Help with last phase of solar install 120 to 240 cabin

suburbancowboy · January 2013

My goal this year is to finish the connection of solar power to my cabin that I finished last year.
The electrician that wired my cabin wired it so that I could use my generator as my power source. The external power cord connects to the 240VAC 20 amp four pronged outlet on the generator.
I have everything designed for my off grid system and have started to buy parts for it. So far I have purchased 3 -250 watt panels. Each month I will purchase another component. I will be running a 24V system. The only piece that I haven't got figured out yet is what inverter to use to connect to the existing 240VAC four pronged cord.

These are the four options I have come up with, all with a down side
1. Outback stackable inverter. - Way more than I want to spend.
2. Step up Transformer - Inefficient and unreliable.
3. Get the electriction to rewire the box to two 120VAC. Not sure if this can be done or the cost.
4. One person told me that I can jury rig a cord so that one single phase could feed the double phase of the 240VAC????

What are you all's thoughts?

Cariboocoot · January 2013

Re: Help with last phase of solar install 120 to 240 cabin

Taken in turn:

1). How much do you want to spend? You may end up spending more in the long run by trying to spend less in the short.

2). Step-up transformers are neither inefficient nor unreliable. Get a good one, not a cheap one.

3). Re-wiring the box to all 120 VAC is simple providing there are no 240 VAC loads: the two Hots are connected together. But this would mean you'd need to change your generator input as well, otherwise its 240 VAC output would be feeding into a dead short.

4). Yes. Basically the same as above (connecting the two hots) only done at the inverters output before the connection to the 240 VAC input to the panel. Essentially the inverter's single hot is 'Y'ed to feed L1 and L2 of an outlet that will accept the plug.

5). Buy a Magnum inverter that has 240 VAC output as standard and a built-in transfer switch. This may be more money than you want to spend, but it's the neatest way to tie up the whole package and make it usable.

I'd be curious to see your pre-purchase plan since it does not seem to have included the inverter.

BB. · January 2013

Re: Help with last phase of solar install 120 to 240 cabin

Regarding the jumpering of the 120/240 box for 120 VAC only.

One caution. A 120/240 box is designed for "split phase" power. Basically a 240 VAC transformer with a "center tap". That gives you L1-L2 240 VAC and L1-Neutral (or L2-Neural) 120 VAC.

The Center Tap (Neutral) in a split phase system will never carry more current than L1 or L2 (running a heavy 120 VAC load). Normally, you have 1/2 the 120 VAC circuits on L1 and the other half on L2--And the current tends to add up to "near zero" on the Neutral.

If you jumper the L1/L2 buses together in the box, if you have any electrical runs that used three wire Romex (for example) Red/Black/White, the Neutral will be the sum of the L1+L2 current instead of the difference L1-L2. This is obviously dangerous.

If your electrician did not wire the building to share a common neutral with both L1/L2 phases, than that is not a problem.

The end question is do you need 240 VAC? Normally, your loads should be small and 120 VAC would be fine (for off grid solar, you don't want to waste power anyway). However, if you have a well pump that is a fair distance from the building, then 240 VAC may be a big help.

Transformer do waste some power--But if you can switch the power input to your (for example) 240 VAC pump transformer, it is usually not an issue (i.e., pressure switch in cabin).

-Bill

inetdog · February 2013

Re: Help with last phase of solar install 120 to 240 cabin

Cariboocoot wrote: »

2). Step-up transformers are neither inefficient nor unreliable. Get a good one, not a cheap one.

One small caution in addition to what Bill said if you look at transformers: Make sure that you get either a 120 to 120 isolation transformer that can handle the full inverter or full generator output (whichever is larger) to feed the second phase OR get a 120 to 240 center-tapped transformer that is rated to deliver full power to either of the two phase outputs alone. A normal power rating on a 120 to 240 transformer of say 2KW can mean that it will only be able to deliver 1KW (or a little more) to each of the two phases. The output winding wires will be too small to take 2KW in at the 120 volt side and deliver the whole 2KW to just one of the two 120 volt outputs.

Cariboocoot · February 2013

Re: Help with last phase of solar install 120 to 240 cabin

inetdog wrote: »

One small caution in addition to what Bill said if you look at transformers: Make sure that you get either a 120 to 120 isolation transformer that can handle the full inverter or full generator output (whichever is larger) to feed the second phase OR get a 120 to 240 center-tapped transformer that is rated to deliver full power to either of the two phase outputs alone. A normal power rating on a 120 to 240 transformer of say 2KW can mean that it will only be able to deliver 1KW (or a little more) to each of the two phases. The output winding wires will be too small to take 2KW in at the 120 volt side and deliver the whole 2KW to just one of the two 120 volt outputs.

I believe in this case he'd be putting a 4kW autotransformer on the output of the inverter so that it would supply 240 VAC the same as the generator. It would then not be connected when the generator is.

suburbancowboy · February 2013

Re: Help with last phase of solar install 120 to 240 cabin

Thanks for all the feedback. I am trying to stay under 1000 for the inverter portion. I think that I will do like the last suggestion and go with a step up transformer connected to a 1500W pure sign wave inverter.

Help with last phase of solar install 120 to 240 cabin

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