NEC demand factors for dryers

The NEC recognizes substantial load diversity among cycling cooking appliances, such as ranges and ovens, but for two clothes dryers,
recognizes none.
Since the appliances are all resistance heating loads, does anyone know why dryers would be treated differently? The only explanation I can think of is that they have much higher duty cycles than ovens or cook tops. Is that the case?
Thanks.
Chuck
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| The NEC recognizes substantial load diversity among cycling cooking | appliances, such as ranges and ovens, but for two clothes dryers, | recognizes none. | | Since the appliances are all resistance heating loads, does anyone | know why dryers would be treated differently? The only explanation I | can think of is that they have much higher duty cycles than ovens or | cook tops. Is that the case?
Two is not not much diversity. Ranges already have a degree of diversity because of multiple burners. So a range with 4 cooktops and 1 oven (now with 8 cooktops and 2 ovens if you have 2 ranges) has a higher count of the number of potential loads to turn on and off. The clothes dryer, however, is a single load. If you have 2 of them you may be expected to use both at the same time sometimes. That's not much diversity. It would be less likely to have all 10 burners on than to have both dryers on.
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By Table 220.54 for Household electric dryers 1 to 4 dryers are calculated at 100 percent, but 5 are permitted to be calculated at 85 percent. It doesn't make sense, but I suppose somewhere along the way someone did a study and came up with those numbers. This Table is PERMITTED and not required. If there was a large family with 6 dryers in one household, I would use 100 percent, but who has ever seen that many dryers in a household or for that matter even 4 dryers? But there are many households with two where they both are used at the same time. Also, as I recall the heating elements for dryers is a single resistive load, but I am wondering how do they acheive drying at a lower temperature without running the element at either 120 volts or 240 volts since there seems to not be any type of on/off or prportional control. With all the new designs out there it is difficult to make a general statement.
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On Sun, 9 Mar 2008 21:28:55 -0700 (PDT), Gerald Newton

They have multiple elements in the ones I have looked at. They also have a thermostat on the exhaust air that will cycle the element in the later stages of drying. That is the "moisture sensor". (analysing the duty cycle)
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Some "moisture sensors" are located within the drum. My Whirlpool has two linear electrodes on the inside drum near the lint filter. The action of wet clothing rubbing across the electrodes is sensed as a current flow and the dryer timer motor is cut off and prevented from advancing as long as the clothes remain damp. There are also thermostats on most dryers that cut the heating element in and out while the blower/tumbler motor continues to run.
Thus, the duty-cycle of the heating element depends on multiple variables including load size and probably load-type (thick blankets vs. light fabric shirts).
Beachcomber
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In article <4fbc52cd-5070-4566-b156- snipped-for-privacy@i12g2000prf.googlegroups.com>, snipped-for-privacy@electrician2.com says...

Temperature control is done the way it usually is, with a thermostat controlling power to the heating element. Lower temperature = lower duty cycle. The wet clothes have a fairly high specific heat (being mostly water) and no need for precise temperature control, so there is no reason to do any finer control than "bang-bang".
-- Keith
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wrote:

The responses are appreciated but don't address the duty cycle differences.
An important difference between an oven and a clothes dryer is that the oven attempts to maintain a fairly constant temperature in a small enclosed chamber. Once the set temperature is reached, additional energy is needed mostly to overcome losses through radiation and conduction. With bang-bang temperature control, the oven shows a "low" ducty cycle once it is up to the set temperature.
A dryer, on the other hand, may have approximately the same peak energy consumption as the oven (5-6 kW), but operates at a much higher duty cycle. Its operation is to actually utilize the heat energy to do work by evaporating water while venting the already heated, water-laden air. A pretty lossy enterprise, energy-wise.
For example, an electric clothes dryer drying 25 one-hour loads per month uses 106 kWh, or an average of 4.24 kWh. This works out to an 85% duty cycle (based on a 5 kW unit) hardly any opportunity for demand factor adjustment.
So the NEC's frugality in allowing demand factor benefits for multiple dryers is well justified.
I should have researched this more thoroughly before posting the question.
Chuck
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