Rev. 2002-09-01, 2003-08-30, -11-14, 2004-03-03,
2007-02-17, -04-02, -06-11, -11-04
[Search on date pattern to find latest changes, more than one may be found.]
|THREE-PHASE||RATE TABLE||VOLTAGE RANGE||RESISTANCE|
The purpose of this page is provide reasonably
accurate information to glassblowers about understanding and
doing their own electrical work.
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.
Certain features of electricity require almost no knowledge of theory. It is possible to function safely knowing only practical things.
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
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]
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.
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
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
|THREE-PHASE - If the voltage of regular house power is examined on an oscilloscope, it looks like a simple sine wave, the smooth curve going up and down. It is off (0 volts) 120 times a second and goes from +120 to -120 on each swing. This is technically called single phase power, a term that is almost never used. Single phase power takes two wires (often a third for safety ground.)|
|When 240 power is required, a second sine wave is introduced exactly the opposite of this one so at the peak, there is 240 volts from peak to peak and zero at the crossing. Good overview (new window).|
|Three phase power has at least three wires (usually 4 for a safety ground) and when the voltage on the three wires is examined over time it is discovered that each of the wires has the voltage going through zero at a different time, evenly dividing the interval graphed like this: because of the timing (and the fact that power comes out of the generator this way) it is possible to deliver power more smoothly and to deliver more power over the same size wires. For this reason, most big power users receive their power as three phase and either divide it up among different single phase uses on the property (carefully balancing the loads between phase to make the power company happy) or using the three phase directly. Most really serious glass fusers and saggers (with kilns measured in feet, not inches) use 3 phase feeds to the kiln. Besides, it is cheaper.|
De Clarke wrote:
"Connie should carefully check your "new"
house, because it seems like an older house, and see whether the
wire matches the breakers. This can be done by squinting at the
tiny print on the wires or by going to a hardware store, smiling
nicely and getting 1 inch samples of solid wire in these gauges.
RATE TABLE 2007, 2006 from source
Power Monthly - Average Retail Price of Electricity to Ultimate Customers by
End-Use Sector, by State
data available. 2007-06-11
Table 5.6.A. Average Retail Price of Electricity to Ultimate Customers by End-Use Sector, by State, February 2007 and 2006
(Cents per kilowatthour) MF-Selected states
|East North Central||9.15||8.74||8.45||7.98||5.89||5.1||6.19||6.18||7.82||7.17|
|West North Central||7.3||7.43||6.2||6.24||4.82||4.64||6.29||6.42||6.27||6.19|
|East South Central||7.71||7.68||7.81||7.67||4.93||4.4||9.8||10.49||6.73||6.38|
|West South Central||10.66||10.74||9.15||9.24||7.01||7.41||8.79||8.67||9.13||9.16|
 See Technical notes for additional information on the Commercial, Industrial and Transportation sectors.
Notes: See Glossary for definitions. Values for 2005 are final. Values for 2006 and 2007 are preliminary estimates based on a cutoff model sample. See Technical Notes for a discussion of the sample design for the Form EIA-826. Utilities and energy service providers may classify commercial and industrial customers based on either NAICS codes or demands or usage falling within specified limits by rate schedule. Changes from year to year in consumer counts, sales and revenues, particularly involving the commercial and industrial consumer sectors, may result from respondent implementation of changes in the definitions of consumers and reclassifications. Retail sales and net generation may not correspond exactly for a particular month for a variety of reasons (i.e., sales data may include imported electricity). Net generation is for the calendar month while retail sales and associated revenue accumulate from bills collected for periods of time (28 to 35 days) that vary dependent upon customer class and consumption occurring in and outside the calendar month. Totals may not equal sum of components because of independent rounding.
Sources: 2006 and 2007: Energy Information Administration, Form EIA-826, "Monthly Electric Sales and Revenue Report with State Distributions Report;" 1992-2005: Form EIA-861, "Annual Electric Power Industry Report."
Wire gauge, tensile-strength, DC-resistance calculation
Ordinarily, the next two gauging systems are used with electrical wire and cables.
Most of conductors is composed with copper. Most important parameter of conductors are cross section which determines DC-resistance and most of current firstname.lastname@example.org Conductor size 16 AWG
area 1.309 mm^2 (square-mm)
area 2583 CM (circular mil)
diameter 58.7 mil (1)
diameter 1.491 mm (1)
DC-resistance 0.0134 Ohm/m
tensile strength 39.26 kgf (2)
weight 11.634 kg/km (Cu)
weight 3.533 kg/km (Al)
Note: lay factor of stranded conductor is assumed 2 %. conductor size 16 AWG
area 1.309 mm^2 (square-mm)
area 2583 CM (circular mil)
diameter 50.8 mil (1) diameter 1.291 mm (1)
DC-resistance 0.0132 Ohm/m
tensile strength 39.26 kgf (2)
weight 11.634 kg/km (Cu)
weight 3.533 kg/km (Al)
"If you are more interested in current carrying ability than physical size, then also remember that a change of 3 AWG numbers equals a doubling or halving of the circular mills (the cross sectional area). Thus, if 10 AWG is safe for 30 amps, then 13 AWG (yeah, hard to find) is ok for 15 amps and 16 AWG is good for 7.5 amps.
The wire gauge is a logarithmic scale base on the cross sectional area of the wire. Each 3-gauge step in size corresponds to a doubling or halving of the cross sectional area. For example, going from 20 gauge to 17 gauge doubles the cross sectional area (which, by the way, halves the DC resistance).
So, one simple result of this is that if you take two strands the same gauge, it's the equivalent of a single wire that's 3 gauges lower. So two 20 gauge strands is equivalent to one 17 gauge. "
Appliances in the United States are required to work over a
range of standard voltages such as 110-125 volts. Sometimes this
is stated as 115 volts, a mid point. According to TXU Electric which bills
me and Oncor which maintains the wires, voltage at my house is to be 120 volts
plus or minus 5% or 6 volts, making the acceptable range 114-126 volts.
>I have to work this out because there is no clear answer (maybe)
but here goes... my problem is to estimate the max current or
yet wattage to allow for a piece of wire in the furnace. In
electrical wiring the manufacturers would say "a 2mm cable