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.
I just had this question asked on my RVelectricity Facebook group about (hot) water heater (hwh) amperage. Since so many of you seem to be tripping circuit breakers, this is a good opportunity to go over the basics of current draw from various RV appliances, starting with the formula to calculate amperage draw from known wattage. I promise you that the math is pretty easy, so don’t get too worried. Once you learn this you can calculate the amperage draw for most any appliance.
Let’s review the basics of volts, watts and amperes and how they interact. The basic formula is volts (E) times Amps (I) is equal to Watts (P). This is normally shown as E x I = P in most formulas. Here’s an Ohm’s Law chart for reference, but you can ignore most of it for this simple lesson.
As you can see from the P (Power) section in the lower left part of the Ohm’s Law chart, P = E x I. Now if we want to calculate amperage (I) and know the voltage and wattage, you can see in the upper left part of the circle that I = P/E or Amperage = Power divided by Voltage. Since your water heater element is likely 1,200 watts, and we know it’s 120 volts, we just have to divide 1,200 watts by 120 volts and find 10 amperes. That’s 1,200 watts /120 volts =10 amps. That’s the direct answer to your question.
To take this further, if we know the wattage that anything is rated for and the voltage it’s connected to, we can easily calculate the amperage it should draw. For example, another question recently came up asking how much amperage a 1,200 watt Instant Pot pulled, and the calculation is the same. That’s 1,200 watts divided by 120 volts = 10 amperes. Pretty cool, eh?
So a 1,500-watt hair dryer works out to 1,500 watts divided by 120 volts equals 12 amperes. And a 100-watt television set would be 100 watts divided by 120 volts equals 0.833 amperes.
Finally, an electric space heater on the 600-watt setting would be 600 watts /120 volts = 5 amperes. But on its 1,200-watt setting it would calculate as 1,200 watts / 120 volts = 10 amperes of current.
And so on, and so on….
Just a quick hint that you can also use this same formula to calculate how much current any 120-volt appliance would draw from your 12-volt batteries. Just plug in 12 volts to the equation and you’re good to go.
So a 1,500-watt hair dryer powered from an inverter would calculate as 1,500 watts / 12 volts = 125 amperes from the battery. Yikes!!! That’s a lot of amperage which would drain your house battery rapidly, probably in 20 or 30 minutes tops.
And a 1,200-watt Instant Pot calculates as 1,200 watts / 12 volts = 100 amperes from your battery. Now, it’s not going to be drawing 1,200 watts once it heats up since it goes into keep-warm mode, but I’ll guess it could completely drain 50% of 100 Ah battery capacity in an hour or so.
Also, as an aside, a 15,000 BTU air conditioner will draw around 14 amperes or so while the compressor is running, but up to 50 amperes or more for less than 1/4 of a second with a stock starting capacitor when the compressor kicks in. That’s what trips a lot of pedestal circuit breakers and overloads generators. Here’s a chart I made of those air conditioner startup currents.
The lesson here is there’s lots of energy-hungry appliances in an RV that can quickly add up to the 30 amperes available from your shore power pedestal connection, which will quickly overload your 2,000-watt inverter. So it’s best to avoid using more than one high-wattage appliance at a time in your RV, unless you have 50-amp shore power plug on your RV and you’re plugged into a 50-amp pedestal.
For more about appliance wattage and amperage, read my article HERE.
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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