. The old 10-2 wire was fed out of the main panel to the crawl space through the conduit for the 150 amp service wire, looks to me that it and an AC line were added after the original house wiring. Does this meet code and/or is it smart? There is an empty knockout on the bottom of my Square D box and I removed several circuits that were added along the way so the service is not overloaded. Any advise is appreciated.

 

Is this a piece of conduit that's open at both ends and is just being used as a support sleeve? Or are you talking about a piece of conduit that enters your panel and has both the incoming service feed and the dryer cable in it?

Either way, just bring the new cable for the dryer in through a separate entrance with its own clamp.

If I understand what you have correctly, possibly not. If you're talking about conduit that your service entrance wiring is running through, what does the other end of that look like? How long is the conduit and how long are the wires? Are they always in conduit or boxes between your meter and your panel?

Even if the branch circuit protection in your panel adds up to 300A or more (and I wouldn't be surprised if it does), there's little chance you can overload your service. Besides, you're not adding a load. You're upgrading the wiring for an existing load.

 

The conduit I was talking about is an open ended sleeve from the bottom of the Square D panel box to just below the subfloor, a total of 48", and runs through a 2 x 4 interior wall.

 

10-2 W/G NM cable (aka romex) to a dryer has NEVER been code compliant and was a hazard, but I have seen it done before by some electricians (using the term electrician loosely). It used to be that dryer and range wiring was permitted to be grounded through the insulated neutral conductor, but it has never been code compliant to use the bare grounding conductor from an NM cable to carry the neutral.

 

If that piece of conduit is connected to your panel on one end, then it is not a sleeve. It is effectively an extension of your panel enclosure, and needs to be closed on the other end. After you get the new dryer cable installed, you should look at how you can correct that situation.