John H. Rodgers on mon 26 oct 98
-- [ From: John H. Rodgers * EMC.Ver #2.5.02 ] --
The number of electrical questions that come up regarding electric kilns
really concerns me. I'm always afraid I am going to hear someones house
burned down or worse, someone got fried.
If you have questions or doubts about your kiln and electricity and simply
don't know- call an electrician. That is the safe way out. If you want to be
knowlegeable about the subject in regards your kiln operation from an
operators standpoint, get a basic book on home wiring. They are easy to read
and understand. Will give you enormous insight with just a little reading.
Heres my $.05 on the subject.
On your kiln ( or any other appliance for that matter) there should be a
data plate or tag that tells you the voltage, the amps, and the watts at
which your kiln is rated for. This is design stuff by the manufacturer. If
you have a multi tier kiln, like a kiln with an extra collar on it, you may
find that it is rated differently than the main Kiln body. Most kilns that
have any size to them at all and are designed for hobby/studio use will be
rated for 208 to 240 Volts. The amperage will vary a bit depending on the
size and make. The bigger Electrics will probably show 45 to 60 amps. Now
there is a relation (i won't do the math here) between the volts(electrical
pressure) , amps( current) and watts(power) and the resistance in the
wire to the movement of the current through the wire. As long as that wire
is big enough the current or amps drawn by your kiln will not be a problem.
Heat created by the resistance of the wire to the current flow will
dissipate as long as the current doesn't exceed the limits of the wire. If
that happens, the wire gets hot and will eventually burn off the insulation
and cause a fire, if there were no circuit breaker. If you were to plug the
kiln directly into the line voltage coming into your house, it would be like
putting the hose directly into the lake to receive water. With great water
pressure(voltage) and because of the small size of the hose(resistance) the
hose will rupture(wire overheat).
Well, in the case of the kiln cord, that is sort of what happens if the
cord is not sized right. And the right size is determined by the
manufacturer. And the electrician goes by the electrical code to be sure to
get the permanent wiring in the house to match the appliance cord. Now as
protection, he adds a circuit breaker. That way, if more current flows
through that breaker than it is capable of carrying, it to will start to get
hot, just like the undersize wire, and the breaker will open, shutting off
the current flow.
Breakers are installed that are rated for different amperages. Most fixed
lights in a house are rated at 15 amps. Appliance receptacles in the wall
are rated at 15. If a wall receptacle is rated at 20 amps, one of the blade
slots in the receptacle must be turned horizontal. The dryer receptacle is
rated at 30, and the Electric Stove receptacle is rated at 45 to 50 amps.
Your kiln will be rated at 45 to 60 amps, depending. Most of the appliances
will not draw the max current they are rated for in actual operation, but
if they do there had better be the proper circuit breaker installed. Equal
to or 5 amps over the equipment rating works and is safe enough. If you are
in a very hot environment where there is poor heat dissipation you may find
it necessary to go to a breaker rated 10 amps above the rating of the kiln.
Don't go above that. Should you have a short somewhere you risk a fire if
the breaker is rated to high to effectively protect the circuit. Personally
on a 50 amp kiln and above I match the kiln amp rating and the circuit
breaker.
When a kiln is plugged into its socket, the maximum amps and voltage is
available to it. But the configuration of the machine prevents it from just
pulling more and more and more which of course would cause everything to get
hotter and hotter and hotter. Including the kiln cord, causing it to burn up
and set the house on fire. When in operation at max settings, ie, High Fire
, etc, the kiln dissipates enought heat to prevent immediate burn out of the
coil, but eventually they will.. At some point the physical limitations of
the kiln materials will be reached, and it simply won't get any hotter,
unless something happens internally to draw more current....like a short,
say in a switch.In which case the current draw will skyrocket, causing the
breaker to heat rapidly and pop open, shutting off the current. The kiln
may get hot enough to cause a meltdown in the kiln, but at some point a coil
will burn out and the heat will begin to drop, thereby being self-limiting.
Or a switch may burn out or short , and current draw will run away, the
breaker will start to get hot, and it will open, cutting the current flow to
the kiln.
So having the kiln, the cord and the breaker matched to each other is
critical.
You can make your own cords by going to an industrial electrical supply
house and buying the male plug for the cable, and getting a cable whose wire
sizes match or exceed the existing cable. The bigger the wire diameter, the
less resistance to the current flow, therefore the less heat is produced.
Wire sizes are quoted in gage sizes. The bigger the number, the smaller the
wire diameter. Typically a kiln wire in a kiln cord is an 8 or 6 gage wire.
The 6 gage wire is larger in diameter than the 8 gage wire. And you need
appropriate connectors for where the cable goes into the kiln control box. I
oversize many times when I need to have an extra long connection. I have
fired my kiln to cone 8 using a 20 foot long cord. But I made sure the
specifications of the cord would accomodate the current given the added
resistance due to extra cord length. Basically I increased the wire diameter
.. But for most, it is simpler....and safer....to get one from the
manufacturer. That way, there is no mistake about the right one unless they
goof.
So, to your house through the service panel comes 240 volts at 200 amps to
drive your appliances. It is split in the circuit breaker box so you have
120 volts on one side of the breaker box and 120 Volts on the other. The
breakers installed on each side are rated at 15 and 20 amps. For equipment
rated above 30 amps, it is rare to be rated at less that 240 volts. So when
it comes to your kiln, the electrician ties both 120 Volt sides together by
using a double gang breaker giving 240 volts for the kiln. Since the panel
amperage is 200 amps, there is plenty. So the electrician installs a breaker
rated for the kiln - 45-60 amp. The kiln could, in event of a short, pull
the full 200 amps. That would definitely cause the insulation to burn off
your appliance cord. But with the breaker in the circuit and rated at or 5
amps above the appliance rating, the minute the current draw reaches or
tries to exceed the rated amperage, the breaker gets hot and opens, shutting
off the current..
I have rambled enough about this. I just hope I have lit one more candle in
the darkeness.
John Rodgers
In Alabama
--
John H. Rodgers on tue 27 oct 98
-- [ From: John H. Rodgers * EMC.Ver #2.5.02 ] --
Sandy, the power company should be able to tell you if you have a 200 amp
service at no charge. Give them a call, before you call an electrician. If
they aren't responsive then call the electrician.
You can gain some insight by looking at the ratings on each of the breakers
in the panel....the amp rating is on the switch handle. Add them all up. If
the total is 150 and you have a 200 amp service, then you have 50 amps to
play with, and can install a fifty amp breaker for your kiln. Just you don't
go over the max of 200...if that is what your service is.....you will be ok.
I hesitate to bring this up, but if you have a total such that your breaker
will exceed the 200 max, you CAN juggle the amp load and maybe put the
breaker in anyway. For example, if you kiln requires a 50 amp breaker, and
you have a total of 180 amps already called for, you are in trouble. But, if
you put in the 50, totaling 230 amps, by NOT running the electric dryer(30
amps) while firing the kiln, you can stay within the 200 amp limit of the
service. I have done that a lot, and just to be safer, I throw the dryer
breaker in the breaker panel to off while firing the kiln.
If you have spare amperage and can install the bigger breaker for the kiln,
but don't seem to have enough physical space to install the bigger breaker,
there are special 1/2 thickness breakers that can be installed to give more
physical room in the panel. Doesn't change the load, just more space
efficient. Have the electrcian replace some of the thicker breakers
controlling the wall plugs and the light fixtures with the thinner ones. A
little extra cost and work, but a lot cheaper than changing out the panel or
adding a breaker panel extension.
If I have a kiln installation that is going to be a significant distance for
the breaker panel, I usually will have the wire pulled from the panel and at
the point where the kiln receptacle will be mounted I have a fuse box
installed with a pull handle, then the receptabcle, then the kiln.
That gives me an extra measure of security.
Hope this helps.
John Rodgers
-------- REPLY, Original message follows --------
Date: Monday, 26-Oct-98 03:36 PM
From: Dwiggins, Sandra \ Internet: (sdwiggin@exchange.nih.gov)
To: John H. Rodgers \ Internet: (inua@quicklink.net)
Subject: RE: Kiln Cords -- and other electrical things
John:
Thanks. This was very helpful to me.I needed to know what I should do to
have my
kiln installed to operate off of the circuit box that houses all the wiring
in
my house. If I understand what you wrote, if I have a service already in
place
that already has the dryer on it on a breaker all its own, and there are
breakers on the service not in use, then another breaker could carry the
line
for the kiln, providing the service is high enough in voltage to match what
the
kiln needs. Is that correct? So what I have to find out is, is my service
actually high enough for the kiln? Yes???
I don't want to have another service installed if I can avoid it, since it
is
quite expensive.
TIA,
Sandy D.
-----Original Message-----
From: John H. Rodgers [SMTP:inua@quicklink.net]
Sent: Monday, October 26, 1998 9:44 AM
To: CLAYART@LSV.UKY.EDU
Subject: Re: Kiln Cords -- and other electrical things
----------------------------Original message----------------------------
-- [ From: John H. Rodgers * EMC.Ver #2.5.02 ] --
The number of electrical questions that come up regarding electric kilns
really concerns me. I'm always afraid I am going to hear someones house
burned down or worse, someone got fried.
If you have questions or doubts about your kiln and electricity and simply
don't know- call an electrician. That is the safe way out. If you want to be
knowlegeable about the subject in regards your kiln operation from an
operators standpoint, get a basic book on home wiring. They are easy to read
and understand. Will give you enormous insight with just a little reading.
Heres my $.05 on the subject.
On your kiln ( or any other appliance for that matter) there should be a
data plate or tag that tells you the voltage, the amps, and the watts at
which your kiln is rated for. This is design stuff by the manufacturer. If
you have a multi tier kiln, like a kiln with an extra collar on it, you may
find that it is rated differently than the main Kiln body. Most kilns that
have any size to them at all and are designed for hobby/studio use will be
rated for 208 to 240 Volts. The amperage will vary a bit depending on the
size and make. The bigger Electrics will probably show 45 to 60 amps. Now
there is a relation (i won't do the math here) between the volts(electrical
pressure) , amps( current) and watts(power) and the resistance in the
wire to the movement of the current through the wire. As long as that wire
is big enough the current or amps drawn by your kiln will not be a problem.
Heat created by the resistance of the wire to the current flow will
dissipate as long as the current doesn't exceed the limits of the wire. If
that happens, the wire gets hot and will eventually burn off the insulation
and cause a fire, if there were no circuit breaker. If you were to plug the
kiln directly into the line voltage coming into your house, it would be like
putting the hose directly into the lake to receive water. With great water
pressure(voltage) and because of the small size of the hose(resistance) the
hose will rupture(wire overheat).
Well, in the case of the kiln cord, that is sort of what happens if the
cord is not sized right. And the right size is determined by the
manufacturer. And the electrician goes by the electrical code to be sure to
get the permanent wiring in the house to match the appliance cord. Now as
protection, he adds a circuit breaker. That way, if more current flows
through that breaker than it is capable of carrying, it to will start to get
hot, just like the undersize wire, and the breaker will open, shutting off
the current flow.
Breakers are installed that are rated for different amperages. Most fixed
lights in a house are rated at 15 amps. Appliance receptacles in the wall
are rated at 15. If a wall receptacle is rated at 20 amps, one of the blade
slots in the receptacle must be turned horizontal. The dryer receptacle is
rated at 30, and the Electric Stove receptacle is rated at 45 to 50 amps.
Your kiln will be rated at 45 to 60 amps, depending. Most of the appliances
will not draw the max current they are rated for in actual operation, but
if they do there had better be the proper circuit breaker installed. Equal
to or 5 amps over the equipment rating works and is safe enough. If you are
in a very hot environment where there is poor heat dissipation you may find
it necessary to go to a breaker rated 10 amps above the rating of the kiln.
Don't go above that. Should you have a short somewhere you risk a fire if
the breaker is rated to high to effectively protect the circuit. Personally
on a 50 amp kiln and above I match the kiln amp rating and the circuit
breaker.
When a kiln is plugged into its socket, the maximum amps and voltage is
available to it. But the configuration of the machine prevents it from just
pulling more and more and more which of course would cause everything to get
hotter and hotter and hotter. Including the kiln cord, causing it to burn up
and set the house on fire. When in operation at max settings, ie, High Fire
, etc, the kiln dissipates enought heat to prevent immediate burn out of the
coil, but eventually they will.. At some point the physical limitations of
the kiln materials will be reached, and it simply won't get any hotter,
unless something happens internally to draw more current....like a short,
say in a switch.In which case the current draw will skyrocket, causing the
breaker to heat rapidly and pop open, shutting off the current. The kiln may
get hot enough to cause a meltdown in the kiln, but at some point a coil
will burn out and the heat will begin to drop, thereby being self-limiting.
Or a switch may burn out or short , and current draw will run away, the
breaker will start to get hot, and it will open, cutting the current flow to
the kiln.
So having the kiln, the cord and the breaker matched to each other is
critical.
You can make your own cords by going to an industrial electrical supply
house and buying the male plug for the cable, and getting a cable whose wire
sizes match or exceed the existing cable. The bigger the wire diameter, the
less resistance to the current flow, therefore the less heat is produced.
Wire sizes are quoted in gage sizes. The bigger the number, the smaller the
wire diameter. Typically a kiln wire in a kiln cord is an 8 or 6 gage wire.
The 6 gage wire is larger in diameter than the 8 gage wire. And you need
appropriate connectors for where the cable goes into the kiln control box. I
oversize many times when I need to have an extra long connection. I have
fired my kiln to cone 8 using a 20 foot long cord. But I made sure the
specifications of the cord would accomodate the current given the added
resistance due to extra cord length. Basically I increased the wire diameter
.. But for most, it is simpler....and safer....to get one from the
manufacturer. That way, there is no mistake about the right one unless they
goof.
So, to your house through the service panel comes 240 volts at 200 amps to
drive your appliances. It is split in the circuit breaker box so you have
120 volts on one side of the breaker box and 120 Volts on the other. The
breakers installed on each side are rated at 15 and 20 amps. For equipment
rated above 30 amps, it is rare to be rated at less that 240 volts. So when
it comes to your kiln, the electrician ties both 120 Volt sides together by
using a double gang breaker giving 240 volts for the kiln. Since the panel
amperage is 200 amps, there is plenty. So the electrician installs a breaker
rated for the kiln - 45-60 amp. The kiln could, in event of a short, pull
the full 200 amps. That would definitely cause the insulation to burn off
your appliance cord. But with the breaker in the circuit and rated at or 5
amps above the appliance rating, the minute the current draw reaches or
tries to exceed the rated amperage, the breaker gets hot and opens, shutting
off the current..
I have rambled enough about this. I just hope I have lit one more candle in
the darkeness.
John Rodgers
In Alabama
--
-------- REPLY, End of original message --------
--
John H. Rodgers on tue 27 oct 98
-- [ From: John H. Rodgers * EMC.Ver #2.5.02 ] --
One thing I left off regarding your question.
Most assurredly you will have 240 volt service to your house. You don't
really need to worry about that. It is the amps that will cause you grief.
The amps are what cause the heat and fires. Not much you can do with the
voltage. It splits down to 120 on each side of the panel to serve your
lights and appliances. Where 240 is needed...like the electric stove or
dryer or kiln, the elctrician installs a breaker and ties both 120 Volt
sides of you breaker panel together to get the 240Volts for a particular
breaker. But the amps have to be right, and the total for ALL breakers
together cannot exceed the total amps for which the panel is rated.
The real fact is that line voltage......anywhere from 1500 Volts to 15,000
volts will go to the transformer on a pole near your house. A transformer on
that pole will step the voltage down, and adjust the amperage that will be
served to the service panel on your house. On older houses the panels didn't
have service breakers. What ever the amperage from the transformer was, that
is what you had. And you protected with breakers directly off of that.
There was a fuse at the transformer to protect the system. Newer homes have
panels with 200 amp master breakers. I don't recall exactly, but it seems
that the amperage from the transformer to the service panel on your house is
well above the 200 amp rating of the service panel. But if you follow the
logic of using a 50 amp breaker to protect your kiln....and your house.....
by the breaker heating up and popping open if it gets to much current draw,
the 200 amp breaker on the service panel protects your house by popping open
if there is to much amp load on the panel itself.
If you had a absolute dead short to ground in your kiln, the kiln in an
instant would start pulling amps. In a split second it would reach 50 amps,
a hair above that the 50 amp breaker heats and opens.
Assume the breaker doesn't open....like maybe it fused together because of
the heat (not likely....they are designed against that) The kiln would just
keep on pulling current ........more, more, more. At 200 amps, the breaker
on the 200 amp service will get hot, and pop open, shutting off everything
in the house. If there isn't a fire already you are lucky.
Assume the worse and the 200 amp breaker doesn't open. The kiln will
continue the draw of amps and it will pull more and more and more. At
somewhere near 600 amps or more a big fuse by the trasformer on the service
pole will open, protecting the transformer and the electrical grid of the
power company, while your house burns down.
Thats an absolute worse case scenario, and probably not possible that way
because something would burn out....of course the burning really could set
the house on fire.
Anyway, hope this has cleared the air a little more.
John Rodgers
In Alabama
--
Tom Wirt on wed 28 oct 98
-----Original Message-----
You can gain some insight by looking at the ratings on each of the breakers
in the panel....the amp rating is on the switch handle. Add them all up. If
the total is 150 and you have a 200 amp service, then you have 50 amps to
play with, and can install a fifty amp breaker for your kiln. Just you don't
go over the max of 200...if that is what your service is.....you will be ok.
Hate to interrupt this, but the adding up of breakers to determine total
service isn't quite right. The individual breakers can add up to more than the
service to the building. The marking on the main breaker (usually at the top or
bottom of the box) tells you what the service is. The box also is supposed to
have the service rating printed on it. The reason all the individual breakers
can add up to more than the service is that there is an assumption that not
every circuit will be in full use at any time. Actually if you had a dryer,
stove, furnace and water heater on a box, you'd have more than 200 amp potential
without any other circuits. The number of circuits provided for, is determined
by the loads in the building. (e.g. 10 outlets count as one 15 amp circuit).
For each circuit, the individual breaker is the protection. For the total
building, the main breaker is the protection (as John notes).
Without going and digging through my copy of the National Electrical Code (NEC),
I would use as a rule of thumb, that your kiln load should not be more than 25%
of the building's service if it's in your home. If you really want to know more
than you want, go to the library, get a copy of the NEC and read article 220.
(Branch Circuit and Feeder Calculations). If you're insomniac, buy your own
copy for nightly reading.
We have three kilns on 200 amp service to the studio...2 at 68 amps and one at
48 amps. This is actually overload (184 amps) for the service. (Load should
never be more than 80% of the service rating-in this case 160 amps (.80 times
200 amps)). But I have checked the wiring and breakers and we aren't getting
heating which is the first sign you're overloading your system. (Another rule
of thumb...if the wires are warm, get it fixed.) Our local power company did
come out and put a bigger transformer up this year because of the amount we use.
There are formulas for figuring what you need to allocate for household use
before adding the extra load for the kiln. As John indicated, your power
company should be glad to do this for free.
Hope this helps.
Tom
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