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

nsaspook

Joined Aug 27, 2009
13,086
My backup power system at 2000 watts is mainly for running the forced air gas heat fan circuits, critical electronics and a few house lights. Usually when I lose power the cold outside will keep the food from spoiling.:)
2016-12-10_11-12-33.jpg
 

cmartinez

Joined Jan 17, 2007
8,220
As long as:
Monterrey is WET heat.. of the worst kind... Interesting fact: It was founded by Jews escaping the Inquisition, and since they couldn't eat pork (which is favored in this climate), Monterrey's favorite regional dish is goat... which also thrives in this weather.
 

be80be

Joined Jul 5, 2008
2,072
Its not much and frist time you forget to turn off the AC or hotwater heater or you try to cook on stove youll burn it up
 

nsaspook

Joined Aug 27, 2009
13,086
Its not much and frist time you forget to turn off the AC or hotwater heater or you try to cook on stove youll burn it up
The large transformer is unlikely to burn-up from a normal overload from a power limited source and good inverters have quick overload protection.
I've never had that problem in the many years my system has been in operation with overloads (it just trips the inverter) and I do use a isolation transformer from my inverter to utility power via a transfer switch.

Nuisance tripping is a problem when running close to the power limit.
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
2,188
We're waiting with bated breath for the answer. :)
I will try to get to it today. Crazy days, trying to get back to normal after the big hurricane scare. :)

In regards to the amount of power being enough; I've been through multiple weeks without power after hurricanes and the 4kw generator was plenty for daily stuff and I'm guessing the 3kw would probably be OK too if it can swing the surge needed to start things. The main loads are 1 or 2 small window unit A/C's (~600w/ea when running), a couple of refrigerators, lights, the house alarm and cameras, and occasionally the computer. My guess is between 2000 and 2500 watts in steady state. Things like central air and hot water are not used. The toaster oven or microwave are by far the largest individual loads and I'm not sure if the 3kw would run those without turning other stuff off first, but that's fine. The 4kw runs all of this fine, but you can definitely tell when someone uses the microwave or the toaster oven.
 

nsaspook

Joined Aug 27, 2009
13,086
I will try to get to it today. Crazy days, trying to get back to normal after the big hurricane scare. :)

In regards to the amount of power being enough; I've been through multiple weeks without power after hurricanes and the 4kw generator was plenty for daily stuff and I'm guessing the 3kw would probably be OK too if it can swing the surge needed to start things. The main loads are 1 or 2 small window unit A/C's (~600w/ea when running), a couple of refrigerators, lights, the house alarm and cameras, and occasionally the computer. My guess is between 2000 and 2500 watts in steady state. Things like central air and hot water are not used. The toaster oven or microwave are by far the largest individual loads and I'm not sure if the 3kw would run those without turning other stuff off first, but that's fine. The 4kw runs all of this fine, but you can definitely tell when someone uses the microwave or the toaster oven.
For the AC units (or anything with a large directly connected line voltage electric motor) you might need surge mitigation if powered by a inverter.
https://forum.allaboutcircuits.com/...se-of-varistor-in-series.160080/#post-1400895
 

Thread Starter

MrSoftware

Joined Oct 29, 2013
2,188
Thanks for the tip. This is an inveter generator and it seems to handle surges well. I normally use it to run a big A/C in a cargo trailer that draws about 2kw in steady state, I'm not sure what the start up current is but the generator handles it fine. The big benefit of the inverter generators is they don't have to spin a specific RPM to keep the frequency up, so when the load is light the engine throttles way down and that's where you get a big boost in efficiency. So if you're just running a couple of lights and charging your cell phone, it's basically idling, which is a huge improvement over a regular 2-pole generator that has to maintain 3600 rpm to provide 60Hz.
 

Reloadron

Joined Jan 15, 2015
7,501
Monterrey is WET heat.. of the worst kind... Interesting fact: It was founded by Jews escaping the Inquisition, and since they couldn't eat pork (which is favored in this climate), Monterrey's favorite regional dish is goat... which also thrives in this weather.
Well live and learn. :)

Ron
 

nsaspook

Joined Aug 27, 2009
13,086
Thanks for the tip. This is an inveter generator and it seems to handle surges well. I normally use it to run a big A/C in a cargo trailer that draws about 2kw in steady state, I'm not sure what the start up current is but the generator handles it fine. The big benefit of the inverter generators is they don't have to spin a specific RPM to keep the frequency up, so when the load is light the engine throttles way down and that's where you get a big boost in efficiency. So if you're just running a couple of lights and charging your cell phone, it's basically idling, which is a huge improvement over a regular 2-pole generator that has to maintain 3600 rpm to provide 60Hz.
I've used the Honda inverter generators before, they are sweet.
 

Lo_volt

Joined Apr 3, 2014
316
Remember to ground the neutral right at the transformer secondary terminal.
Max.
Just a note: As long as the neutral and ground are not switched (and I don't see any reason you would switch either), the neutral will still be bonded to the ground at the main breaker panel. You don't want an extra neutral-ground connection.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
With ref to DWG post #1, the secondary for the new supply will be isolated and the earth GND reference will be lost.
Per NEC when using an Isolation transformer the neutral will be re-referenced to ground.
As per X2.
Max.
 

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MisterBill2

Joined Jan 23, 2018
18,179
With ref to DWG post #1, the secondary for the new supply will be isolated and the earth GND reference will be lost.
Per NEC when using an Isolation transformer the neutral will be re-referenced to ground.
Max.
If the output is re-referenced to ground then it is no longer an ISOLATION transformer.
 

Lo_volt

Joined Apr 3, 2014
316
If he's connected the transformer to his house wiring, the neutral and ground should already be bonded together at the main panel.

In one company that I worked for (I'm in the US), they had a 50 Hz power converter that they hard wired into the building. It was a properly completed job with its own breaker panel, conduit and outlets and it was used to test their product on the production line at 50 Hz. Since the output of that converter was isolated, they DID need to re-reference neutral to ground.

I'm not disagreeing, I'm just saying that neutral and ground may already be connected and won't need to be connected twice.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
The transformer secondary Ov C.T. referencing to earth ground in the OP drawing is shown earth connected, as it should.
However, grounding should be done right at the Transformer C.T. connection, again, per NEC/CEC.
Max.
 
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