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Engine Block Heater


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I have charged the batteries and had the block heater on the same plug with no problems. The block heater need only be plugged in if you plan on starting the engine within 3- 4 hrs at 8 degrees F, any colder and you might want to add some time.

2004 Freightliner m2 106  2015 DRV lx450 Fullhouse  2019 Indian Springfield 2014 Yamaha 950 V-Star

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Block heaters typically use 1000 watts 8.6 amps (should be in the engine manual). Maintaining a battery might use 1 amp (probably less then 1/4 amp once is it totally charged). I have started my 7.3L diesel and my 12.8 L diesel down to 0*F without a block heater.


Why would you run the block heater unless your going to use it? If your heading out, then yes the 120 vac will probably work but that is a function of what else you may have on the circuit, how far away it is, or is it a dedicated circuit.

Very good reply.

Good travelin !...............Kirk

Full-time 11+ years...... Now seasonal travelers.
Kirk & Pam's Great RV Adventure



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Without knowing the block heaters wattage Id venture a pure "guess" a typical 120 VAC 15 Amp branch receptacle circuit (provided its NOT supplying any other high current loads or electric heaters etc and the run isn't excessively long) can power BOTH an engine block heater PLUS maintain a trickle maintenance charge to your batteries. Of course, an also typical 20 amp branch circuit would do likewise, but allow room for more current.


Lets run some numbers: If the engine block heater were 1000 to 1200 watts, even at 1200, that's around 10 amps of receptacle circuit current draw.


As far as the battery charger, if yours are already basically charged and its only necessary to provide say a low 2 amp trickle maintenance charger (like a small battery tender might do) that's something like 27/28 watts which corresponds over on the AC input side (accounting for inefficiencies and heat loss) to lets use 30 watts, which is only around 30/120 = 1/4 amp at 120 volts.


HOWEVER if you had say a 100 amp charger and your batteries were badly discharged??? such could require around 11 or so amps and if so those 11 plus 10 amps for the heater (if its 1200 watts, maybe its only 1000???) THATS TOO MUCH FOR A TYPICAL RESIDENTIAL 15 OR EVEN 20 AMP RECEPTACLE BRANCH CIRCUIT. My practice was not to exceed 80% max continuous load on a branch circuit meaning 12 amps max on a 15 amp circuit or 16 amps max on a 20 amp circuit. If you had a 20 amp circuit and the block heater were 1000 watts you could "get by" (100 watt heater plus 1000 amp charger running at max) but that's cutting it closer then I like, but I'm NOT saying it cant still "work" as it (charger) may not ever draw any full 100 amps for very long at all. (A very likely scenario)


BOTTOM LINE ANSWER: For a 1000 or 1200 watt block heater plus a low current 2 amp trickle maintenance battery charge YES a 120 Volt 15 amp receptacle branch circuit will work (or a 20 amp even better) as long as there's no excess voltage drop problem cause by a very long run. But if you were pumping like 100 amps into your batteries (which I doubt unless they were badly discharged when you plugged in plus you have a 100 amp capacity charger which is actually all being used into your batteries) then you could trip a 15 amp circuit breaker subject to how long and how much actual current. I would suggest a 20 amp branch circuit versus only a 15 if such were the case..............BUT even it will trip drawing 21 amps or even 20 over a long time period..........But it depends on wattage and current draw which I just don't know sitting here.


If I were fabricating such a circuit I would have it dedicated for ONLY the RV heater and charger and of course use a GFCI if its outdoors (may be subject to nuisance trips) and it would be a 20 amp circuit possibly with 10 gauge wire if the ruin is excessive in length.


Best I have to offer not knowing the block heater wattage and your battery charger capacity and state of batteries, sorry.


John T

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What is your purpose for powering the block heater constantly? to answer your question, it will run from a 120VAC circuit.


2000 Winnebago Ultimate Freedom USQ40JD, ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG retired;Good Sam Life member,FMCA ." And so, my fellow Americans: ask not what your country can do for you--ask what you can do for your country.  John F. Kennedy 20 Jan 1961


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