120V from a 240V circuit

[brad.walker]

I have a 240V circuit in my garage. But, I would like to pull a 120V off this feed line. My thinking is to use an industrial transformer that is rated for this. My needs are small, meaning the circuit would be a standard 120V 15A outlet. Mostly to run a power tools and some lights.

Something like this from Hubbell/ACME - Low Voltage Distribution Transformer .

This should work, right?

Thanks.

-brad w.

 

[wyliesdiesels]

Do you need the 240v circuit for anything else? If not, why not just convert it to a 120v circuit?

 

[nadogail]

Some of us manage to complicate everything.

 

[brad.walker]

Do you need the 240v circuit for anything else? If not, why not just convert it to a 120v circuit?

Good question. I did try to think about that and maybe you can help me.. The motor for my door runs off the 240V circuit.

I would assume that I could pull off 1 of the hot lines from the 240V to the 120V circuit and tie the neutral of the 120V circuit to ground. But, that doesn't seem like the safe thing to do, at least to me. The way I understand things that means the ground could go hot and that's not correct. Even though neutral is tied to ground back at the circuit breaker.

The other thing that bothers me is the 240V circuit is wired with 10/2 cable. That means 2 hots, a ground and no neutral. So what then?

 

[sparky 1971]

Deleted. I made a rookie mistake and didn't read the whole thing.

 

[sparky 1971]

Good question. I did try to think about that and maybe you can help me.. The motor for my door runs off the 240V circuit.

I would assume that I could pull off 1 of the hot lines from the 240V to the 120V circuit and tie the neutral of the 120V circuit to ground. But, that doesn't seem like the safe thing to do, at least to me. The way I understand things that means the ground could go hot and that's not correct. Even though neutral is tied to ground back at the circuit breaker.

The other thing that bothers me is the 240V circuit is wired with 10/2 cable. That means 2 hots, a ground and no neutral. So what then?

There is no neutral so you can't make 120 out of it and keep the 240.

You might be able to convert the door opener to 120. All of the 240 volt door openers I've wired could have been 120 by moving a jumper around.

 

[tfi racing]

How huge is this garage that it is unreasonable just to run a new circuit, or add on to a nearby receptacle?

 

[beemerphile]

How huge is this garage that it is unreasonable just to run a new circuit, or add on to a nearby receptacle?

It's funny that how hard (or expensive) it is to do something correctly becomes the justification for doing it wrong - kind of like the five second rule for dropping food on the floor.

 

[Innovate1]

The transformer would allow you to generate 120V from the 240 and avoid using a ground for a neutral. Still seems like a lot of effort/expense when there are other options. Converting the opener to 120V seems like a good plan if possible.

If you really are set on the transformer plan it might be better to convert the circuit to 120V and use a transformer to boost that to 240 for the door motor. You could use a 120:120V transformer and connect one side of the output to the new neutral. If phased correctly you will have 240V between the other output and the input hot wire. And the transformer only handles half the total power.

 

[pbon]

It's funny that how hard (or expensive) it is to do something correctly becomes the justification for doing it wrong - kind of like the five second rule for dropping food on the floor.

As I get older, I add seconds to the 5 second rule. It is still a race between me and the dogs, but they are older and slower as well.

 

[brad.walker]

The transformer would allow you to generate 120V from the 240 and avoid using a ground for a neutral. Still seems like a lot of effort/expense when there are other options. Converting the opener to 120V seems like a good plan if possible.

If you really are set on the transformer plan it might be better to convert the circuit to 120V and use a transformer to boost that to 240 for the door motor. You could use a 120:120V transformer and connect one side of the output to the new neutral. If phased correctly you will have 240V between the other output and the input hot wire. And the transformer only handles half the total power.

Several people have asked why not just convert the circuit to a 120V and use it that way. I did try to think about this. The garage is 40' by 30'. The garage height is about 14'. The door is a steel folding door that weighs about 2,500 lbs. Hence, the reason the motor runs on 240V. I don't think I have the option, at the motor, of switching it to 120V as it might not be enough power to open the door nor is this an option on the motor.

I'm not the original owner. But, I can't understand why the electrician didn't pull a cable like 10/3 originally, then just leave the neutral "unconnected". Maybe it was cost? I just don't know.

I do know that using ground for a neutral is not a smart thing to do, even if it will work. That's why I'm looking at the transformer method.

Maybe I'm just overthinking this and should just accept this.

 

[mike93lx]

How many hp is the opener? You should have a hell of a spring setup for that door

 

[sparky 1971]

This is getting weird. 40X30X14 with a 2500 lb door and there's no 120 to plug in a tool or run lights? Is there a panel?

 

[Innovate1]

This is getting weird. 40X30X14 with a 2500 lb door and there's no 120 to plug in a tool or run lights? Is there a panel?

Maybe a bifold door? With those there are no springs and more of a winch to open. I know they are used on some plane hangers. Still does seem like strange combination of factors...

 

[brad.walker]

This is getting weird. 40X30X14 with a 2500 lb door and there's no 120 to plug in a tool or run lights? Is there a panel?

It's basically a steel building, i.e. an airplane hangar. The door is a steel bi-fold type. Yes, there is a sub-panel with my 240V circuit off of it. I could probably run an additional 120 circuit from it.

I'm just trying to keep the hassle factor low and balance the cost of transformer vs. pull new circuit.

Just an FYI, but I do all my work to code and I always get it inspected, if required. For me, I like the peace of mind it provides. And if there is ever an insurance claim, the ins. company can't use my work as a reason to deny the claim.

 

[mike93lx]

So you just have a 10/2 feeding this sub? What kind of wire is that (nm-b, uf-b, thwn in conduit, etc)?

How far is the run? Might be time for an upgrade

 

[brad.walker]

So you just have a 10/2 feeding this sub? What kind of wire is that (nm-b, uf-b, thwn in conduit, etc)?

How far is the run? Might be time for an upgrade

The sub-panel has service entrance cable coming into it. I have no idea what it's sized at.

The circuit to my door motor, from the sub-panel, is 240V and it's sized at 10/2. The circuit length is about 75 feet.

 

[BillK]

Brad,
I am thinking the cost of a breaker and the material to run a circuit almost anywhere in the building is going to be less than what you pay for a transformer etc. And a lot less hassle. Plus you could put a few additional outlets along the run to make it a lot more useful.

I think you are way overthinking it.

 

[brad.walker]

Brad,
I am thinking the cost of a breaker and the material to run a circuit almost anywhere in the building is going to be less than what you pay for a transformer etc. And a lot less hassle. Plus you could put a few additional outlets along the run to make it a lot more useful.

I think you are way overthinking it.

Yea, you're probably correct (I'm an engineer by training). Thanks to everyone for helping me work through this.

-brad w.

 

[sparky 1971]

Maybe a bifold door? With those there are no springs and more of a winch to open. I know they are used on some plane hangers. Still does seem like strange combination of factors...

It doesn't matter what type of door it has. The point of my post was basically asking if there was no 120 available. It turns out there is a sub, but I'm having a hard time believing there are no 120 receptacles in there. There's gotta be one somewhere that an extension cord could be run to.

 

[sparky 1971]

I'm not the original owner. But, I can't understand why the electrician didn't pull a cable like 10/3 originally, then just leave the neutral "unconnected". Maybe it was cost? I just don't know

There is no reason to take a neutral to the door opener. It would be no different than taking an extra hot wire for every 120 circuit.

I do know that using ground for a neutral is not a smart thing to do, even if it will work. That's why I'm looking at the transformer method.

You're right about that. Are there no 120 volt outlets in the hangar? I'd start by putting one under the sub panel and buying a drop cord.

Maybe I'm just overthinking this and should just accept this.

 

[TRWham]

... But, I can't understand why the electrician didn't pull a cable like 10/3 originally, then just leave the neutral "unconnected". Maybe it was cost? I just don't know...

I cannot understand why you cannot understand. The spec for that part of the job would have been to install a branch circuit for the garage door. If that door only needs 240 V power, why do you think he should provision for an unneeded neutral? Cost is a perfectly valid reason to provide only what is needed. Undefined "future proofing" is a Garage Journal fantasy that does not always apply in the real world especially if no one is paying for it.

 

[Innovate1]

Just run a new circuit and be done with it. Probably less cost. Maybe a little more work (doubtful!). And you have a more standard setup. You are likely to want to run more tools as time goes on so it will have future benefits beyond immediate use. Add a few extra outlets while you are at it.

 

[acer66]

Undefined "future proofing" is a Garage Journal fantasy

Blasphemy!!!

 

[dave*99]

The transformer would allow you to generate 120V from the 240 and avoid using a ground for a neutral. Still seems like a lot of effort/expense when there are other options. Converting the opener to 120V seems like a good plan if possible.

If you really are set on the transformer plan it might be better to convert the circuit to 120V and use a transformer to boost that to 240 for the door motor. You could use a 120:120V transformer and connect one side of the output to the new neutral. If phased correctly you will have 240V between the other output and the input hot wire. And the transformer only handles half the total power.

The existing 240 circuit is run with 10ga. So that is a 30A circuit. And he has a large motor running a heavy door, which is what necessitated 240V@30A. Boosting 120V to serve a 240V@30A load would require a 60A feed for the 120V side. And that won't come from an existing 10ga wire.

 

[alfredeneuman]

Any transformer over 9A would require both primary and secondary fused disconnects and ground rods....Another added expense.

 

[mikedodge]

Just run a new circuit from the panel and be done with it. If cost is a factor put the plug right by the panel and use an extension cord.

 

[Innovate1]

The existing 240 circuit is run with 10ga. So that is a 30A circuit. And he has a large motor running a heavy door, which is what necessitated 240V@30A. Boosting 120V to serve a 240V@30A load would require a 60A feed for the 120V side. And that won't come from an existing 10ga wire.

Absolutely right. I was thinking of a common opener for a typical size/weight door so didn't think through the power and current requirements. The door details came later...

 

[wyliesdiesels]

Good question. I did try to think about that and maybe you can help me.. The motor for my door runs off the 240V circuit.

I would assume that I could pull off 1 of the hot lines from the 240V to the 120V circuit and tie the neutral of the 120V circuit to ground. But, that doesn't seem like the safe thing to do, at least to me. The way I understand things that means the ground could go hot and that's not correct. Even though neutral is tied to ground back at the circuit breaker.

The other thing that bothers me is the 240V circuit is wired with 10/2 cable. That means 2 hots, a ground and no neutral. So what then?

no no no. dont do that. not only is it against code, but also very dangerous.... good way to kill someone

 

[no704]

I want to see this subpanel.

 

[Metal-Marc]

DId anyone mention to run a new circuit yet?

 

[theoldwizard1]

no no no. dont do that. not only is it against code, but also very dangerous.... good way to kill someone

So why did all electric stoves and electric dryer from the 50s though the 80s just use ground as neutral, or did they not have a ground at all ? They all had 120V lights and timers.

Never heard anyone dying from this !

 

[sparky 1971]

So why did all electric stoves and electric dryer from the 50s though the 80s just use ground as neutral, or did they not have a ground at all ? They all had 120V lights and timers.

Never heard anyone dying from this !

They didn't use a ground. They had a neutral and the frame was bonded to the neutral in the appliance. It was that way up until either the 1993 or 1996 code came out. Dryers didn't use 10-2, they used 10-3 without a ground. Ranges used 8-3, 6-3, or SE cable.

I'm in the middle of an argument over this same thing with some idiot on FB right now. I think he gave up trying to convince me that 240 appliances use the ground to make 120 because he vanished when I showed him that a 10-30R is 125/250 volt and if w!hat he claims is cc true, we could have just used a 6-30 instead.

 

[theoldwizard1]

They didn't use a ground. They had a neutral and the frame was bonded to the neutral in the appliance. It was that way up until either the 1993 or 1996 code came out. Dryers didn't use 10-2, they used 10-3 without a ground. Ranges used 8-3, 6-3, or SE cable.

I do understand that those devices did NOT have grounded (third prong) convenience outlets. Of course now you have multiple bonds.

 

[Metal-Marc]

So why did all electric stoves and electric dryer from the 50s though the 80s just use ground as neutral, or did they not have a ground at all ? They all had 120V lights and timers.

Never heard anyone dying from this !

They can't tell their story because of their dead status.

 

[theoldwizard1]

They can't tell their story because of their dead status.

Mother-in-law is very much alive at 88. ALWAYS cooked on an electric stove and used an electric dryer.

 

[sparky 1971]

So why did all electric stoves and electric dryer from the 50s though the 80s just use ground as neutral, or did they not have a ground at all ? They all had 120V lights and timers.

Never heard anyone dying from this !

I do understand that those devices did NOT have grounded (third prong) convenience outlets. Of course now you have multiple bonds.

You already stated they used the ground for a neutral and this is wrong. They did have a grounded terminal, that's the neutral. They did not, however, have a grounding terminal. They used the neutral to make the 120 volts, and in the event of a ground fault, the frame was bonded to the neutral to carry the fault current and trip the breaker. That's why 10-30 and 10-50 receptacles were used instead of 6-30 and 6-50. On the latter, the third pin is bonded to the yoke and could potentially introduce neutral current to the face plate or a metal conduit system. The former receptacles, the third pin was not bonded to the yoke, making it a neutral terminal that couldn't cause harm if installed properly.

We all know what 120 is, the same for 240. Dryers and ranges are 120/240, officially 125/250, that's because they have an actual neutral that is used, and it might not have a ground.

And they don't have multiple bonds. They come from the factory with one bonding wire from the frame to the neutral. That jumper gets removed when a four wire cord is used.

 

[sparky 1971]

Mother-in-law is very much alive at 88. ALWAYS cooked on an electric stove and used an electric dryer.

And in the old days if there had been a ground fault, it would have been cleared because the frame was bonded to the neutral. There was probably never a fault though and that's why she's still alive and kicking.

 

[Norcal]

I do understand that those devices did NOT have grounded (third prong) convenience outlets. Of course now you have multiple bonds.

Convenience receptacles were quite common on old ranges, my old Frigidaire Flair range from 1965 had one, the introduction of GFCI requirements I think is one thing that led to them to be discontinued.

 

[mikedodge]

DId anyone mention to run a new circuit yet?

At least 3 times now.