| Author | Message | ||
| Debo Cox (Debo)
Registered Member Username: Debo Post Number: 28 Registered: 1-2009 Posted From: 149.168.204.10  Rating: N/A |
Hi guys, I'm starting to work on my house electrical system a little bit in my MC9 conversion and I have a generator connection question. Through the breaker box, I plan to hook one roof air to one leg of the 240 volt output (120 volts), and the other to the other 120 volt leg as is generally-accepted practice. I understand the need to balance the load. My question is this... How close does this balance need to be? Do I always need to run both rooftop airs at the same time? I'm guessing running one without the other for any length of time would burn out a generator winding. Is this correct, or am I way off-base? What is the toleration limit for this "balance"? Thanks in advance for your help. | ||
| Bill Gerrie (Bill_gerrie)
Registered Member Username: Bill_gerrie Post Number: 380 Registered: 3-2006 Posted From: 67.142.171.27  Rating: N/A |
Debo There are some guys on here that are more knowledgable then I am to answer you but I have always been told that connecting the gen set in a 120 volt configuration is best to balance the load. It allows the full amperage output available to all loads at any time. Of course wire size must be considered to handle the extra amperage to the distribution panel. Connected this way the gen set is never unbalanced. Bill | ||
| Sean Welsh (Sean)
Registered Member Username: Sean Post Number: 1175 Registered: 1-2003 Posted From: 67.142.130.23  Rating: N/A |
The loads do not need to be in perfect balance at all times, just balanced overall over the long term. IOTW, if you have an air conditioner on each leg, and you use both air conditioners about the same amount of time overall, you'll be fine, but if you use one of them 100% of the time, and the other one only 5% of the time, one winding will get more use than the other. As far as re-strapping the genny for 120, that only makes sense on generators up to about 5kW. Above that, you need to balance the loads across two legs anyway, because that's the way shore power is provided. -Sean http://OurOdyssey.BlogSpot.com | ||
| Tom Christman (Tchristman)
Registered Member Username: Tchristman Post Number: 254 Registered: 1-2006 Posted From: 66.218.33.156 Rating: N/A |
I have a 10kw Powertech that is wired straight 120vac 4 wire. Hence-no loads to balance ever. I am restricted to one leg of the 50amp shore power, but when stopped, 6,000 watts is plenty. I like the fact that I don't ever have to think about balancing the load, my new Wrico 12kw is also wired straight 120vac. Besides-how do you balance 3 A/C's? Good Luck, TomC | ||
| Debo Cox (Debo)
Registered Member Username: Debo Post Number: 29 Registered: 1-2009 Posted From: 149.168.204.10 Rating: N/A |
OK guys, thanks for the clarification. I'm still planning to wire things up the same way, but now have a better understanding about the load balancing business. I want to be able to use the full capability of the generator, even if it means I have to pay a little more attention to balancing things out. In my case with two roof airs, it will work fine. Just to add to the knowldege base, I spoke with an electrician friend of mine about how a single-phase generator can produce two legs of power, and he explained to me like this: Think of the two 120 volt outputs of the 120/240 plug as not "on" at the same time because of the sine wave nature of alternating current. That's what the power is doing - alternating quickly between the two 120 volt legs. Hope this helps someone understand a little more. It did me. Thanks again for the input. Debo | ||
| Buswarrior (Buswarrior)
Registered Member Username: Buswarrior Post Number: 1926 Registered: 12-2000 Posted From: 174.91.143.159  Rating: N/A |
The way to get all the power out of a smaller generator is to wire it straight 120. Wiring a smaller generator 120/240 can limit your choices, with spare power left over that you cannot access. With an AC on each leg of 240, another heavy load, like a big microwave, might trip a breaker, yet the loads of two AC and the microwave do not add up to your whole generator total. Each leg of the 240 only has half the power available and then they trip. A straight wired 120 will give you the whole capacity in whatever combination you attach to it. or something like that. happy coaching! buswarrior | ||
| Jim Wilke (Jim Bob) (Pd41044039)
Registered Member Username: Pd41044039 Post Number: 563 Registered: 2-2001 Posted From: 184.0.3.170  Rating: N/A |
If you wire your genset for 120 only (assuming you don't have any 240 volt items) you will be able to use 100 percent of the output with no balance problem. The typical small genset has two output windings coils. When connected for 120/240 each can produce 1/2 of the total output. Trouble comes when you have a genset rated for 50 amps and you have 35 on one winding and 15 on the other. The overloaded coil may fail. You might trip the output breaker when you have less than a full load on the machine. Genset builders often use an oversized breaker on the machine to avoid nuisance tripping when the machine is in a hot place or a momentary load (like when your A/C compressor starts). The easy way is 120 only. Then those things don't happen. | ||
| Tim Brandt (Timb)
Registered Member Username: Timb Post Number: 568 Registered: 10-2003 Posted From: 66.165.176.62  Rating: N/A |
I run a 15KW wired 240/120 but my large draw items like rooftop ac and stove are all 240 | ||
| Jim Wilke (Jim Bob) (Pd41044039)
Registered Member Username: Pd41044039 Post Number: 565 Registered: 2-2001 Posted From: 184.0.3.170 Rating: N/A |
Tim, what kind of roof airs are 240V? I knew some basement airs were but have never seen rooftop units like that. | ||
| Jack Conrad (Jackconrad)
Registered Member Username: Jackconrad Post Number: 1341 Registered: 12-2000 Posted From: 75.202.147.60  Rating: N/A |
Per the recommendation of PowerTech, we wired our PowerTech for 120 only. This is what they recommend unless you have a need for 240. They said balance should be within 15 or 20% (I forget which). With an AC on each side I would be balanced, but turning on the water heater would put me out of balance. So far, we are happy with the 120 only. YMMV Jack | ||
| Tim Brandt (Timb)
Registered Member Username: Timb Post Number: 569 Registered: 10-2003 Posted From: 66.165.176.62 Rating: N/A |
Jim, They came with the bus but they are old Dometic units. The previous owner special ordered them perhaps it's a European standard | ||
| Ken Pridmore (Kenny)
Registered Member Username: Kenny Post Number: 10 Registered: 10-2006 Posted From: 66.188.25.34 Rating: N/A |
When wiring for 220 volts, a more important reason for balancing loads has to due with loading the engine. When using 120 or a balanced 220 configuration, the engine is rotating and driving a balanced, symetrical or equally spaced load from the generator head. If when using 220 the load becomes unbalanced, the engine is now driving an imbalanced or non-symetrical load. This imbalanced condition can excite resonance frequiencies in the generator driveline causing excessive vibration and eventual engine / generator shaft failure. According to Wrico, their recommendation when using 220 is not to imbalance more than 60-70% Kenny | ||
| Jim Wilke (Jim Bob) (Pd41044039)
Registered Member Username: Pd41044039 Post Number: 566 Registered: 2-2001 Posted From: 184.0.3.170 Rating: N/A |
Thanks Tim. It just reinforces that we should never assume anything! | ||
| Debo Cox (Debo)
Registered Member Username: Debo Post Number: 30 Registered: 1-2009 Posted From: 76.5.186.152 Rating: N/A |
Well guys, this has been very informative for me and has made me re-think my 120/240 wiring scheme. I've pretty much decided to go the "straight 120" route with the generator after considering all of the options and weighing some of the variables. I don't really have a specific need for 240, and worrying about always having the load balanced to keep everything happy is something that would nag me every time I was running the thing. Thanks for the input. | ||
| Sean Welsh (Sean)
Registered Member Username: Sean Post Number: 1177 Registered: 1-2003 Posted From: 67.142.130.37 Rating: N/A |
Debo, Just remember now to double all your wire sizes, transfer switch, and main breaker. Also remember to de-rate the transfer capacity by 25% if you are splitting the feed across two legs. Everything is a trade-off, and there is a price to be paid for wiring straight 120 in larger sizes. -Sean http://OurOdyssey.BlogSpot.com | ||
| FloridaCliff (Floridacracker)
Registered Member Username: Floridacracker Post Number: 437 Registered: 7-2004 Posted From: 68.204.16.36 Rating: N/A |
Sean, Could you elaborate on the trade off.. Thanks Cliff | ||
| Sean Welsh (Sean)
Registered Member Username: Sean Post Number: 1178 Registered: 1-2003 Posted From: 67.142.130.20 Rating: N/A |
Sure... Halving the voltage doubles the current, which means you'll need bigger breakers, wires, switches, etc., as I wrote. I did a column on this in BCM a few months back. Basically, on a coach, generators smaller than 4kW should always be strapped for 120v, and generators 12kW and larger should always be strapped for 240/120 split phase. Between those numbers, generators between 4 and 12 kW, which voltage you choose can depend on a number of factors, one of which will be the cost and availability of parts. Take, for example, a 10kW generator set, a rather common size. 10kW is about 42 amps at 240 volts, but it's a whopping 84 amps at 120 volts. Because breakers don't come in those sizes, the required generator output breaker would be a 40-amp two-pole for a 240-volt set, or an 80-amp single-pole for a 120-volt set. With the 40-amp 240-volt setup, you would have the option of wiring that directly (through the 40-amp breaker) to a NEMA 14-50R receptacle into which you could just plug your shore cord, a simple and inexpensive way to switch between shore and generator inputs. With the 80-amp setup, you could not use this method, and would need a transfer switch. Any transfer switch you get for this application would also need to be rated for 80 amps, but as you are probably aware, most such devices made for the RV market are 50 amps (or worse, 30 amps). An 80-amp switch is an industrial item, much larger and more expensive than the commonly available 50-amp models. Then there is the matter of wire sizing. The 40-amp setup would require only #8 wire (in THHN, for most normal temperatures). The 80-amp setup would require #4. If you've ever tried to pull three #4's through conduit, you'll know what a bear that can be. Also, you'll need much larger conduit for three #4 than you will for four #8. Lastly, the rig will still need to have a split-phase entrance, because shore power at those ratings is 240 volts. So now you also have to have a way to connect your split-phase shore power to this system designed for 80-amps single-phase. Several ways to do that, but it is an additional layer of complexity. By contrast, a 6kW generator produces only 50 amps wired for 120. So if you already have a 50-amp split-phase electrical system, whether with a transfer switch or with a cord-and-plug, it is a simple matter to just gang the two legs of the input together between the transfer mechanism and the generator breaker. For these reasons, if it were me, I would strap for straight 120 up to 6kW, and for 240 above 10kW. In between, you need to do the math, and figure out what your priorities are. -Sean http://OurOdyssey.BlogSpot.com | ||
| Debo Cox (Debo)
Registered Member Username: Debo Post Number: 31 Registered: 1-2009 Posted From: 149.168.204.10 Rating: N/A |
I just realized I never stated the capacity of my generator either, so let me do that here. It's a 6kW, so that's why I was a little on the fence about what to do. Thanks for all of the good information guys. | ||
| marvin pack (Gomer)
Registered Member Username: Gomer Post Number: 1004 Registered: 3-2007 Posted From: 71.55.13.161  Rating: N/A |
Good Post Sean!! That has helped me also and if it doesn't help others, they ain't reedin this thang!! | ||
| Tom Caffrey (Pvcces)
Registered Member Username: Pvcces Post Number: 1338 Registered: 5-2001 Posted From: 65.73.106.211 Rating: N/A |
Sean, I think you shed some much needed light on this subject. Thanks for the time that you put in on it. Tom Caffrey PD4106-2576 Suncatcher Ketchikan, Alaska | ||
| Buswarrior (Buswarrior)
Registered Member Username: Buswarrior Post Number: 1929 Registered: 12-2000 Posted From: 174.91.143.159 Rating: N/A |
Lovely explanation, thank you! happy coaching! buswarrior |