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7.3L IDI Diesels (Not Power Strokes) Technical discussion of topics related to vehicles powered by the 7.3 Liter In-Direct Injection Navistar engines.

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Old 12-06-2005, 02:57 PM   #1 (permalink)
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Block heater extension cord

Just wondering what size of extension cord I need to run the block heater? Will probably run 25 to 40 ft cord? How many amps does the heater draw?
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Old 12-06-2005, 03:02 PM   #2 (permalink)
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Re: Block heater extension cord

I was wondering the same thing, I'll throw a test cord on mine today and run an amp check. I own an electric motor sales and service business, so one of the fringe benefits of that is I have lots of cool toys to play with! I'll let you know. I'm sure they take more when cold due to higher resistance, but I don't imagine they change a drastic amount as long as they are mostly cool when starting. I'll check in a little while and post the results! Crexrun
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Old 12-06-2005, 03:15 PM   #3 (permalink)
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Re: Block heater extension cord

When in doubt, use a 12ga. 10ga would be a little large and $. 12ga should be at your recepticle depending on age of structure.
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Old 12-06-2005, 03:30 PM   #4 (permalink)
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Re: Block heater extension cord

yeah - just use the shortest 12ga you can get away with.
i like the better quality cords like 'yellow jackets' & the like.
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Old 12-06-2005, 04:15 PM   #5 (permalink)
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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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Old 12-06-2005, 06:48 PM   #6 (permalink)
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Re: Block heater extension cord

Hey crexrun, nice post!

[img]/ubbthreads/images/graemlins/wink.gif[/img]
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Old 12-06-2005, 07:34 PM   #7 (permalink)
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Re: Block heater extension cord

I use 16 or 14 gauge cord. 25' to 50' (depending on where I park it) I plug in my '88 anytime that the nighttime low will be below 40-45 degrees. I can start the engine without it having the block heater on overnite, but it's just so much easier on the starter if it's been plugged in.
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Old 12-06-2005, 08:45 PM   #8 (permalink)
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Re: Block heater extension cord

Nice post crexrun everything you said makes sense but one thing I noticed is when I use a heavier cord the lights in my barn seem to only flicker when I plug my truck in but when I use a smaller cord the lights seem to dim plus for the money I rather be safe then sorry [img]/ubbthreads/images/graemlins/shrug.gif[/img] [img]/ubbthreads/images/graemlins/warmsmile.gif[/img]
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Old 12-07-2005, 08:10 AM   #9 (permalink)
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Re: Block heater extension cord

Thanks guys, I just wanted to get the information, for myself too. As far as the lights dimming, electricity is a strange animal. It's one of those things that "shouldn't" but it does, know what I mean? Personally, when I get a cord, I always get a heavy one, just because, but I just wanted to find out if it was necessary. The only reason that the lights would dim is the resistance in the lighter cord in conjunction with the heater makes more resistance, hence slightly more load, so in your case, I'd go heavier too, but a lighter (16-14g) wouldn't be a safety issue in my opinion, there must be quite a bit of draw on that circuit already to cause any flicker or dimming, or possibly some resistance in the barn wiring. Nothin worth worrying about, it could be as simple as an old outlet. Well, I suppose, back to the old grind, have a good day, Crexrun
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Old 12-07-2005, 10:35 AM   #10 (permalink)
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Re: Block heater extension cord

I just go to the depot and buy the thickest 1 they have they have a nifty blue one that stays flexable when really cold
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Old 12-07-2005, 04:17 PM   #11 (permalink)
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Re: Block heater extension cord

Thats great crexrun the info is appreaciated I use 25' 12 gauge cords for my trucks just for safety the barn and wiring is new so its not an age problem I was just pointing out how a heavier cord is better with out going in to the tech part [img]/ubbthreads/images/graemlins/shrug.gif[/img] [img]/ubbthreads/images/graemlins/warmsmile.gif[/img]
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Old 12-07-2005, 07:28 PM   #12 (permalink)
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Re: Block heater extension cord

I gotcha. I like heavy cords too. Just seems like I never have one around when I need one! Crexrun
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Old 12-07-2005, 10:45 PM   #13 (permalink)
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Re: Block heater extension cord

What gauge are the common orange ones with 3 prongs 2 hot and a ground...like we use for Christams lights and power tools? Taht's what I have been usinga nd no indications of an unsafe condition.
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Old 12-07-2005, 10:59 PM   #14 (permalink)
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Re: Block heater extension cord

more than likely a 14ga cord, JD.
it'll be ok, but feel how warm it is after 2-3 hrs with the BH on... that is called resistance [img]/ubbthreads/images/graemlins/phoney.gif[/img]
again, i like a big fat HD 12ga ext. cord for 120v use. regardless of length (i have everything from 9ft to 100ft cords).

edit:
i don't think you can find 10ga 120v cords in stores... if you thought it was necessary (200 ft run?), i bet you'd have to make it yourself.
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Old 12-10-2005, 02:09 AM   #15 (permalink)
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Re: Block heater extension cord

I have several 100+ foot 10 gauge 3 conductor ex cords on the service truck. FOR the life of me I cant remember where I got them? white cap or grainger maybe? Thanks to osha I have to have these if its a decent sized jobsite where BS rules are king. They even came with the "proper" twist lock connectors on the ends. I had to make small pigtails to acutally use them anywhere other than the damn temporary junction box or generator.
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