Question About Actual Generator Output

AlanRT · February 2009

Crewzer wrote a very informative summary about "actual" generator output in this thread:

http://forum.solar-electric.com/showthread.php?t=2078

I'd like some clarification because I'm still a little confused on the question of actual generator output when derating for the 120 volt outlet.

We have a client with a small Honda 3,500 watt gasoline generator. Unfortunately, I don't yet have the model number. Using Crewzer's numbers, I'm calculating this generator can put out a maximum of ~ 2,950 watts @ 240 volts, or about 12.0 amps. Am I safe to assume the 120 volt outlet can put out no more than ~ 10 amps?

How is the 240 to 120 derating calculated? Is the maximum 120 volt capacity 80% of the 240 volt capacity?

Following on this, what would be a "standard" configuration for the Max AC2 In and Max Charging Amps settings for a Xantrex SW2524 that would avoid stressing this size of a generator too much? I'm assuming no more than 7 to 8 amps AC @ 120 volts? The generator will run continuously on some days up to five hours.

mike95490 · February 2009

Re: Question About Actual Generator Output

A generator with 240V output, will only have HALF that output available on each of the 120V outlets (each 120V breaker should control a single or pair of outlets.)

It's because the Generator winding is like a center tap 240V transformer.

You can get rated power from the single 240 outlet (read the fine print for de-rating over time, altitude and temperature), or you can get half power from each 120V side, at the same time.

> Am I safe to assume the 120 volt outlet can put out no more than ~ 10 amps?

In your case, I would think that 6 amps is the max continuous from 1 leg, your manual and the ckt bkr on the generator should indicate what's what.

For better life of the engine, 80% (of the continuous - not surge) load should be all you pull.

And, unload the generator, and let it cool for a couple of minutes, before you shut it down, or else the windings may melt their insulation from "superheating" when the cooling stops. Also good for the motor too. (like a car that "boils over" when you stop at a gas station on a long trip.)

Battery charging. Depending on your batteries, You should strive to charge around their max permissible rate, to get the most "mileage" for your fuel.
Varys between 10 - 20% of battery capacity, varys by brand/chemistry.

Your battery charger's "Power Factor" rating may affect how much power it can pull from a generator, sometimes it's less than what it can pull from the grid, which appears as an "unlimited" source. A higher power factor (PF) is better, perfect is 1.

(PS I stand corrected on the avaib current, it would NOT be half of the original current, if the total 240V winding is good for 15A, then each 120V leg is good for 15A.
Late night, no coffee)

AlanRT · February 2009

Re: Question About Actual Generator Output

Thanks for that.

Now a related question that's giving me a brain fart.

If 2,950 watts at 240 volts = 12 amps, and one of 120 volt taps can supply one half or 6 amps, then 120 volts X 6 amps = only 720 watts per side?

Both sides = 1,440 watts total watts available? Where are the "missing" watts? (I know I'm going to feel like an idiot when I hear the answer.)

Mangas · February 2009

Re: Question About Actual Generator Output

If you have a propane unit at altitude derate it some more.

AlanRT · February 2009

Re: Question About Actual Generator Output

It's gasoline.

Also, when looking at the specs for a typical 3,500 watt Honda generator, it reads:

3500W max.
120/240V
(29.2/14.6A)

3000W rated
120/240V
(25.0/12.5A)

So where are they getting 25 total amps at 120 volts? Is this spread out across all the plugs: 2-120 volt/20 amp standard duplex, 1-120 volt/30 amp 3-prong twist lock, and 1-240 volt/30 amp 4-prong twist lock?

crewzer · February 2009

Re: Question About Actual Generator Output

Alan,

Don't know if this is the right manual, but check pages 18-20.

http://www.hondapowerequipment.com/pdf/manuals/31ZB4770.pdf

HTH,
Jim / crewzer

Ecnerwal · February 2009

Re: Question About Actual Generator Output

You have a number of things confabulating to confuse your issue. Sticking to the numbers you give (12A, 240V). 12A x 240V is 2880W, or 2880VA.

A generator which is driving 240V out at 12 amps can drive 12 amps on each 120V leg, at the same time. Mike is missing the point that half power at half voltage is the same amperage.

However, it's not very good for it to drive 12 amps on one and nothing on the other (unbalanced). Not "just won't work" but not mechanically good in the long term.

Some generators can be wired to actually put out EITHER (not at the same time) 12 amps at 240 V OR 24A at 120V. If you have one of those, and you have a 120V inverter, that would be the way to wire it up - keeps the load mechanically balanced. If the generator is the same as the one crewzer posted a manual link for, there's a switch for 120V only operation. Some others (usually older) have leads that can be reconnected for either mode of operation.

A third possibility is to take something like an X240, or a rather less expensive (possibly not quite as efficient, possibly that's just marketing) generic 3000VA (3.0 KVA) 120/240 transformer and run the generator (one that can't be wired for 120V only) into the 240 end, and slurp 120 out the other end. Sure, you only technically need 2880VA, but 3000VA is the typical size actually sold. This keeps the mechanical end of things balanced well and involves only a small loss (a few watts to magnetize the core), and that only while the generator is running.

AlanRT · February 2009

Re: Question About Actual Generator Output

Very helpful. I'll confirm model number this morning, and whether they have the 120V ONLY switch.

Unfortunately, I'm up in California at the moment and the gen is down on the Sea of Cortez East of Cabo San Lucas, Baja Mexico.

Cariboocoot · February 2009

Re: Question About Actual Generator Output

Mangas wrote: »

If you have a propane unit at altitude derate it some more.

Not just propane: high altitude decreases HP in all internal combustion engines. So so extremes in temperature, variations in fuel quality, and age. Decrease in HP = decrease in max. wattage.

The best thing to do is read the owner's manual. It should tell you exactly what the generator is rated at under what circumstances, including 120/240 differences and run/peak wattage. Then discount that figure for real world performance.

BB. · February 2009

Re: Question About Actual Generator Output

AlanRT wrote: »

So where are they getting 25 total amps at 120 volts?

Is this spread out across all the plugs:

2-120 volt/20 amp standard duplex,
1-120 volt/30 amp 3-prong twist lock
1-240 volt/30 amp 4-prong twist lock?

Since, in your case, the generator can provide maximum power at 120 or 240 VAC... Just watch the currents.

The 120 VAC duplex is limited to 15-20 amps maximum by NEC electric code and design (used to be 15 amp maximum, but is now 20 amps maximum for a standard North American outlet).

The other two outlets are rated at 30 amps--so they will allow full load on either outlet through one (properly rated/sized) cord.

When running your loads--just add up all of the current and make sure that it does not add up to more than 25 amps (for the 120 volt loads), and that you do not exceed 15-20 amps on the duplex receptacles (probably will pop a circuit breaker--no damage or danger).

For the 4 prong 240 volt receptacle--it can supply full current at either 120 VAC (one leg fully loaded to 25 amps; or 240 VAC with both legs at 12.5 amps maximum). So, you have two limits to look at here--the maximum current (25 amps max) and the maximum power (120 vac * 12.5 amps max). You can exceed neither.

And just so you understand, say you have two 120 VAC loads at 12.5 amps... You can plug both into a 120 VAC receptacle/leg and get 25 amp draw. Or you can put one load on the "A" hot to neutral leg, and put load "B" on the other hot to neutral leg--and you will now be drawing 12.5 amps at 240 VAC--still maximum power rating. When you plug between the two A/B phases, they share the power flow between the two "hot legs" and no current will flow through the "neutral leg"--Sort of like connecting your loads in series or parallel.

There is no difference to the 120 VAC loads which way you connect them... But for the wiring, the current flow it less (12.5 amps vs 25 amps) so there is less voltage drop and wasted heat in the plugs/cord/generator internals--a good thing overall.

-Bill

mike95490 · February 2009

Re: Question About Actual Generator Output

BB. wrote: »

The 120 VAC duplex is limited to 15-20 amps maximum by NEC electric code and design (used to be 15 amp maximum, but is now 20 amps maximum for a standard North American outlet).

There are actually 2 different receptacles.

One is the common |.| (best I can do in ASCII) @ 15 A
(NEMA 5-15) about 49 cents at big box stores

The other is |.╠ with the addition of a extra sideways slot.
These have better metals and beefier spring tension, to clean oxide from the plug, and they are rated 20A, they also take 15A plugs too.
(NEMA 5-20) about $6 at big box stores.

examples:
http://en.wikipedia.org/wiki/Domestic_AC_power_plugs_and_sockets#Type_B

Hospital grade is a whole different beast.

BB. · February 2009

Re: Question About Actual Generator Output

mike90045 wrote: »

Hospital grade is a whole different beast.

I always wanted to know--what is the difference with hospital grade receptacles besides orange plastic with a green dot?

-Bill

what, it doesn't cost way more?

sorry about this bill as i hit edit rather than quote. duh.:p

mike95490 · February 2009

Re: Question About Actual Generator Output

Hospital Grade:

(My guess - very tough connector contacts and low leakage plastic layout - water drips out, not in)

http://www.ul.com/regulators/ReceptacleGrades_Kurten_JF2005.pdf

Bottom of page 2 -
grade receptacles are specially designed and are subject to additional requirements of the standards. These include additional grounding reliability, assembly integrity, strength and durability....

And to withstand a cord yankout, with out breaking.

So the janitor can unplug the ventilator, and run the floor polisher, without damage to the outlet.

BB. · February 2009

Re: Question About Actual Generator Output

Thank you Mike!

-Bill

Lefty Wright · February 2009

Re: Question About Actual Generator Output

Bill, the green dot designates hospital grade. They can be any color.

Orange colored recepticles are those with isolated grounds. In these the ground lug is not electrically connected to the the receptile's yoke.

If you find an orange recepticle with a green dot it's a hospital grade with isolated ground.

BB. · February 2009

Re: Question About Actual Generator Output

Thank you LW,

I was being a little silly in my first reply. I always try to find the "best" sockets out there from the local home center. The cheap $0.99 sockets and light switch should never be sold in the first place. The fail so quickly--and the socket plastic breaks if the plugs are not pulled straight out.

I know that hospitals run isolated grounds... Sometimes I am not sure if that is a good thing or not. Sometimes ran into issues with ground loops in larger office buildings where they would sometimes run isolated grounds--would burn out RS 232 signal path/grounds for the old dumb terminals sometimes ("computer" plugged into isolated "computer circuit" and terminal plugged into local "junk collection" circuit--and local/remote grounds where at different potential).

-Bill

Question About Actual Generator Output

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