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  1. #1
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    Default Can I replace a two-prong power cord with a 3-prong power cord?

    We have one of those oil filled convection heaters (made by Delonghi, ? circa 1989) that I would like to get working safely. It works fine...no oil leaks that I can see, and the switches and thermostat seem to work OK. It heats up nicely.

    The problem is that the power cord is quite deteriorated, and should be replaced. It is a two-prong polarized version. The outside chassis of the heater is metal, and there is no insulated inner core between the inside wiring and the outside metal case, as I would expect to see in modern appliances with a two-prong plug.

    What I would like to do is to replace the power cord with a three-prong version, and attach the ground wire to a convenient point inside the metal chassis.

    Would this be a reasonable and safe thing to do? Am I correct in concluding that the outside metal chassis of the appliance should be the part that is grounded??

  2. #2
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    Default Re: Can I replace a two-prong power cord with a 3-prong power cord?

    You have n Ohm meter or continuity tester right?

    I would check continuity between the metal case and the neutral side of the cord feeding the heater. If you do have continuity and can not figure out how to break the connection inside the heater then I would not install the three wire cord as you describe because you will share current on both the ground and neutral back to the panel.

    If not then I think your plan would be okay.

  3. #3
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    Default Re: Can I replace a two-prong power cord with a 3-prong power cord?

    Hey kentvw. Thanks for the response.

    The question (for me at least) is now entirely academic, as it turns out there is an oil leak from the unit after all. It is scrap. We've gone out and bought a new one.

    However, I still would like to be clear on the answer anyway. I can certainly check for continuity between the metal chassis and the neutral wire in the plug. I assume that if continuity is found, then that would be abnormal, or at least not meeting current safety standards. Would that be due to an internal fault in the wiring, or by deliberate design of an older appliance?

    Am I correct that modern appliances should never rely on using the neutral conductor for case grounding? In that situation, if an open neutral fault developed, then the appliance would not work, but the entire outside case would be live.

    To meet current safety standards, the outside case should be bonded to ground via the third prong. If a connection developed between the hot and the metal case, then the current would dissipate through the ground wire. If a connection developed between the neutral and the metal case, then current would flow through both the neutral and the ground. In both cases, if the appliance was plugged into a GFCI protected circuit, then the GFCI would trip.

    The alternative is that the outside case could be electrically isolated from the inner circuitry if there is a two-prong plug, making the ground prong unnecessary.

    Am I correct?

  4. #4
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    Default Re: Can I replace a two-prong power cord with a 3-prong power cord?

    My best guess is that the heaters heating coils and associated electrical are “double insulated” since it just has a two prong plug.

    But I can also tell you that until recently we were allowed by the US NEC to use the neutral conductor and bond the metal cases of dryers and ranges to the neutral and ground it that way as long as it was taken back to a panel where the ground and neutral were bonded together. These are 220 volt appliances. I’ve never known of a 120 volt appliance to have this exception.

    Okay, I have a question or two for you. I have been considering an oil filled heater myself. How many watts? Is it just low, medium, high or does your new one have a thermostat like the old one? How big of a space are you heating?

    I ask because I am ripping and stripping back economically even more this year than years past. I have a small 10’x11’ study off of my kitchen/living area that can be closed off with French doors. I figure I can pretty much spend my evenings in there surrounded by phone, small TV, PC, all of my books and a couple of wing back chairs. I would like to stay at a comfortable temp in there but the rest of my home could stay at 62/64 F pretty much around the clock. I figure I only need about 1000watts at 120volts to get my heating needs covered.

  5. #5
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    Default Re: Can I replace a two-prong power cord with a 3-prong power cord?

    I believe the exception for grounding ranges and dryers through the neutral is permitted to allow for the wiring in older homes which usually has only a three-prong receptacle for the range and dryer. Even the writers of the code realize it is political suicide to require anyone to rewire their house just because they bought a new range.

    I don't know when the code changed, but it now requires a four-prong receptacle for ranges and dryers in new installation or remodels. You can still use a three-prong cord if all you are doing is replacing the range or dryer.

  6. #6
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    Default Re: Can I replace a two-prong power cord with a 3-prong power cord?

    Thanks for the answers...very helpful as usual.

    This old heater I was talking about might have a double insulated heating element inside the oil gallery, but the rest of the electrics are wide open inside the metal case. If it is double insulated it sure is not obvious. That's why I was concerned and asked the question regarding grounding the chassis when I replaced the power cord. As I mentioned, I junked it when I discovered an oil leak...I'm guessing the heater was 20 years old. I bought a new version of the same heater, and the basic design looks like it has not changed much...except that the power cord has the grounding prong as well. I suspect the manufacturers just updated the design to modern standards.

    So...kentvw: here is what I found about space heaters: the manufacturer Delonghi has a good reputation (Italian design...usually good). I checked some of the consumer reports on the web. Also, our friends have one of the oil heaters and they like it. We bought a Delonghi HHP1500 Micathermic space heater to heat a room about 12' x 15'. The heater has two settings (750W and 1500W) and is thermostat controlled. We also bought a Delonghi MG15EW oil-filled radiator heater to replace the old one that started this discussion. It has 3 power settings (700W, 800W or both: 1500W) and is also thermostat controlled. It heats a room 10' x 10'. We live in Vancouver, BC (climate similar to Seattle) in a 60 year-old house that is poorly insulated.

    Here are my impressions of both heaters after a few days use: the Micathermic heater has a bigger footprint, but is flatter in profile. It heats up quickly and starts putting out heat immediately, while the oil-filled radiator takes a while to warm up. The oil filled heater provides only convective heat, while the micathermic heater provides both convective and radiant heat. I would expect the oil-filled heater is more energy efficient, and is best at keeping a room at a steady temp, while the radiant micathermic heater seems better at quickly warming up a cold room. They were the same price ($70 CAD).

    The micathermic heater has only a two-prong cord, so I expect the electrical innards are double insulated. The new oil-filled radiator has a 3-prong grounded plug, as discussed above.

    So...my initial impressions are that either heater would be good for your 10x12 study. If you want to heat up the room quickly, go for the micathermic model. If you want to keep it at a steady temp for long periods, go for the oil-filled radiator. Or buy both at a big-box store, try them out and then take one back. These are just my initial opinions after only a couple of days use. Ask around and check the web for consumer reports and differing opinions. Let us know what you decide. Hope this helps.

  7. #7
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    Default Re: Can I replace a two-prong power cord with a 3-prong power cord?

    Incidentally, the reason that 125V portable space heaters are limited to 1500W is because it's expected that they are going to be plugged into 15A circuits, which have a maximum capacity of 1875W (125V x 15A). Since for a continuous load you're supposed to derate the circuit to 80%, this gives you a maximum 1500W. Hair dryers, on the other hand, go up to 1875W because they are not considered continuous load appliances, and aren't going to be left unattended.

    If you want to go with a higher wattage heater, you'll have to go with a 250V heater instead, which of course will require a dedicated circuit.

    (You'll notice that I used 125V instead of 120V or even 110V. Electrical wiring devices are NEMA-rated for 125V or 250V. This includes receptacles, switches, breakers, fuses, and fixtures.)

  8. #8
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    Default Re: Can I replace a two-prong power cord with a 3-prong power cord?

    (You'll notice that I used 125V instead of 120V or even 110V. Electrical wiring devices are NEMA-rated for 125V or 250V. This includes receptacles, switches, breakers, fuses, and fixtures.)[/QUOTE]

    NEMA ratings on the above mentioned devices have little to nothing to do with calculations.

    I'll roll with recomendations made in Annex D of NFPA 70 and the thousands of engineered load calculations I look at over the course of any given year and they are 120/240, 120Y/208, and 277/480.

  9. #9
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    Default Re: Can I replace a two-prong power cord with a 3-prong power cord?

    Tell us fencepost. Please back your post regarding voltage and calculations...... Why? Why I ask do you continue after all these years to offer your dis - information?

  10. #10
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    Default Re: Can I replace a two-prong power cord with a 3-prong power cord?

    Kent ... give some slack .... apparently fencepost is a licensed plumber .
    "" an ounce of perception -- a pound of obscure "
    - Rush

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