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The purpose of this page is provide reasonably
accurate information to glassblowers about understanding and
doing their own electrical work.
If you are not comfortable
dealing with electrical wiring, don't do it!
But you may still be
able to learn from this page information that may prevent a fire
or help you understand the inspectors, contractors, and workers
that you have to deal with.
As far as possible, information provided here
complies with the NEC 1999 (National Electric Code for the United
States created and maintained to guide cities, who mostly use the
code intact, in maintaining standards that avoid fires from
electrical failure.) [There was an update in 2002, which I have not reviewed as
of 2003-11. A copy costs about $60 and lots and lots of people will sell you the
book, or a CD-ROM version, etc. I have not yet found a web site with a
summary of changes.]
SAFETY - The electricity
used in glassblowing can kill or seriously injure people in the
shop, just as it can in the home, but often home equipment comes
with safety features that have to be designed into glass shop
equipment. Features designed to increase safety should not be
defeated or bypassed and rules for safe wiring should not be
ignored. All cases should be grounded and all circuits with the
remotest chance of human contact should be protected with a GFCI. When working with power connections, the
electricity should be cut off, yet the worker should still behave
as though it might be electrically live (as it may be if a fault
exists) and use gloves and insulated tools while testing for
voltage on connections.
PRACTICAL
Certain features of electricity require almost no
knowledge of theory. It is possible to function safely knowing
only practical things.
While USA 60 cycle power is very accurate, the voltage
delivered is approximate and can range from 110 to 125. Below 110 is
considered a brownout. Most of the time the voltage is between 115 and 120.
For this reason, you will see reference to house power being 110, 115, or
120 (and rarely 125). Double voltage for driers and ranges is exactly twice
and thus may be 220, 230, 240 (or 250). Strictly speaking, one should
always pair the low and high accurately, 110/220, 115/230, or 120/240, but
casual usage will often say 120/220, etc. There are other voltage
levels associated with 3 phase, such as 208, that will be ignored here.
I will use 110/220.
All wiring connections must have a solid
mechanical connection - just twisting wires and taping
them is not acceptable. The mechanical connection may be
a wire nut, a split bolt, or a terminal connection. In
the old days, wires were twisted and soldered before
wrapping in tape, which is not acceptable now.
All electrical connections must be
protected from outside electrical contact - they must be
insulated. In the case of a wire nut, the plastic cone
provides the insulation if the wire ends were not
stripped off too long - no copper should be showing.
Split bolts are normally taped over. Terminal connections
must be contained in a box that prevents physical damage and contact.
All electrical wiring must be protected
from physical damage and all connections must be enclosed
or out of reach. In the most practical terms, this means
that wiring must be enclosed in conduit or if Romex,
placed so that it can not be crushed or stepped on. Wire running
over joists, for example, must have wood blocks so stepping in the area does
not crush the insulation. Ends
of wiring runs must be placed in junction boxes so there
are no visible wire nuts or terminals.
Wiring must conform to standard rules as
to size of wires for current being carried. Current is
the casual name for the "stuff" [electrons] that moves in
wires to do work. The measure of current is amps and
certain size wires are required to carry each level of
amps so the wire will not overheat. The most common amp levels and their matching wires
are given just below. The theory
further down discusses why.
15 amps 14 gauge
20 amps 12 gauge
30 amps 10 gauge
50 amps 6 gauge
Each size of wire must be protected with
a matching circuit breaker (or fuse.) These devices are
carefully designed to carry the current specified and to
trip (or blow) rather quickly if the current goes higher.
The list above is the most common breaker sizes also. The
purpose is to protect from fire. If wire is too small for
the breaker used, the wire will get hot and may start a
fire. It is always wrong to replace a breaker that is
constantly tripping due to overload with a larger one. [Rarely
it may be useful to reduce the size of a breaker to
protect the equipment at the far end of a larger wire.]
With age, a breaker may start tripping at lower than its
rated amperage and should be replaced with the same size.
There are rules about how many
connections can be made inside a junction box and how
many wires can pass through. The bigger the box, the more
wires and connections. Again, the goal is to avoid
overheating. The exact numbers are in the Code. Most boxes have
the number of cubic inches stamped on them and that can be used with a
general rule of wires and connections. "1.
Count the number of wires for the box. Don't count outlet/switch
pigtails and count all ground wires as one. 2. Take that number, add
one for each cable clamp, and two for each device
(like a switch or outlet). 3. If the box contains only 14-gauge
wires, multiply the total by 2 cubic inches. Or, for 12-gauge wires,
multiply the total by 2.25 cubic inches."
Hometime - How-To - Project Help - Electrical
When a device is to be unplugged instead of hard wired, there are
standard connectors matched to the amps and volts of the
connection. These are called plugs and sockets/outlets or
male and female connectors. The shape of the plug on the removable part declares "This
device needs no more than this amount of
power" while the matching outlet declares "I
can safely deliver up to this amount of power." The
common household outlet with two parallel prongs and U
shaped ground prong is rated at 15 amps and 120 volts.
Most people have seen the 30 amp and 50 amp 240 volt
plugs used on dryers and ranges. There are many other
pairs of plugs and sockets matched to levels of amps and
volts. The different arrangements are intended to prevent
plugging into the wrong power.
[A major change occurred
with the 1999 NEC - before this version, it was allowed
that a drier or a range needing 240 volts might use a
three prong plug - two current carrying lines and a
combined neutral and safety ground. These plugs had two
flat blades arranged in a V with the ground prong between
the legs of the V. All new appliances and all new
construction under the 1999 NEC must use a 4 prong plug,
separating the white neutral wire from the green or bare
safety ground. All recently built appliances are easy to
convert to the new plug; older housing may require
serious rewiring to bring in the fourth wire.]
A junction box must never have power from
two different breakers going though it - that is when a
breaker is tripped off a worker should be able to open
all the connections in a box connected to that breaker
and be sure that no power is coming from another breaker
that is still on. Similarly, if two outlets side-by-side
are served by different breakers, they must be in
separate boxes, not paired in a double wide box.
European 220 volt power is not the same wiring as
American 220. In American 220, the two hot wires exchange phase 60
times a second and are 110 volts above and below ground at maximum. In
European, the hot wire is 220 volts above ground at maximum and the neutral
wire is at or near ground.
THEORY
Electricity travels through conductive materials
in the form of moving electrons. Depending on the physical
structure of the material, the electrons may move freely or meet
with more resistance. When resistance occurs, energy is lost as
heat. This may be a good thing, if a heating element is desired,
or a bad thing if the goal getting as many electrons as possible
out the far end of the wire with as much energy as possible..
The common measurement of electron flow is done
in units of AMPS (which in electronic circuits may be scaled down
to milliamps 1/1000 Amp, abbreviated ma.) The force driving the
electrons is defined in VOLTS (or millivolts or kilovolts). Power
is measured in WATTS, defined as Volts times Amps and power does
work. The shorthand formula for power is W=EI, where E is the
symbol for voltage no matter what the units, and I is current.
There are very specific relationships of the electron to the volt
and the amp, but such details can be researched elsewhere.
Resistance is measured in OHMS. There is a very
specific relationship of Ohms to Volts and Amps, E=IR. That is,
voltage equals current times resistance. The formula can be
rearranged using the rules of algebra - I=E/R or R=E/I. To give
specific examples, a 120 volt light bulb that draws 1 amp will
have a resistance of 1/120 ohm. Since watts are related to volts
and amps, it is possible to determine wattage from volts and
resistance.
All normally used materials have some resistance,
therefore there is some heating and some voltage loss. Silver,
aluminum, and copper have fairly low resistance and copper is
used because silver costs too much and aluminum has other
problems. If wire is small diameter it has higher resistance and
loses more voltage over a given length. Therefore, standards set
the size of wire to be used for normal runs so that voltage does
not drop below an arbitrary level.
Standard wire ratings for reasonable runs in a
house
15 amp 14 gauge 0.0641" diameter
20 amp 12 gauge 0.0808"
30 amp 10 gauge 0.1019"
50 amp 6 gauge (actually 55 amp)
Voltage does not matter here, 12 volt, 120 or 240 (although a
volt of loss in 12 volt is a lot more important than at 240)
If you go and look at extension power cords you
will see good examples of the trade off of length and power. A
common cheaply sold 16 gauge 100 foot cord will only deliver 10
amps out the end - pulling more will drop the voltage too far to
run the tools properly. A 50 foot cord will deliver 13 amps and a
25 foot cord 15 amps. To deliver 15 amps at the end of a 100 foot
cord requires a 14 gauge cord. [On the other hand, a 12 gauge
cord should deliver 20 amps, but it is commonly equipped with a 15
amp parallel blade plug and therefore can only be reported as
being rated at 15 amps, but it will have less voltage drop at the
end of the cord than a 14 gauge.
Two goals are in mind here: avoid fires from
heating the wire by overloading it and avoid voltage loss over a
long wire with a load at the end. Long runs of wire should be
sized larger. A light weight 16 gauge
extension cord 100 feet long (under $10) will only deliver 10 amps
out the end (even though the plugs are rated 15 amp) because
there is too much voltage drop at higher amps. A circular saw run
on this cord will burn out very quickly because as the saw bogs down, its back
voltage falls and more amps try to get through, increasing heating. To get a full 15 amps
out of a 100 foot cord, a 12 gauge (about $45) must be used.
[This site Copper
wire has a lot of info, with a lot of typos also.]
AWG dia circ open cable ft/lb ohms/
mils mils air A Amp bare 1000'
10 101.9 10380 55 33 31.82 1.018
12 80.8 6530 41 23 50.59 1.619
14 64.1 4107 32 17 80.44 2.575
Mils are .001". "open air A" is a continuous
rating for a single conductor with insulation in open air.
"cable
amp" is for in multiple conductor cables. Disregard the
amperage ratings for household use.
AMPS - A measure of how many
electrons are being pushed through the wire. An amp is to
electricity like a gallon is to water. Modern house service is
commonly 200 amps. Circuit breakers commonly limit service to 15,
20, 30 or 50 amps on each circuit.
OHMS - A measure of resistance of wire;
resistance being the blockage of electron movement. Resistance is measured by applying a voltage and
measuring the current flow - 1 ohm lets 1 amp flow under pressure
of 1 volt. Some people have a problem with resistance because we
are so used to increasing the pressure to change flow - opening a
valve to force more water through a fixed resistance. With
electricity, the voltage is normally fixed, commonly at about 115
volts, so we change the resistance to get more current. If the
material (say copper) is not changed, then more area (bigger
diameter) decreases resistance. A longer wire increases
resistance. Changing material can also change resistance - iron
has more resistance than copper which has more resistance than
silver. Heating elements are made of alloys of iron, among other things.
VOLTS - A measure of the
pressure available to push electrons through a wire. A volt is to
electricity like pounds per square inch (psi) is to water.
American house power is specified to be 110-125 volts and
appliances must work over that range. At my house, about 117
volts is common most of the time (I just went and measured it and
found it to be range from 118.7 to 119.2 at noon on 2000-8-9. I
expect it to go down as the temp goes above the 94 it is right
now). Below 110 is considered a brown out condition, lights are
visibly dimmer.
WATTS - A measure of the
power being used or required, the product of amps and volts. For
a light bulb, 120 volts times 0.833 amps yields 100 watts. From a
modern 15 amp outlet running a heater, 1875 watts is commonly
specified; this is 15 amps times 125 volts and will be lower if
the voltage is the more common 115-120.
KILOWATT-HOURS -
Billing in the U.S. is commonly done in kilowatt-hours where kilo
means 1000. (KWH*3.6=MegaJoule in metric.) It is the product of
watts and time. A thousand watt heater left on for an hour uses a
kilowatt hour; a 100 watt light bulb left on for 10 hours also
uses a kilowatt hour. In the United States, the cost of a
kilowatt-hour varies around the country depending on how
electricity is generated and within a city depending on rules of
the service contract (big businesses who agree to have their
power cut off in an emergency get a lower rate as reward.) My
electric bill last month was based on 7.78 cents ($0.0778) per
kilowatt hour and my average usage was $5.98 per day. I did no
glassblowing last month. [In 2007-06, Texans are paying 13.5 cents per kwh.]
Down below is a table
showing rates in various states (selected as having glassblowers
mostly) and regions. Note how low the rate is in Washington state
with hydroelectric power. The electric cost of running an
operation in California or Massachusetts would be more than twice
as high as Washington state. [these figures before the
deregulation disaster in CA]
"On a national level, the price of electricity sold by
utilities in 1998 averaged 6.75 cents per kilowatt-hour.... In the
residential sector, on a cents-per-kilowatt-hour basis, the price
fell to 8.27 cents during the year from 8.43 cents in 1997. Both
the commercial and the industrial reported lower electricity
prices in 1998 at 7.43 and 4.50 cents per kilowatt-hour sold,
respectively. This decline in the price of electricity was a
result of the lower cost for fossil fuels and rate reductions in
response to the emerging environment of competition in the
industry. "
DOE report at http://www.eia.doe.gov/cneaf/electricity/epav1/epav1_sum.html
A serious glassblower should negotiate for lower rates.
SERVICE - House and
business wiring begins at the service entrance, but only strange
people like me do it themselves so you may wish to skip through this section.
Virtually all homes and small business sites are supplied with
power in the form of three wires that deliver 220-240 volts.
Business sites may have higher voltages (440 or more) and three-phase that let more power be
delivered over the same size wires. This power is delivered to a
service entrance, usually a head raised above the roof on high on
the side of the building (the height is set by the Code.) Wires
bring the power down to a meter base where usage is measured for
billing and then wires take the power through a main breaker,
which may inside or outside the breaker panel. High voltage
appliances, like air conditioning and kilns, remote from the
breaker box, must have a cutoff switch at the unit for safety of
workers, so there is no risk from someone turning the breaker back on
while they are working.
BREAKERS - The purpose of
a breaker or fuse is to keep the wiring from overheating and
causing a fire. Therefore replacing a fuse or breaker with a
higher rating just because it keeps blowing is a no-no. Circuit
breakers have replaced twist in fuses as the primary protection
in all new construction. Cartridge fuses are still used in inline
switching panels. A circuit breaker is a heat activated switch: a
tiny amount of the current flowing through the breaker is used to
generate heat and when the heat gets high enough, a metal latch
trips and the breaker trips. (For this reason, a breaker can
"just wear out", although normally this takes decades.)
Breakers are designed to hold a load at their rated amperage
forever and to trip faster and faster as the amp goes over the
limit. It may take a minute or more for a 20 amp breaker to trip
at 21 amps and a fraction of a second at 30 amps (the exact
timing is set by standards like UL (Underwriters Laboratory.))
Under the Code, if a breaker box has more than
six breakers, it must have a master breaker to cut off all power
to the box with one switch. This breaker can be inside the box or
separate. A box is rated at the size of the master breaker, 125
amp, 200 amp or more. The number of breakers that can be put in
the box is actually determined, according to the code, by the
number of amps and the rating of the breakers. The code
recognizes that not all equipment will be used at the same time
and not every circuit will be used at full load, so it allows a
certain amount of overload - more apparent capacity than the main
breaker will supply, but it places a limit on this. When I moved
in this house it had 2 breakers in a 6 breaker box with no master
and a 60 amp meter base. Now it has a box with 20 slots for
breakers, 18 of which are filled, 2 with 15 amp (1 a GFCI), 4 with 220 volt 40 amp double wide breakers for the
garage and to glass center in the yard, 2 with 220v 30 amp double wide for the
air conditioner, about 10 with double 20's and the balance with
single 20's. No matter how you add that up, there is a lot more
than 200 amps in the breakers. I have over-wired this small (1100 sq.ft.) house. There are, for example, 5 distinct 20 amp circuits
available around the edges of a 30 by 30 foot area in the back
yard and 5 separate circuits in the kitchen. (The dishwasher
circuit has an outlet on it, as does the gas oven electrical. The
first does not usually run if we need power for a griddle, the
latter draws little power any time.)
Normally, wiring design is done from the other
end, because it is recommended that most large appliances have a
circuit and a breaker to themselves. The amperage of the
appliance is stated (20 amps for disposal, 30 amps 240 volts for
a drier, 50 amps 240 volts for a range) and the length of the run
determines the wire size, usually the numbers given above. Then a
breaker is installed to protect the wire and the unit. Because of
the cost of copper wire, over wiring is rarely done. However, 15
amp outlets are commonly wired with 12 gauge wire and 20 amp
breakers because the code says you can put up to 6 15 amp outlets
on such a circuit. If a circuit were run with 6 gauge wire (50
amp service) but a dryer or kiln only calling for 30 amp service
were installed, then installing a 30 amp breaker would be
appropriate to protect the innards and connection wiring of the
appliance. A note of the higher rated wire should be placed on or
near the breaker for future use.
OUTLETS - Outlets are the
devices mounted to the wall (or floor) to plug in devices. Often
units with heavy current usage, such as kilns or air
conditioners, are direct wired (directly to terminals in a box),
but code requires that a cutoff be easily at hand. Outlets are
coded by the size and shape of the slots according to a published
standard. Usually higher amperage connectors are heavier metal or
have more contact area (as in twist lock connectors.)
An outlet announces by its shape that it can
supply a given amperage and voltage. The matching connector on
equipment declares its need for that voltage and less than that
amperage. It is always best to use the proper connectors in glass
work to avoid confusion and possible destruction of equipment. A
lower voltage outlet (120) should never be used on a higher
voltage circuit (240) As long as the breaker protects the wire,
there is somewhat more flexibility with higher or lower amp
outlets.
PLUGS - Plugs and outlets are
designed to work together. As it says under outlets above, there
is a standard that specifies the shape of a plug to carry a
specific current and voltage. Physical requirements, such as a
twist lock to hold the connection may also affect the choice. In
the past, dryers and ranges operated under an exception to the
NEC that permitted two hots and a wire that served as both
neutral and safety ground. This was eliminated in the 1999
revision of the code, and now all dryers and ranges come with
four prong plugs (two hots, neutral and ground) and all new
construction must use this pattern. I mention this because people
often wire their 220 annealers using these common and less costly
dryer/range connectors.
There is a product that conducts heat and keeps oxygen in the air away
from connections to reduce corrosion and increases in resistance which result in
the plug getting hot to the touch and aging it. Sold as anti-seize
compound or conductive paste or silicone grease is commonly sold for coating
aluminum terminals, spark plugs, and aluminum light bulb bases. Permatex is one
brand. It is an messy product when used on plugs when these are pulled. It
needs to be applied to the prongs before insertion into the socket. 2007-11-04
GFCI -The purpose of a GFCI is
to protect people from a small current leakage (4-6 ma.) across
the heart that is large enough to set it into fibrillation. It is
not a circuit breaker, which deals in amps and protects the
wiring from overheating. A circuit in the GFCI detects that the
current flowing out is different from that flowing back, with the
assumption being that the missing current may be flowing through
a body, across the heart, to another ground. Mine tripped when a heating coil grounded part way
around. Installing a GFCI at the circuit breaker box can be very
expensive (i.e. a plain breaker costing $4.99 may be matched with a $45.99 price
for the GFCI.) Protection for up to 20 amps can be had much cheaper
by installing a GFCI outlet (normally 15 amps at the outlet for
about $10) most of which protect and feed through 20 amps to
other much less expensive outlets, 15 or 20 amps. 2/27/96
For higher current one choice is Leviton High Current GFCI, part #8895
link, up to
80 amp/
240 volts 1 or 3 phase with
contactor,
2009-12-04
The sensitivity of a GFCI is based on tests which demonstrated a
healthy adult could stand that current across the heart without
the heart stopping. Today I was setting up the heating element
used for preheating my furnace and touched the tip of the
thermocouple to check its location. Something was at 120 volts
and I got a brief shock. The GFCI tripped. I don't know what
might have happened if I did not have the GFCI, anything from a
sharp jerk of muscles to severe damage and burns. Several years
ago, I got a fairly full shock when I touched the contacts on a
wall switch because the plate was off. I barely recall the muscle
pain and spasm, but it was considerable. I don't expect to give
up using GFCI's to find out how it felt.. 11/10/96
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