Converting 1500W 120VAC Space Heater to 48VDC: Resistance and Current Calculations

Looking to convert a 120 vac 1500 watt space heater to 48 vdc.   That means that the current information is:
Voltage = 120 vac
Wattage = 1500
Current = 12.5 amps
LOAD Resistance = 9.6 ohms.
I want to convert to operate on a 48 vdc system. Am I correct in assuming the resistance load doesn't care about the type of voltage AC or DC?  If the input is changed from 120 to 48 then is the following assumption correct?
Voltage =  48 vdc
Wattage = 1500
Current= 31.25 amps (I = E/R)
Load Resistance = 9.6 ohms
I understand that if the original heater has a fan, that it will not work on DC.  It will have to be converted. 
Please advise/comment!
Thanks,
Sid

Hi Sid-You are correct on all counts.  The heating element doesn't care if the voltage is AC or DC, and the fan will have to be replaced.What kind of heating element are we talking about here? I certainly would not recommend stepping the voltage up to operate a 120 volt heater. I would find a lower voltage heating element, or possibly find a way to split the existing element into parts that could be reconnected in parallel.   Can you provide more information about the heater or post pictures?-Rick

Thanks for your reply Rick!  I don’t have a 120 vac heater yet. Was going to buy one.  I’m thinking a 1000 to 1500 watt heater.  My calculations were for 1500. This is for my golf cart.  There is a 48 volt heater commercially available, but it’s about $300 and it’s only about 500 watts.  
I’ve seen IR heaters that might work and they don’t have fans.  I’m curious if I just apply 48 vdc to an off the shelf 120vac, what will happen?Interested in your (and others) opinion.  

Sid - not quite correct as Rick said, the resistance of your heater at 48V will be 1.536 Ohms, not 9.6 as you stated. (48V / 31.25A)if you take your original element and divide it into 6 equal parts and connect them in parallel, the resistance would be 9.6 / 6 = 1.6 ohms.This would give you (on 48V)   V2  / R   =   2304 / 1.6   =   1440 W - I don't think you'd notice much difference.  You could maybe put taps in the existing element without breaking it.If your fan motor is across the 120V, it would probably be an AC motor and you'd need to replace it with a DC motor.  However if it is in series with the heater or part of it, it would probably be a brush motor (hairdryers usually have this arrangement) and you could possibly work it in with one of the new elements to work ok.

I thanks David.  Perhaps to simplify my question:What will be the result if I hook a 120vac 1500 watt heater to 48vdc?  I know the fan will not likely work, but IR heaters don’t have a fan.  What would be the current draw?  What will be the heat output?  Will the heater continue to function?Thanks for the feedback!

Sid,At 120V, your 9.6 ohm heating element will draw 12.5 amps from the power source which will give you 1500 W.At 48V, the same 9.6 ohm heating element will only draw 5 amps from the power source which only gives 240 W. As David pointed out, in order to get 1500 W of heat output, you will need a much lower resistance heating element. If it's possible to divide the existing heating element into 6 parts in parallel then you could do it, otherwise it's not likely to work.There is also the problem of making sure that you have heavy enough wire to source 31.25 amps from your 48 volt supply.

Sid, a picture would help.  You mention this is an IR heater.  I am thinking now of a heater I saw in a warehouse, it had coiled wire in glass tubes and glowed a very dull red, but boy did it put out heat.  If it's one of those, it would be impractical to tap the element(s) as I suggested.  But I may be wrong here.  As Elizabeth says, running it at 48V would result in a much reduced output.If I am right, your only option would be to get a 1500W 48VDC - 120VAC inverter, but a very expensive way to go....

Sid,
As an alternative, you could get four 12V 150W cheap car heaters like this:
https://www.walmart.com/ip/Greensen-2IN1-Port...ter-Defroster-Demister-12V-150W/925275146Wire them in series and you will have a 48V 600W heater. You'd want to have them all be the same and work out some way to make sure that they are all turned on at the same time so you don't burn them out through over-voltage. One advantage is that you should be able to position them better than if you just had one big heater. 600 W should be enough for a golf cart but, if not, you could add another string of them.

Elizabeth - you're hot on the case   First time I've seen something like this, but 2 banks of 4 of these would give 1200 W - not much less than the original 1500W.  And for only $80.  A steal....

Great comments Elizabeth and David!  I’m afraid 4-8 12v heaters wouldn’t leave room for me in the golf cart!  
If I remember correctly, the internal resistance of a heater (I.e.nichrome wire) varies with the applied voltage.  Therefore , the resistance of the 120 volt coil will change with 48v applied. (Am I thinking correctly?)I think I’m going to buy a cheap 120 volt space heater and apply 48 volts dc instead and try to measure the outcome.  If I survive, I’ll report my results.(I haven’t even mentioned the fact that 1500 watts may deplete my batteries and leave me stranded on the course somewhere!)

Sid a golf cart is not large and 1500W is more likely to cook you than warm you.  I'd suggest that one or two of Elizabeth's 150W heaters would be adequate.  You can get modules that step down from 48V to 12V but as you'd need 12.5A at 12V it would have to be beefy.You'd be better off as you say running a 120V heater at 48V - it would not put out as much heat but probably enough for your golf cart.  Your fan would be the problem but you could maybe replace the motor.Another thought.  Your golf cart probably has 4 x 12V batteries in series.  You could run a 12V heater off one battery only. However you'd then run that battery down a LOT quicker than the other 3.  Maybe get a switching arrangement to put the heater on any of the 4 batteries and use them in turn.Final suggestion - give up golf in the winter and take up electronics! 

Sid - re your comment:> If I remember correctly, the internal resistance of a heater (I.e.nichrome wire) varies with the applied voltage.  Therefore , the resistance of the 120 volt coil will change with 48v applied. (Am I thinking correctly?)
You are, but the effect will be that at 48V the wire will not heat up as much, it will have lower resistance and will allow more current to pass.  However the effect will not be marked, Elizabeth calculated you'd get 240W off a 12V 1500W heater run at 48V, in practice you might get 250 or 260W.  So don't worry about this factor.

Sorry, but despite extensive training and teaching in electronics, golf is my passion!  I’ve been using a propane heater that isn’t too much heat. Golf carts don’t seal very well, plus opening the door frequently exchanges air often.  Just don’t like portable propane heater taking up room not to mention the possibility of being burnt.  My cart has 6-8volt batteries so tapping 12vdc is a little more complicated.  I’m gonna try a ~$20 electric space heater to see what happens.  (I may not need 1500 watts, so whatever the output it may suffice. )

....or you could just buy an Electric Golf Cart Heater.  Note that the store-bought version is 480 watts.
-Rick

Rick... nice detective work there.  But it says "Doesn't drain your cart's batteries" but lower down "10A @ 48vDC (MAX) - Heating Element".  Do you know how that works?   Maybe it's one of those much touted devices that generates more than you put into it...    But seriously, that's probably the best solution so far.  Sid is obviously not the only winter golfing nut around!

The one for sale is over $300.  That’s why I’m trying to be more creative!

Sid yes, I did have a sharp intake of breath when I saw the price.  So go back to Elizabeth's Walmart heaters, 4 of them are not going to take up that much space and will give you 600 watts which I think should be more than enough, and they'll be 40 bucks. They're fan heaters so will warm up a volume quickly. As Elizabeth says, you need to have them all on heat, if you had one on fan then most of the 48V would appear on the one on fan and likely burn out the fan in that one.  You could probably make a mounting bracket that would take 2 of 4 on top of one another with not much effort.This would take 12.5A at 48V so you'd need to make sure your batteries are beefy enough, and not use them except when you need.  Else you might be looking at a long cold walk back to the clubhouse....Worth a try maybe?