Electrical Engineering - NEC demand factors for dryers

Author

NEC demand factors for dryers

Chuck

2008-03-09, 9:25 pm

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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phil-news-nospam@ipal.net

2008-03-10, 3:25 am

On Sun, 09 Mar 2008 21:39:24 -0400 Chuck <nospam@nospam.at.all> wrote:

| 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.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-03-09-2207@ipal.net |
|------------------------------------/-------------------------------------|

Gerald Newton

2008-03-10, 3:25 am

On Mar 9, 5:39=A0pm, Chuck <nos...@nospam.at.all> wrote:
> 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
>
> ----=3D=3D Posted via droptable.com - Unlimited-Unrestricted-Secure Usenet=
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! 120,000+ Newsgroups
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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.

gfretwell@aol.com

2008-03-10, 1:25 pm

On Sun, 9 Mar 2008 21:28:55 -0700 (PDT), Gerald Newton
<electrician@electrician2.com> wrote:

>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.

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)

Beachcomber

2008-03-10, 5:25 pm

>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)

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

krw

2008-03-10, 8:25 pm

In article <4fbc52cd-5070-4566-b156-
0725cbf6b80d@i12g2000prf.googlegroups.com>,=20
electrician@electrician2.com says...
> On Mar 9, 5:39=A0pm, Chuck <nos...@nospam.at.all> wrote:
net News=3D=3D----http://www.droptable.comThe #1 Newsgroup Service in the W=
orld! 120,000+ Newsgroups[color=darkred]
=3D----[color=darkred]
>=20
> 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.

Temperature control is done the way it usually is, with a thermostat=20
controlling power to the heating element. Lower temperature =3D lower=20
duty cycle. The wet clothes have a fairly high specific heat (being=20
mostly water) and no need for precise temperature control, so there=20
is no reason to do any finer control than "bang-bang".

--=20
Keith

Chuck

2008-03-12, 5:25 pm

On Sun, 09 Mar 2008 21:39:24 -0400, Chuck <nospam@nospam.at.all>
wrote:

>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?
>

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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dirskyster@gmail.com

2008-03-13, 1:25 pm

On 3=D4=C210=C8=D5, =C9=CF=CE=E79=CA=B139=B7=D6, Chuck <nos...@nospam.at.all=
> wrote:
> 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
>
> ----=3D=3D Posted via droptable.com - Unlimited-Unrestricted-Secure Usenet=
News=3D=3D----http://www.droptable.comThe #1 Newsgroup Service in the World=
! 120,000+ Newsgroups
> ----=3D East and West-Coast Server Farms - Total Privacy via Encryption =
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