First off, I'm a homeowner with limited experience. I have done some home wiring, but I have not messed with 240 that much.
Here is what I'm trying to accomplish... I have a 3hp pump that I'm using for temporary irrigation from my pond. I will only need this until my seed comes up and through summer. So, about 3 months. I have a direct bury 12-2 wire connected to a double pole 30 amp breaker and connected to the pump single phase 230. Right now if I want to turn on my sprinkler system, I turn on the breaker and everything comes on. However, I would like to add a controller to set it to come on at different times automatically, and also be able to control it using my phone. In order to do this, I need a relay and a controller. The problem is... The controller that I want, plugs into a 120 outlet and steps down to 24v via a transformer. I don't have a 120 outlet down there and I only have the one wire. I'm wondering if I can add an outdoor receptacle by coming off of one leg of the 240 and ground. There would be no neutral. The controller would activate the relay via a 24v coil. all connections will be in waterproof enclosures.
Can I do this?
Is it dangerous?
How much risk considering the controller only operates on 24v?

Here is a pic of what I want to do.....

 

Not allowed in N.A. and also potentially dangerous.
A small 240v/120v control transformer could do it.
Max.

 

"I don't have a 120 outlet down there and I only have the one wire"

? What does that mean?

 

Any chance of running another wire ( red ), from breaker box to relay box ? If so red & black -hot, white neutral, & green ground, subject to comments from Max.

 

Is the breaker box out at the end of the 12-2 run?

Do you have a neutral in the breaker box?

Can you run a neutral out to motor? This way in the future you'll have 220 and 120 there.

Might be worth the trouble.

 

A universal 24VDC plug pack may be used, the output wired into the controller 24V supply. As long as the controller does not use the AC to run the solenoids.
You may get away with an old laptop 19V supply. Check that the supply used is a 100 -240V AC input one.

It is even possible your controller has a 240V option. Have a look inside.

 

From what I gather the OP needs 120v but only has one conductor available.
Which does not leave too many options.
Max.

 

Thanks for the replies. I will try to clarify some points. The breaker box is in my garage at my house. The pump is 150 away down by the pond. The 10-2 wire was previously buried when the house was built in anticipation of adding the pump, thus, I cannot add another wire to mix. My plan was to put the relay and controller down by the pump, because that is where I have the best access to the wiring. The wiring at the house, comes up from the ground in conduit, through the foundation, into the crawlspace and then into the breaker box. I suppose, I could try to wire the controller and relay in up at the house, by running the controller wires to a junction box in the crawlspace?

 

From my understanding of wiring 220-240v is only (two) 120v hot wires and a ground in a house system. So one leg and a ground = 120v.
When you hook up a 220-240v air conditioner in the breaker box it's a breaker with two hot legs with the ground wire to ground it. Some wiring harnesses have a green wire (making 4 wires total) but guess what it also goes to ground. Black=hot, White and Green=ground
Why not just splice it meaning use one leg of the 220-240v
So I say yes to your question.
Brzrkr

 

Do the wiring in the garage. You can plug the controller into an available outlet in the garage. Make sure your relay can handle the 3 HP pump.

 

Do the wiring in the garage. You can plug the controller into an available outlet in the garage. Make sure your relay can handle the 3 HP pump.

I had pretty much came to the same conclusion. Here is the relay I am planning on using.
Munro StartBox 24 Volt Pump Start Relay | MPSR24
https://www.sprinklerwarehouse.com/...lays/murno-irrigation-pump-start-relay-mpsr24

This should work... correct?

 

Not allowed in N.A. and also potentially dangerous.
A small 240v/120v control transformer could do it.
Max.

Exactly!

Never use the ground wire for load current!

From my understanding of wiring 220-240v is only (two) 120v hot wires and a ground in a house system. So one leg and a ground = 120v.
When you hook up a 220-240v air conditioner in the breaker box it's a breaker with two hot legs with the ground wire to ground it. Some wiring harnesses have a green wire (making 4 wires total) but guess what it also goes to ground. Black=hot, White and Green=ground
Why not just splice it meaning use one leg of the 220-240v
So I say yes to your question.
Brzrkr

Please don't answer questions that involve safety hazards and code compliance if you don't know the subject matter well enough. Guesses are fine when debugging a harmless low voltage project, but not when dealing with mains power.

The reasons why neutral and ground must be kept separate are too involved for short simple forum posts, but they're well documented. Ground wires and neutral wires should *not* be used interchangeably even though they appear to be at the same voltage. If you don't want to research and learn, you'll just have to take my word for it.

 

When you hook up a 220-240v air conditioner in the breaker box it's a breaker with two hot legs with the ground wire to ground it. Some wiring harnesses have a green wire (making 4 wires total) but guess what it also goes to ground. Black=hot, White and Green=ground
Why not just splice it meaning use one leg of the 220-240v
So I say yes to your question.

He can't do that in the US. He has two hots and ground. US electrical code only allows neutral to be bonded at the power panel. He can't use the ground wire that was routed with the 220VAC.

Using a transformer to step 220VAC down to 110VAC is the only thing he can do without pulling more wires.

 

From my understanding of wiring 220-240v is only (two) 120v hot wires and a ground in a house system. So one leg and a ground = 120v.
When you hook up a 220-240v air conditioner in the breaker box it's a breaker with two hot legs with the ground wire to ground it. Some wiring harnesses have a green wire (making 4 wires total) but guess what it also goes to ground. Black=hot, White and Green=ground
Why not just splice it meaning use one leg of the 220-240v
So I say yes to your question.
Brzrkr

Ground wire and Green is the same thing, or should be.
And it is verboten, in N.A. at least to use the earth ground as a conductor.
Max.

 

I'm sorry but in the USA there is no single wire 220-240v in a house unless it is added or converted. You say I'm wrong but I disagree. In the USA you get 220-240v by using two hots wires and a ground. My house has only three wires coming down the main. each side is only 110-120v whatever you want to call it. You can't get a single wire to put out 220-240v The last wire you all call the earth ground still goes to the ground in the panel and it also is hooked up just to ground the panel.

go stick a meter in a 220-240 plug and check each side it's only 110-120v
Brzrkr

 

My understanding of the wiring there is the 240V is across 2 x 120V hot wires.
The ground is not involved in the power and should have no current flowing through it. Of course, hooking from one hot to ground will give you 120V as the ground and neutral are connected at the power switchboard. But it is a big no-no to do that!

 

But the differentiation is the term ground and neutral, you cannot use each indiscriminately, although they are at the same potential, if you are missing a neutral, it is not allowed by NFPA70, (NEC) & CEC to use earth ground in place of a neutral to obtain a return conductor.
Max.

 

But it is a big no-no to do that!

Worse than a big no-no. Illegal in North America.

 

I shudder to see some things uninformed folk do to power wiring at times.
Some years ago, a stupid guy had the 240V active (here in Oz) strung around as an electric fence over his fish pond to keep cats out.
I'll leave the result to your imagination!

 

I'm sorry but in the USA there is no single wire 220-240v in a house unless it is added or converted. You say I'm wrong but I disagree. In the USA you get 220-240v by using two hots wires and a ground. My house has only three wires coming down the main. each side is only 110-120v whatever you want to call it. You can't get a single wire to put out 220-240v The last wire you all call the earth ground still goes to the ground in the panel and it also is hooked up just to ground the panel.

go stick a meter in a 220-240 plug and check each side it's only 110-120v
Brzrkr

The "one wire" referred to earlier was one romex 12-2 cable, which actually has a total of three conductors if you count the ground. As such, it is suitable for providing two opposite polarity 120V lines (hot legs) and one ground, which for safety reasons is not allowed to carry any current. So, at the end of the line, you can get 240V between the two 120 lines, but you can't use either 120V by itself. You'd need a separate neutral wire to do that. If you use the ground wire as a neutral, you've defeated the safety grounding aspects of it. Modern electrical codes in the USA forbid such installations, and have for quite a while (decades? I'm not sure.)

As for the wires at the service entrance to your house, yes there are three: two lines and a ground, no neutral. The neutral is derived from the ground at the breaker panel. The neutral and ground are not allowed to be bound together anywhere downstream of the breaker panel, nor are you allowed to run 120/240 circuits without a ground (so 120/240 circuits require 4 wires now, not 3.)

If you don't agree with these rules take it up with the NEC. If you want to rewire your house wily nilly, I can't stop you (but please share your address so I know you never by your house. I've gotten very tired of fixing other people's wiring errors!)

Whatever you do, please stop giving bad advice here regarding high voltage circuits. You're obviously not a licensed electrician, nor are you up to date on electrical codes, so just sit out of questions like these and make the world a safer place.