Join Date: Nov 2005
Location: Aberdeen, SD
Feedback Score: 0 reviews
Re: Block heater extension cord
OK, here's the official results- you could run mine off of a string of christmas lights.
I have been an electrician for 5 years, and have seen about everything. So, I'm pretty careful when it comes to stuff, and I repair power tools and electric motors for a living now. So, here's the results.
Start test ambient air temp, 7F above zero.
Resistance when cold (sitting for 3 hours) 14.7 ohms between the leads on the cord, clean connections, digital multimeter
Amp draw- 7.2 amps
That equals out to 864 watts when cold at 120.
After 15 min, it was drawing 7.0 amps.
And after 1 hour, 6.8 amps
resistance dropped to 12.8 ohms
and the wattage equates out to 816 watts.
After 1 full hour, That element is probably close to as hot as it will get, it will heat the fluid in teh immediate vicinity to the same temp as heating element, and then it will radiate out from there.
So, in closing, my stock (I'm pretty sure) block heater draws 7.2 amps max, well within the range of any common extension cord of I'd say less than 50 feet.
16 gauge wire=15 amps, 30C ambient
14 gauge wire=20 amps, at 75C wire temp
12 gauge wire-25 amps
10 gauge wire=30 amps
That is for THHN stranded wire in conduit, it was the only table I had handy. Standard household Romex wire standard is 14g- 15 amps, and 12g-20 amps, 10g-30 amps.
Of course voltage drops over distance, but you will still have very near your starting voltage even on a 100 foot cord, as long as it's in good condition. Voltage drop is usually more important in things like motor starting.
So, I'd say use a good, cold weather cord of at least 16g (which most are) and if it's farther, step up to 14g.
And one last note, not that it would hurt anything to use a 10g cord, but look at the cord coming out of the front of yoru truck, that's maybe a 14, maybe a 16. Hope this helps someone, Crexrun
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