Changed a 120v feed to 220v and then back

[PeterN]

Many years ago I ran a 12-2 line to an outside building and installed in a load center to split the circuits. All 120V line. Over time I've added a few more 120v draws and noted a drop in voltage. I'm looking for advice on what effect changing this over to a 220v circuit down to the shed and then splitting it in a breaker panel back to multiple 120v circuits with breakers. I don't have any 220v use at this time. Let's say I originally had 20A of capacity when running the 120v line. Now changing to 220v, can I assume that I still would have 20A of capacity per leg or is there more/less loss with 220v vs 120v?

Pete

 

[exranger06]

Going to 240V will essentially give you twice as much amperage for 120V circuits. You'll have 20A per hot leg. You'll have to change the wiring from the house to the shed, though. A 12/2 wire only has 3 conductors: 1 black wire (hot leg) 1 white wire (neutral) and a bare ground wire. You need an additional wire (2 hot legs, 1 neutral, 1 ground). Also, if this 12/2 wire is NM-b (Romex) cable, it's the wrong wire. NM-b is for dry locations only, not outdoors/underground or wet locations. You need to use UF cable or, if there's conduit continuously from the main panel to the shed panel, THWN wire.

Also, if you're replacing wire anyway, you can install larger wire for more ampacity.

 

[PeterN]

The wire that is in the ground is 12-2 UF wire. I might be misunderstanding the 240v circuit in this instance but I was under the impression that a 240v circuit only needs the neutral when the appliance requires a 120v & 240v input.

 

[exranger06]

If you were powering only a 240V appliance, like a 240V electric motor for example, then yes you only need 2 hot legs and ground, no neutral. If you were powering an appliance that needs 240V and 120V, like an electric stove, then you need 2 hot legs, ground, AND a neutral. If you're powering a bunch of 120V circuits, like feeding a subpanel that can/will power 240V circuits as well as 120V circuits, you need 2 hot legs, ground, AND a neutral. A subpanel circuit is no different than a 240V/120V electric stove circuit: it's going to power 240V AND 120V stuff, therefore it gets wired the same as an electric stove circuit.

 

[micromind]

Since it's 12/2, you won't be able to connect it for 240 and use 120 at the other end as it is but you can connect it 240 and install a transformer at the other end. It'll need have a 240 primary (120/240 or 240/480 will work) and a 120/240 secondary (most likely 4 wires, easily connected for 120/240).

3KVA will give you 12.5 amps per leg, 5KVA will be up to 25 amps per leg but you're limited to 40 amps total, 32 continuous.

 

[PeterN]

If you were powering only a 240V appliance, like a 240V electric motor for example, then yes you only need 2 hot legs and ground, no neutral. If you were powering an appliance that needs 240V and 120V, like an electric stove, then you need 2 hot legs, ground, AND a neutral. If you're powering a bunch of 120V circuits, like feeding a subpanel that can/will power 240V circuits as well as 120V circuits, you need 2 hot legs, ground, AND a neutral. A subpanel circuit is no different than a 240V/120V electric stove circuit: it's going to power 240V AND 120V stuff, therefore it gets wired the same as an electric stove circuit.

I think I'm learning something here. I don't plan on powering any 240V circuits from the intended subpanel, just 120v. Do I still need a ground and a neutral?

 

[exranger06]

I think I'm learning something here. I don't plan on powering any 240V circuits from the intended subpanel, just 120v. Do I still need a ground and a neutral?

Yes, you still need ground and neutral. The only reason 240V circuits don't need a neutral is because the current going down one hot leg will be the same as the current on the other hot leg. So if you had a 240V electric motor that draws 10A, you have 10A flowing down one hot leg, and the same 10A flowing on the other hot leg back to the panel. The current leaving the panel HAS to be the same as the current returning to the panel.

A 120V circuit works the same way, except it uses one hot leg and a neutral: If there is 10A flowing down the hot leg, that same 10A is flowing through the neutral.

Now, if you have a 240V circuit (2 hot legs) that's supplying a bunch of 120V circuits, you're gonna have one hot leg supplying some of those 120V circuits, and the other hot leg supplying the others. The amount of current on one hot leg is basically never going to be exactly the same as the current on the other hot leg. The neutral is there to carry the difference in current between the 2 hot legs. So for example, if one hot leg has 13A of current going through it, and the other hot leg has 7A of current, the neutral will have 6A of current (13A - 7A = 6A).

On a 20A circuit, you can have up to 20A on each hot leg (this is how a 240V circuit essentially doubles the amperage of a 120V circuit). So if you plug a lot of stuff in to one of the hot legs, and you're drawing 18A on one hot leg, you'll also have 18A on the neutral wire. A regular 120V 20A circuit would be pretty much maxed out at this point. But a 240V circuit has a whole other hot leg to use. So if you plug a bunch MORE stuff in (on the other hot leg) and you're drawing 17A on the other hot leg, you have only 1A of current on the neutral (18A - 17A = 1A).

But note how you always have current on the neutral, whether there's stuff plugged in to both hot legs or just one. The only time you'd have no current on the neutral is if you happen to be drawing the exact same amount of current on both hot legs at the same time.

So, yes, you absolutely need a neutral to feed 120V loads.

 

[PeterN]

Good explanation. Looks like I need to bury another wire of the correct configuration. Thanks.

 

[dave*99]

Good explanation. Looks like I need to bury another wire of the correct configuration. Thanks.

And now you enter the GJ twilight zone.

The discussion will rapidly move toward using conduit and larger cable and a larger service etc. You will get suggestions for 30,60,90 and 100A. Sit back and hold on. There is nothing you can do to stop it.

You will learn about THWN, MHF even some X rated cable. And always remember, in the USA it's 120 and 240. Not 110 and 220.

 

[wyliesdiesels]

Good explanation. Looks like I need to bury another wire of the correct configuration. Thanks.

not quite. code does not permit the adding of a separate circuit conductor like that since you have a jacketed cable. also, the only wire you could probably add would be THWN and that has to be in conduit indoors

it would be far easier to switch to 240v feed at the main panel, then add a transformer in the building to step down to 120v to feed the subpanel.

 

[Max]

not quite. code does not permit the adding of a separate circuit conductor like that since you have a jacketed cable. also, the only wire you could probably add would be THWN and that has to be in conduit indoors

it would be far easier to switch to 240v feed at the main panel, then add a transformer in the building to step down to 120v to feed the subpanel.

That’s an interesting idea. I assume that the transformer is center tapped to form the neutral, and the new neutral has to be grounded as well? It seems like the transformer might be expensive though.

 

[johnre]

And always remember, in the us it's 120 and 240. Not 110 and 220.

Officially, 117 and 234. C'mon. get it right .....

But the worst error I've seen in some circles is "120 and 220" - through some weird number system that is applied such that even the expected factor of two doesn't apply.

 

[micromind]

That’s an interesting idea. I assume that the transformer is center tapped to form the neutral, and the new neutral has to be grounded as well? It seems like the transformer might be expensive though.

Yes, most likely it'll have 4 secondary wires, you'll need to connect them properly to get 120/240, the diagram with the transformer will show how. If there isn't a diagram, there are several of us here that can tell you which wires to connect. Very likely it'll be X1 to phase A, X2 and X3 connected together to form the center tap (this will be both neutral and ground) and X4 will be phase B.

You'll need to connect the center-tap to ground but only in one place. It can be at the transformer or the panel but not both.

Since this is what is called a 'separately derived system', you'll need to connect a ground rod to the ground wire or the ground bus in the panel, it doesn't matter which one. I usually connect to the bus, it's easier.......

 

[Max]

Yes, most likely it'll have 4 secondary wires, you'll need to connect them properly to get 120/240, the diagram with the transformer will show how. If there isn't a diagram, there are several of us here that can tell you which wires to connect. Very likely it'll be X1 to phase A, X2 and X3 connected together to form the center tap (this will be both neutral and ground) and X4 will be phase B.

You'll need to connect the center-tap to ground but only in one place. It can be at the transformer or the panel but not both.

Since this is what is called a 'separately derived system', you'll need to connect a ground rod to the ground wire or the ground bus in the panel, it doesn't matter which one. I usually connect to the bus, it's easier.......

Thank you. I’m familiar with the transformer aspects but not the NEC requirements around it.

 

[wyliesdiesels]

That’s an interesting idea. I assume that the transformer is center tapped to form the neutral, and the new neutral has to be grounded as well? It seems like the transformer might be expensive though.

that is correct. its called a separately derived system. neutral is bonded & connected to grounding electrodes