wiring 120vac elements to 240vac

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HDNB
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wiring 120vac elements to 240vac

Post by HDNB »

Anybody smart enough to do the math?

as an adventure in electricity, i'm looking to use 2 120VAC elements, but use my 200vac controller to run them. my brain sez run them in series...but someone here actually knows tha math and it's not me.
SO:

2 x1500 watt elements. they measure 35 ohms. in parallel, 17 and in series i'm showing 62.

i ran a single element up to about 5 amps 240vac, on my controller and it was working nicely, i didn't want to crank up the current anymore than that as the heat was sufficient and i didn't want to send the whole thing up in a shower of sparks and smoke.

the plan is to use 2, to reduce the watt density on the elements. in series at 62 ohms, it will be easier load on the ssr....the ssr is rated to 40 amps and i have pretty fine control over it....

anybody know what i need to know, without the experiment ending badly? like can i run more current through the two in series at 62ohms at a higher current to get the same heat?
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Re: wiring 120vac elements to 240vac

Post by zed255 »

120V elements will be fine wired in series on 240V so long as they are the same wattage. Unequal wattages would skew the voltage split, overdriving the lower wattage and under driving the higher wattage.

The current is common in a series circuit but voltage sharing relies on equal resistances. Ohms law, baby.
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Re: wiring 120vac elements to 240vac

Post by HDNB »

they are the same insofar as the manufacturing will allow. and the resistance values are the same according to my fluke.

any idea of how much gain i can put into them before the factory smoke comes out? i'm thinking about 7.5A at 240vac?
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Re: wiring 120vac elements to 240vac

Post by sambedded »

HDNB wrote: Tue Oct 20, 2020 7:33 pm

any idea of how much gain i can put into them before the factory smoke comes out? i'm thinking about 7.5A at 240vac?
If you connect them in series - up to 12.5 Amp will be completely OK
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Re: wiring 120vac elements to 240vac

Post by Yummyrum »

HDNB wrote: Tue Oct 20, 2020 5:01 pm 2 x1500 watt elements. they measure 35 ohms. in parallel, 17 and in series i'm showing 62.
This doesn’t make sense HDNB .

1500Watts/120Volts= 12.5Amps

120Volts / 12.5Amps = 9.6 Ohms per element .

That should be 19.2 ohms if both in series .
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Re: wiring 120vac elements to 240vac

Post by still_stirrin »

Agree with yummy, the resistance of each of the elements rated for 120VAC should be roughly 9.6 ohms:
VxV / P = 120 x 120 / 1500 = 14400/1500 = 9.6 ohms.

Here’s a chart that relates Ohm’s Law:
Ohm’s Law
Ohm’s Law
50C1920B-478D-44CC-A6DE-74B73A0C6C06.gif (7.8 KiB) Viewed 1529 times
If you’re planning to run the elements from your 220VAC (240VAC peak) source, then you would connect the elements in series, with them becoming a “voltage divider” circuit. That would make each element subject to 1/2 the line voltage, or 1/2 x 240VAC. That would still draw 12.5 amps and make each element dissipate its rated power (1.5kW).

To check, you’d input 220VAC x 12.5 amps = 2750 watts of energy into the boiler, or 3kW if full rail voltage (240VAC) was provided.
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Re: wiring 120vac elements to 240vac

Post by still_stirrin »

HDNB wrote: Tue Oct 20, 2020 5:01 pm Anybody smart enough to do the math?

2 x1500 UNKNOWN watt elements. they measure 35 ohms (each). (And) in parallel, 17 (ohms,) and in series i'm showing 62 (ohms)...can i run more current through the two in series at 62ohms at a higher current to get the same heat?
If running in series, your elements would draw: 240 V / 62 ohms = 3.87 amps. That is only: 240x240 / 62 ohms = 929 watts!

But, as you’ve said, they are 120VAC rated, so you should not run them in parallel. The dielectric breakdown strength may not tolerate that voltage and they very well could burn out at that current draw. The resistance would skyrocket as the resistance to current flow caused breakdown of the conductors, ie - very small gage wires.

incidently, the current draw would be 240 V / 35 ohms = 6.86 amps through EACH element. That is 240x240 / 35 ohms = 1646 watts (for each element). But your power circuit would be providing 2 x 6.86 amps = 13.72 amps, and that is: 240 V x 13.72 amps = 3293 watts.

But, if your elements are not rated for the 240VAC source, it could make the elements dangerous to operate in parallel.
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Re: wiring 120vac elements to 240vac

Post by sambedded »

HDNB wrote: Tue Oct 20, 2020 5:01 pm Anybody smart enough to do the math?

i'm looking to use 2 120VAC elements ..

2 x1500 watt elements. they measure 35 ohms. i
As it was said already 35 Ohms for 120V/1500Watt element is incorrect. However 35 Ohm is a valid cold resistance for 240V/1500Watt element.
Could you double check element's spec?
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Re: wiring 120vac elements to 240vac

Post by jonnys_spirit »

I work with vacuum tubes a bit and need to solve resistance of the heater element in a vacuum tube under operating conditions because the resistance is different when heated and when cold.

I've got an ammeter on my still controller and also note that the amps wander a bit then settle as I change the power setting which is an indication of the same behavior - as the element temp varies the resistance changes until it stabilizes (even with small changes in the power setting). Not sure exactly how much it varies on a still element but there is some variance.

Just an observation.

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Re: wiring 120vac elements to 240vac

Post by HDNB »

Well you guys still know what yer talking about. Rechecked the spec and they are 450w at 120v, so using the math above it looks like I can run just under 4 Amos at 240 for a total of 900 w and not blow anything up
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Re: wiring 120vac elements to 240vac

Post by sambedded »

HDNB wrote: Wed Oct 21, 2020 3:00 pm they are 450w at 120v, so using the math above it looks like I can run just under 4 Amos at 240 for a total of 900 w and not blow anything up
3.75 Amp to be precise. Although they will probably withstand 4 amps as well.
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Re: wiring 120vac elements to 240vac

Post by jonnys_spirit »

Interesting point - two equal resisters in series to gnd (or neutral) make up whet they call a voltage divider. Where at the junction the voltage is 1/2. The same amount of amps go through both resistors and power = Vdrop * Amps so the drop across each resistor (element) is approx 120V which would make 3.75A for 450W. Measure actual voltage in circuit operation (carefully!) to get a more accurate max amp value and wattage output. Your line voltage may likely be less than 240V in practice.

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Re: wiring 120vac elements to 240vac

Post by still_stirrin »

HDNB wrote: Wed Oct 21, 2020 3:00 pm Well you guys still know what yer talking about. Rechecked the spec and they are 450w at 120v, so using the math above it looks like I can run just under 4 Amos at 240 for a total of 900 w and not blow anything up
Not much power there. I’d look for some other elements. In fact, 4.5kW/240VAC elements are about the same price and will give you a lot more heating possibilities for single or double elements, series or parallel. Plus, they’re ULWD foldback design.
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Re: wiring 120vac elements to 240vac

Post by HDNB »

heh heh heh. the experiment ended badly.

i wired them series and had no problems, full output on the ssr was using 3.8a at 240v....but was not enough heat for the application.

sooo....being a curious fool i wired parallel and ran them 5a....yay! lots of heat....8a wooo! smokin' hot.... 10A and it was all over but the crying.

on the bright side they we old used elements and now i can go shopping and get a nice new 240 5500w and get on with it...

Thanks for the advice guys, you knew what would happen and now i do too!
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Re: wiring 120vac elements to 240vac

Post by still_stirrin »

Told ya.

Yer lucky you didn’t burn out your controller....or did you?

I ran the numbers and gave you a caution. Your results are because you chose to disregard what you read. Just lucky you didn’t do more (serious) damage.
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