Creating split phase 120V from 120V with transformer, safety concerns?

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,555
I'm currently using a generator that supplies 120V split phase (240V with center neutral) to power my entire house after storms, and it works great. However I would like to have the option of using my smaller 120V-only inverter generator instead because it's much better on fuel, however I need 120V split phase to safely power both branches of the house. Are there any safety concerns with creating split phase 120V using a transformer as shown below assuming the transformer is of adequate size? The center tap would be tied to earth through the home breaker panel. It looks good to me but I just wanted to run it by the crowd in case I'm missing a safety concern.

Also does anyone know off hand of a transformer with this configuration that is capable of sustained 3kW minimum?


upload_2019-9-3_13-38-50.png
 

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cmartinez

Joined Jan 17, 2007
6,451
I'm currently using a generator that supplies 120V split phase (240V with center neutral) to power my entire house after storms, and it works great. However I would like to have the option of using my smaller 120V-only inverter generator instead because it's much better on fuel, however I need 120V split phase to safely power both branches of the house. Are there any safety concerns with creating split phase 120V using a transformer as shown below assuming the transformer is of adequate size? The center tap would be tied to earth through the home breaker panel. It looks good to me but I just wanted to run it by the crowd in case I'm missing a safety concern.

Also does anyone know off hand of a transformer with this configuration that is capable of sustained 3kW minimum?


View attachment 185368
If your main interest is fuel efficiency, keep in mind that an average transformer is 95% efficient.
 

crutschow

Joined Mar 14, 2008
23,294
Why do you need split phase to "safely" power the house?
Can't you just power both sides from the 120V with the other side connected to neutral?
Or do you need 220V for some appliances?
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,555
95% is fine, the difference in generator efficiency is probably >20% at light loads.

@crutschow I need split phase because the house is wired with multi wire branch circuits, so if I power both legs with the same phase there's a chance the neutrals could be overloaded.

@nsaspook Thanks, that looks like it will work great if I'm reading the datasheet correctly (below)!

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Comments

1. Transfer switch safety?
2. VA not Watts
3. Transformer size. Makes sense.
4. Overload protection?

5) Neutral and ground is always a PITA. Whole house is different than stand-alone generator.
With whole house the ground-neutral bond is broken at the generator.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,555
Thanks guys. The mains are disconnected when the generator is connected. This is the generator, 2800watts continuous with 3500watts 10-second surge, the specs don't list a VA rating so it would appear I need to go up on the transformer size to account for PF < 1. The generator itself has a breaker on it, and the connection to the house also has a (larger than the small generator can supply so maybe not ideal) breaker.

The generator ground is typically connected to the house ground through the power cable, but with an isolation transformer in-between I would have to jump the ground around the transformer (generator ground, transformer neutral and house ground all tied). Is this proper?
 
A stand-alone generator has a neutral-ground bond at the generator. No connection to earth.

A whole hose generator has to break that bond and run ground and neutral to the panel/transfer switch.
This makes the neural-ground-earth-water pipes etc the zero reference point for the house.

If you do panel switching (breaker back feed), The breakers should be mechanically interlocked where the house and aux breaker CANNOT be on at the same time. There is a company that does this.
 

MaxHeadRoom

Joined Jul 18, 2013
18,999
The new or secondary neutral terminal is where the earth GND reference is connected to and should have a conductor back to the normal service ground at the panel or source of the GND, metalic water pipe, ground rod etc.
ref: NEC
Max.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,555
Thanks guys, I think we're on the same page. The current 240V generator runs 3 wires to the house, 2 hots and a ground that connects to the house ground. If I add the isolation transformer, I would connect the generator ground to the transformer secondary neutral, then the transformer secondary neutral to the house ground.

Thanks for the tip on the interlock kit, I see several of them online, I'll check them out.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,555
With multi wire branch circuits, two out of phase legs share a neutral. So if I powered both legs with the same phase, and both legs are loaded, the neutral would get double its rated current.

For example, if one branch (3 wires, 2 out of phase hots and a shared neutral) had 2 similar window unit A/C's on it, then with both loaded the neutral would have close to 0A current. However if both legs were changed to be the same phase, and if each leg was close to its maximum capacity, then the neutral would be carrying nearly 2x its rated current, instead of ~0A.

https://www.twielectric.com/safety-and-energy-saving-tips/national-electrical-code-multiwire-branch-circuit/
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
18,999
Remeber if both L1 and L2 are carrying a current and sharing a neutral, the current in the neutral is the difference Not the Sum!
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
18,999
I see you edited your post to incorporate changes that specified L1 - L2 in phase with each other! :( ;)
Max.
 

crutschow

Joined Mar 14, 2008
23,294
Remeber if both L1 and L2 are carrying a current and sharing a neutral, the current in the neutral is the difference
That's true for a normal 240-120 split phase, but not if you connect 120 to both phases, as I suggested.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
1,555
That's true for a normal 240-120 split phase, but not if you connect 120 to both phases, as I suggested.
I'm not following.. If I'm not mistaken, the walls are wired with multiple hots (L1 and L2) sharing a neutral. So if I change both L1 and L2 at the box to be the same phase, the shared neutrals in the walls can be overloaded. Am I not understanding multi wire branch circuits correctly?
 

crutschow

Joined Mar 14, 2008
23,294
If I'm not mistaken, the walls are wired with multiple hots (L1 and L2) sharing a neutral.
All the USA house wiring I've seen uses a 3-wire cable to the outlets -- hot, neutral, and ground.
I've never seen a shared neutral connection, which would require a 4-wire cable, expect to a stove or electric clothes dryer that uses 240V.
So the only neutral sharing occurs at the breaker box from there to the main's transformer.
 
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