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Author Topic: 8kw generator wiring  (Read 4274 times)
Kitt
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« on: August 20, 2012, 11:43:48 AM »

I'm looking for some expert guidance and I couldn't find exactly what I was looking for by searching. This closest I could find was: http://www.busconversions.com/bbs/index.php?topic=9732.0

I have an 8kw Cat/Powertech generator (http://powertech-gen.com/wp-content/uploads/2010/04/CSI-7-8-OPERATORS-MANUAL-Exciter.pdf) currently wired for 120v. The existing wiring from the generator to the transfer switch is undersized (8/2 wire) so I am replacing it. Here's the situation:

The generator can be wired for 120v (66 amps via two bridged 35 amp breakers) or 240v (33 amps via two 35 am breakers). My transfer switch is rated to 50 amps. It and the main panel are wired for 120v/240v. If I set up the generator for 240v I can run 8 gauge (8/3) wire, but that limits my load sharing (3 AC units, inverter/charger, water heater, microwave, etc.). If I wire it for 120v then I could be over the transfer switch limit for a single leg.

I've read, but have no evidence of it otherwise, that running these generators in 240v and uneven loads on the two legs can cause the voltage to be a little different on the two legs (like 130v on one and 110v on the other). If that's the case, is it an issue I should worry about?

So do I:

1) Leave the generator on 120v, wire it with 6/2 cable, and bridge the black/red on the input of the transfer switch to share power between the two input legs?

2) Do the above, but replace the generator output breaker to a single 50 amp?

3) Go with 240v and hope for the best with the loads on each leg? I haven't checked to see what the draw of the AC units are so I really have no idea if I could run two AC units on one 33 amp leg.

4) Something else creative that I haven't thought of yet...?

Thank everyone.
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« Reply #1 on: August 20, 2012, 11:46:05 AM »

Heres an option, if you get no others: http://www.smokstak.com/forum/forumdisplay.php?f=6
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« Reply #2 on: August 20, 2012, 12:09:16 PM »

Powertech's recommendation on that size generator is to go with 120 volt unless you have 240 volts loads.  I have that same generator and I wired it for 120 volt.  Powertech also recommends putting a jumper between the two 35 amp breakers in the unit when going to 120 volt.

There is a page in the manual that shows the wiring diagram when converted to 120 volt.  The change is not hard.  Your most expense would be for new wire. 
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Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
Geoff
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« Reply #3 on: August 20, 2012, 12:15:36 PM »

There is no reason to wire the genset for 240 unless you need to run a 240v load.  The only things I can think of might be a 240v clothes dryer, or a welder.  I have mine set up on a single wire to run both sides of the breaker box @120v, but when I am plugged into 50 amp service I have 240 available from the breaker box.
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Geoff
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Kitt
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« Reply #4 on: August 20, 2012, 02:29:41 PM »

I'm fine with leaving the generator at 120v as it solves the load issue between the two legs. The question just remains as to the best way to wire it into the transfer switch.
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« Reply #5 on: August 20, 2012, 03:00:15 PM »

I'm fine with leaving the generator at 120v as it solves the load issue between the two legs. The question just remains as to the best way to wire it into the transfer switch.

Mine was wired 240 volt to the 50 amp transfer switch at teh start.  I used the same wires when I went to 120 volt.  Powertech suggests a jumper between the breakers to even out the load.  You'll even see the jumper on the 120 volt wiring diagram.
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Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
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« Reply #6 on: August 20, 2012, 05:05:35 PM »

There is no reason to wire the genset for 240 unless you need to run a 240v load.  The only things I can think of might be a 240v clothes dryer, or a welder.  I have mine set up on a single wire to run both sides of the breaker box @120v, but when I am plugged into 50 amp service I have 240 available from the breaker box.

I know what you want but in order to do what I/we wanted I had to order the parts and make my own transfer switch.

Here is what mine does:

1.  Shore power is 50 amp with two circuits to the main breaker box (240v).
2.  If shore power is unplugged, and generator is running-- both sides of breaker box run off a single 120v feed from the generator.  It will not transfer to shore power if the generator is running.
3.  No generator, with a 30 amp adaptor to the 50 amp land line both sides of breaker box are on same circut (120v).
4. If I forget to turn off my my generator and hook up to shore power the transfer switch won't work (transfer/repeat as above in #2.).
5.  When plugged into a weak 30 amp campground circuit the Trace SW2512MC inverter will pick up the extra load of running two air conditioners so I can have two airs while on weak 30 amp service.

Sounds cool, hum?

I don't have a wiring diagram, I just looked the wires and switches and hooked it up.  10 years and it still works as described.

--Geoff


  
« Last Edit: August 20, 2012, 05:08:52 PM by Geoff » Logged

Geoff
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« Reply #7 on: August 20, 2012, 06:45:38 PM »

If you install the jumper between the breakers at the generator all that does is evenly distribute the load between the two windings. If the coach is wired for 50 amp service including a 50 amp transfer switch, you will never pull higher than 50 amps through either leg of the transfer switch so long as the distribution panel is wired/balanced properly. If you have two hots from the genny to the ts, each one will carry half the load at all times if you place the jumper between the hots at the ts. (on the generator input side of the switch only!). this way you will only be able to pull a max of 33 amps on each hot leg from the genny. If you are already wired with 2 hot wires from the genny to the ts, I would just put the jumper at the ts instead of the genny breaker. You accomplish the same thing, just in a different place.
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« Reply #8 on: August 20, 2012, 11:52:52 PM »

...
I've read, but have no evidence of it otherwise, that running these generators in 240v and uneven loads on the two legs can cause the voltage to be a little different on the two legs (like 130v on one and 110v on the other). If that's the case, is it an issue I should worry about?


Not really, as long as you make some effort to balance loads between legs.  For example, if you have two air conditioners, put one on each side.  The loads do not need to be identical.

Quote
...
1) Leave the generator on 120v, wire it with 6/2 cable, and bridge the black/red on the input of the transfer switch to share power between the two input legs?


That is not an option.  If you do end up drawing a full 66 amps, you will overload your 50-amp transfer switch.  Also, #6 will be marginal for 66 amps.  Remember that even though each of your two existing "hot" legs coming from the generator will be carrying 35 amps or less, when wired for 120, the neutral will be carrying the sum of the total current, or 66 amps.  You would need to upsize the transfer switch and the wires to run the unit on 120 at the full 66 amps.

Quote
2) Do the above, but replace the generator output breaker to a single 50 amp?


That' an acceptable solution, as it will ensure that the neural current is 50 amps or less.  You could also put another panel in between the generator and transfer switch, to make use of the 16 amps you would otherwise leave on the table this way, for example to run an extra batter charger.

Quote
3) Go with 240v and hope for the best with the loads on each leg? I haven't checked to see what the draw of the AC units are so I really have no idea if I could run two AC units on one 33 amp leg.
...


This method allows you to use the full output of the generator, but creates the additional problem of load management.  You will not be able to run two ACs per leg, so this would also limit you to two ACs total.

Probably the least expensive solution that provides the most flexibility is to wire the genny for 120 and put a two-position panel between the breaker and the transfer switch.  Supply the transfer switch through a 50-amp breaker and add a 15-amp breaker for another dedicated circuit that only works when the generator is running.  That will give you a full 50 amps without having to balance loads and without having to replace your transfer switch and hardware.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com
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Kitt
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« Reply #9 on: August 29, 2012, 02:42:55 PM »

Thanks Sean! That really helps. I may do the sub panel idea - I hadn't thought of that.
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Geoff
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« Reply #10 on: August 29, 2012, 05:08:18 PM »

Thanks Sean! That really helps. I may do the sub panel idea - I hadn't thought of that.

Yup, like I said on the other board, you have to be able to put the whole wiring diagram (gen power, 50 amp shore power/ 30 to 50 amp adaptors) in your head to figure the wires out.  And it is not a sub-panel.  Its in your transfer switch.

--Geoff
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Geoff
'82 RTS AZ
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