Electrical Breaker 'cheat'?

Hi all,
This is a code question more than anything, but I thought it might be worth looking at.
I recently had a 240V electrical heating circuit added. #12 wire, 20A breaker. Max load is 3840W, but I want to drive 4500W. Unfortunately, 25A breakers aren't available for my panel (this would fix the problem) and I have no additional circuits left in my panel.
One other solution is to use 30A breakers and switch to #10 wire. Problem is that one heater was already in-place (and on a different floor) so I'd have to demolish the house to re-wire it. Is it acceptable to run #10 wire from the panel to the first thermostat (3000W), and keep the existing #12 wire for the remaining 1500W (which is branched from the first thermostat)?
Under normal conditions, current will never exceed wire maximums. However, if the 1500W heater had a fault, current through that section could potentially exceed the #12 capacity (30A) before tripping. Doesn't sound safe to me, but I wonder if there is anything in the code that would allow it.
FWIW, I have an alternate solution that will balance loads between heating circuits..it just isn't pretty because I need to create (and maintain access to) a junction in an existing circuit.
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Borrall Wonnell wrote: ...

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In a word, "No" (assuming you're intending to switch to a 30A breaker which your posting doesn't explicitly state but I infer is the reason for posing the question).
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The "legal" way would be to put 20a overcurrent protection between the stat and the 1500w heater in the 12ga wire circuit. (with the 10ga to the stat) Doing that in an estheticly acceptable way may be tough. If you can live with a small 2 pole breaker box there it is easy.
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On Thu, 9 Dec 2010 10:27:58 -0800 (PST), Borrall Wonnell

What you suggest is probably safe (#10 to the stat #12 to a 1500w heater) but it is not legal. Arguing on the safety side I can say the 1500w load is well within the ampacity of the 12ga wire and a short will still safely be cleared by a 30a breaker. Unfortunately there is really no code language that allows this.
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A short circuit will trip breakes with much smaller wire than the code requires. That is only part of the problem. If there is a wiring problem that causes the smaller wire to draw more current, but not enough to trip the larger breaker, the wire can over heat and causes a fire.
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Have you looked at possibly swapping out some of your other breakers with a few of those single slot, double breakers to create some more room in your box.
http://cdn.oneprojectcloser.com/wp-content/uploads/2009/10/split-breaker-in-a-breaker-panel.jpg
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On Thu, 9 Dec 2010 10:27:58 -0800 (PST), Borrall Wonnell

According to the code, this is NOT legal. It's also a fire hazzard.
Just get an addon box and put it next to your main panel. You can buy them for 2 4 or 8 breakers. I'd suggest at least a 4 breaker one. Then use 2 slots for your dual 30A breaker and you'll still have 2 left for something else in the future. Run at least #8 wire from your main to that box. #6 is better. (It's only about 3 feet of wire).
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On Dec 13, 1:29am, snipped-for-privacy@someplace.com wrote:

Update: I went with the cheapest solution. I swapped two of my 500W heaters for 300W units and transferred one of them to another heating circuit. Both circuits are now loaded at 3800W, just below the 80% limit. This required one extra junction box and 30' of wire, but the heaters were a couple of bucks cheaper. On the downside, I have absolutely no headroom for future expansion. If the next owner wants to put in that hottub (or whatever), he'll have to pay dearly for it :)
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