• posted

Hi all,

Does anyone know with which wattage I should use in calculating the load requirement for a backup generator that I am going to install. I would like to hook up a hot water heater that has 2 elements "rated"(?) at 4500 wattts, giving a grand total of 9000 watts. This would be out of reach for the 7500 running watt generator that I plan to install. But, if I calculate the wattage based on 240Volts times the 30 amp circuit that it is on, I come up with 7200 watts...well within limits of the generator.

My question is: can I hook this up to the generator without tripping the thing? Of course, with the hot water heater being the only piece of equipment running at the time.

Sincerely,

Scott

• posted

Any info on whether both elements ever come on simultaneously? A load of

4500W could be given a 30A circuit breaker, a load of 9000W would not. FYI, typically a load on a breaker is limited to 80% of the breaker's rated current. j
• posted

j,

I was just in the middle of a long reply to you when I came accros the answer (I think)...

It is 4.5 Kw per phase leg... each on on leg of the 30A breaker. So now my question is:

Does the generator provide 7Kw TOTAL power, or is that a per phase leg number. I guess I'm off to call the manufacturer (unless you know :)).

• posted

Look at the power rating. The heaters would draw 9000/240 =37.5A The breaker would trip after a little while as it will only handle 30A on a continuous basis The 30 A figure is an upper allowable steady state limit limit and has nothing to do with the actual current taken by the load.

• posted

No problem. Caveat emptor, though.

I believe your latest line of thinking is a little bit off on this.

A 30A 2 pole breaker on a 120/240 V service will allow a maximum of 30A to a load at 240V. This is, as you calculated, 7200W. If you want to think of it as 'per leg', then you are sending 30A at 120V 'per leg', or 3600W 'per leg' for a total of 7200W. So it's the same either way, as it must be, when you reflect that it is ultimately the same situation you are describing either way. The load is maximum 7200W. Actually, let's say it is max

5760W, because we only load breakers to 80% (breakers are sized according to the load, such that the load is not more than 80% of the breaker rating).

I suspect that your HWT only uses 4500W at any time, if it is on a 30A 2 pole breaker on a 120/240V system. If I had a 4500W load on a 240V circuit, I'd calculate the load as 18.75A, and multiply by 125% (so that the load is a maximum of 80% of the ampacity) to get 23.4A. So a 23.4A breaker would be perfect. But you might find they are all out of those at Home Depot. I would go to the next available breaker size and use a 30A breaker (let's pretend 25A breakers don't exist). So you see, you have a HWT labeled with

4500W elements on a 30A 2 pole 240V breaker, which suggests that only one element will be on at a time. If all 9000W of heating came on, your breaker would probably trip in seconds or minutes. I assume it does not. A HWT using only one heating element at a time may not be uncommon, but don't take my word for it. If you're going to call someone, call the HWT people first.

If you have a generator rated at 7500W, that will be the total power it can put out. Be careful here, generator sizing is not necessarily simply a matter of "the amount of load I have is less than the size of the generator so I'm ok". That is a large load to step on all in one shot. Your load is pure resistive with no motor load, so I expect you'll be fine, your voltages will just go a little crazy for a couple seconds when the load comes on. But, that won't bother a hot water tank, which is just a resistor.. If you call a Cat or Cummins or whomever distributor they can probably set you straight in about 30 seconds. Anyone in sales can help you. Let us know what they say.

j
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snipped

Most water heaters that are on 30 amp breakers use one element at a time. Top for quicker recovery, bottom for heating up the volume.

Yes the generator supplies 7kw of power on both legs not each leg.

An generator should never be loaded beyond 80% of full load. Todays generators have an peak/surge number that gives some people an false sense of capasity. Plan your load on 80% of full load, not the peak number and you will have many hours of happy water heating.

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My last electric water heater would only run one element at a time. The bottom one was interlocked such that it could not be on when the top one was. When the top one reached temperature, it would switch off allowing the bottom one to switch on.

I imagine this is typical.

Tim

• posted

install. I would like

at 4500 wattts,

reach for the 7500

calculate the

is on, I come up

That sure is interesting about thier 9000 watt rating. I have no idea whats up with that.

On the gen set issue... here is gen set, lawn mower story. My bruddah is a world class motorcycle racer, (now into sprint cars)...and all of a sudden his lawn mower and gen set wouldnt run... on new gas right from the glorious gas station on the corner as usual.

So he put race gas in them (\$4.50 a galon). and they ran perfect... He thinks the gas companies are putting mouse pee in the gasoline so people will get lower gas mileage and have to buy more or the cheapo gas.... so accordingly I think you should get a 10kw gen set just on general principles because its probably not going to give you an honest 10k...probably more like 8k.

We have the same thing with pumps... the manufactured pump seldom does as well as the one the pump company gives the testing agencies.. that one is all polished up and carefully optimized... the one you get was thrown together by drunks.

Phil Scott

tripping the

piece of

• posted

Bottom line (I hope)...

I'm going with a maximum of 7200 watts for the water heater. That's all the system is set up to handle as it is now (240V X 30A). I'm gettin a 7500 watt generator, so it should be able to handle it.

Thanks for the input to my quandry...

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If you generator is 7500 watt under load your going to experence some serious voltage dips when you throw an 7200 watt load on it. You really should be loading the generator to no more than 6000 watts (80% of full output). Hey it is your unit and you paid for it.

snipped

• posted

SQLit,

I'm going under the assumption that that the 7200 watts is the max that the hot water tank will draw, and that if it were to do so that it would trip the 30A breaker on the generator. I don't believe that from what I've seen after doing a check on the amount of current that the hot water tank draws during normal operation (approx: 18 A) that it will even come close to the

7200 watt limit (or, within the 80% you mentinon).

Thanks for playing devils advocate...it helps me make sure I'm heading down the right path... :)

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