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Author Topic: Another 120/240 generator wiring question  (Read 915 times)
Uglydog56
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« on: August 21, 2012, 09:50:19 AM »

All the recent electrical discussions have made ponder changing my setup.  I have a 5k generator, which is small, but adequate for my needs currently.  It is currently wired to provide 240v, because my bus had an apartment washer/dryer that needed it.  My wife decided she wanted a pantry more than a washer, so I removed it.  Now I no longer require 240v, and since my genny is small, I was going to rewire it to provide 120v.  But I'm thinking that I want a 240v outlet in the back and on the side in a bay to run the mig welder, which is seems I need to do quite often at home and would like to retain that ability.  How would one wire it up so there's a 240v outlet, but everything else is 120?  I have a 50A automatic transfer switch between genny/shore power and main panel.  My main panel is a square d that backfeeds through 50A 240v main breaker that are in the top left of it.  Would I need to build one of those manual transfer boxes out of contactors upstream of the auto transfer switch, or would you put one of those auto transformers powered from the main panel, or how would you do it?  I'm sure there's a simple way to do it, that's still a bit more permanent than the two receptacle / one cord thing.  Maybe a breaker panel with a sliding bar thingy?  Thanks.
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Rick A. Cone
Silverdale, WA
66 Crowny Crown "The Ark"
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« Reply #1 on: August 21, 2012, 09:57:09 AM »

Unfortunately, there is no way to do both, that is to give you full capacity at 120 v and still have available 240 v, unless you use an external 120/240 transformer.
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« Reply #2 on: August 21, 2012, 10:22:58 AM »

I guess I am missing something.  Your genset is wired to put out 240, so it powered your panel with two legs of 120.  This ran all your circuits at 120 except the one for the dryer, which used a two pole breaker to provide 240 for the washer/dryer.  Having removed the washer/dryer, you are left with the one unused 240 circuit that you now could use for your welder.  Normally, you could just put one receptacle on that 240 circuit.  However, I would be tempted to cheat and put both on it knowing that I would only be using one at a time.

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Oonrahnjay
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« Reply #3 on: August 21, 2012, 02:17:06 PM »

Yeah, Rick, you're set up about as well as you can be now.  For your 240V circuit, just run your wiring from the 240V supply and site your outlet where you want it.  When you want to use the welder, start your generator, plug in the welder, and it'll work fine.  For your 120V needs, you already have a system where the two legs are split into 120V circuits and these should work just fine.  There are really no drawbacks (such as unusual efficiency losses or safety issues, etc.) with this setup.

There are a couple of items (and I'm sure that you know them but let's discuss them for clarity if someone looks for this thread in the archives in the future) that should be considered.  First, you'll need to be careful of your power loads total since you have a 5KW gennie.  You won't be able to pull 240V for your welder from the generator and run air conditioners at the same time, for instance.  But that's more of an issue about the size of your generator (although it's a factor that everyone should be aware of when adding big loads at any time).  Another item will be "load balancing" - if you're running a pair of air conditioners, have each one on a different "leg" of the 120V.  Plan similarly for other large 120V loads.  But with good planning as you're thinking about setting up your loads, that won't be an issue.

I'm also assuming that your "shore power" is set up for 240V in and then legs are split.  The difference here is that you'll have more available watts but the factors are pretty much the same.  Unless you have very special needs (like the wife wants a 120V dryer again in the future), the same setup that works for your 240V gennie will work for the shore power.  

What you have is about the best and simplest setup for what you need and what you have (i.e. a 5kw generator).  Of course, the usual good design for breakers, transfer switch design, wiring logic, wire sizes, etc. applies but you haven't indicated that you have any issues on those items as your system is now.  And since you have different amounts of power available from the generator and shore power, you'll want to consider breakers/fuses carefully, but this is just ordinary good design; the same for wiring sizes, you'll need to plan for the maximum load up to the maximum available power and be sure that wiring accommodates it but is also appropriate for the "lower wattage" supply.

Starting from 240V and splitting to 120V is simple (as long as the basic good design criteria are met); going the other way is more complicated and expensive.  It's even more logical to have 240V from your generator is you're set up for 240V from shore power.

But, as far as I can see, you're already set up about as well as you can be.  And there's nothing wrong with a 5Kw generator if you don't need more - lots of people have met their power needs with less.
« Last Edit: August 21, 2012, 02:31:52 PM by Oonrahnjay » Logged

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« Reply #4 on: August 22, 2012, 05:35:57 PM »

I originally had my  8kW genset wired for 240v-- 120v on two legs, but since I run three roof airs, microwave/convection oven/ sometimes water heater I decided to do a single wire hookup to the transfer switch.  I was also thinking about running a 240v welder when I did that.  But in 12 years I have never needed to run a 240v welder on the road.  Plus I figured if by some chance I needed 240v from my generator it would be very easy to change the connections at the generator output to 240v than worry about load balance 99.9 of the time when all I need is 120v.
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Geoff
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Uglydog56
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« Reply #5 on: August 23, 2012, 07:46:57 AM »

Thanks, guys for your input.  The best part about this advice is, everybody's telling me not to change anything! 
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Rick A. Cone
Silverdale, WA
66 Crowny Crown "The Ark"
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