By Mike Sokol
Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.
My wife has one of those electric hot-water kettles which really heats the water up fast. But when I run it from my Honda 2000 generator the gas engine cranks up to full speed just to boil water. And even if we’re on 30-amp shore power and try to run the electric kettle and the microwave at the same time, the circuit breaker for the kitchen outlet pops off. How do I know how much power my water kettle is drawing? Could there be something wrong with it? —Mustang Sammy
Dear Mustang Sammy,
I suspect there’s nothing wrong with your electric water kettle, since water takes a LOT of energy to heat up quickly. In fact, my wife has a very similar electric kettle which draws around 12 amps of current when it’s heating the water. Why so much current? Well, water doesn’t want to heat up easily and has a lot of latent heat.
Let’s consider something like this 7-cup electric kettle from Magic Chef. In the specifications you’ll find that it’s rated for 1,500 watts of power. That is, from the moment you plug it in and it begins to heat the water, it’s going to be drawing 1,500 watts from the 120-volt outlet.
This is just as much wattage as your electric water heater might draw. But instead of having to heat up 6 gallons of water, you’re only heating up a quart or so. That’s why a kettle heats up the water much faster than a water heater tank. You’re essentially storing energy in the form of heat.
Let’s do a little math using Ohm’s Law to figure out just how much current that takes. Since volts times amperes equals watts, then watts divided by volts equals amperes (e.g., 1,500 watts / 120 volts = 12 amperes).
Now, 12 amps of current (1,500 watts) is a pretty good load for any 2,000-watt generator to supply, especially when you consider that your 2,000-watt generator is probably rated for 1,800 watts continuous power, with 2,000 watts for only short bursts of power.
Where’s the juice?
Any 2,000-watt inverter generator in eco-throttle mode is going to step up its engine speed to full throttle just to power your tea kettle for the 5 minutes or so that it takes to bring the water up to boiling. Again, that’s all quite normal since water needs a lot of energy to heat up.
Now, to your question about trying to run a microwave and an electric tea kettle at the same time on a single breaker in your RV when on shore power. That’s not going to work because your microwave (even a non-convection version) could be drawing up to 1,200 watts of power all by itself, and 1,200 / 120 = 10 amperes of current just for the microwave.
So if we add the 12 amps of current for the electric kettle to the 10 amps of current for the microwave, we end up with 22 amps on that breaker.
And remember that even if you have a 20-amp circuit breaker and 12-gauge wire, it’s not rated for a full 20 amps of continuous current. You have to derate it to 80% of stated capacity, which works out to 16 amperes of maximum current continuously for extended periods (10 minutes or more).
Is this normal?
So it’s normal for the 15-amp circuit breaker in your RV power panel to trip after 5 or 10 minutes of running an electric water kettle and a microwave at the same time. Many new RV users think that RV power is just like electric power in their home. But your house probably has a 200-amp, 240-volt service which is capable of supplying 48,000 watts of power, while the 30-amp pedestal outlet for your RV can only supply a total of 3,600 watts of power. That’s why you have to ration electric power in an RV compared to your house.
But wait, there’s more (or less)…
We just did calculations based on 30-amp shore power. What if you’re plugged into a 50-amp or 20-amp outlet? Well, you have to adjust your available power accordingly. A quick bit of math suggests that 20 amps of current times 120 volts equals 2,400 watts of power. So that’s all the power you have if you’re mooch-docking from a 20-amp outlet.
But a 50-amp, 240-volt outlet is really two separate 50-amp lines at 120 volts each, which adds up to 100-amperes of current at 120 volts. So if we multiply 100 amps of current times 120 volts we come up with 12,000 watts of power. That’s 5 times the amount of power from a 20-amp outlet, and 3.33 times the amount of power available from a 30-amp shore power outlet. See why we like 50-amp shore power when we can get it? You can run more appliances at the same time on a 50-amp shore power hookup.
That’s a wrap…
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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The wattage rating of the kettle should be found on the bottom of it. All appliances have their ratings listed somewhere.
My Forester RV has 30 amp service. As I read Mike’s comparison of 20, 30 and 50 amp outlets I wondered, if I use a 30 amp to 50 amp adapter on my cord to plug into the 50 amp outlet on the pedestal will I increase the power available to my RV?
No you will not. Your RV is still limited to the max amperage of your power panel main breaker, as well as the 30 amp max of your 10 gauge shore power cord.
In addition to Mike’s answer- there are 2 reasons you may want to be plugged into the 50a. receptacle. If you load your use to over 30 amps- it’s likely the breaker in your rig will trip- rather than the one side of the 50 at the pedestal. This would be my choice since it’s much easier to reset the one in your rv instead of having to walk outside. Also, the wider blades on the 50 a. side do a better job of carrying heavier loads and dissipate heat better also. I believe they should redesign the tt 30 plug and receptacles for these reasons. Those are the ones that wear out much more quickly in parks and you’ve probably noted for yourself how loosely they seem to mate with your plug.
Minor correction, latent heat is the energy needed for a phase change in a substance, freezing and boiling in water for instance. Water has a very high specific heat (capacity), meaning that it ‘holds’ a lot of energy when heated and thus takes a lot to heat. Water is a very strange substance.
We sometimes dry camp with solar panels plus we occasionally find ourselves having only 30 amps. We have a 37′ Fifth Wheel so energy management is crucial. I have made a list of what each appliance draws by turning everything off and turning on one thing at a time: water heater, fridge, microwave, coffee maker, fireplace, ACs, hair dryer, etc. etc. We have a gauge but most appliance manuals have this info as well. Our fridge and water heater have a propane option and we have an EasyStart on one of the ACs. It all helps.
Thank you for this explanation. I had the same issue.
2000 watt Honda inverter generator is only rated for 1600 watts of continuous use, barely enough for the 1500 watt device when running at full throttle. Just be thankful there is no significant start up surge current with the tea kettle.