Conductor ampacity
The smaller the wire, the greater the
resistance for any given length, all other factors being
equal. A wire with greater resistance will dissipate a
greater amount of heat energy for any given amount of
current, the power being equal to P=I2R.
Dissipated power in a resistance manifests
itself in the form of heat, and excessive heat can be
damaging to a wire (not to mention objects near the wire!),
especially considering the fact that most wires are
insulated with a plastic or rubber coating, which can melt
and burn. Thin wires will, therefore, tolerate less current
than thick wires, all other factors being equal. A
conductor's current-carrying limit is known as its
ampacity.
Primarily for reasons of safety, certain
standards for electrical wiring have been established within
the United States, and are specified in the National
Electrical Code (NEC). Typical NEC wire ampacity tables will
show allowable maximum currents for different sizes and
applications of wire. Though the melting point of copper
theoretically imposes a limit on wire ampacity, the
materials commonly employed for insulating conductors melt
at temperatures far below the melting point of copper, and
so practical ampacity ratings are based on the thermal
limits of the insulation. Voltage dropped as a result
of excessive wire resistance is also a factor in sizing
conductors for their use in circuits, but this consideration
is better assessed through more complex means (which we will
cover in this chapter). A table derived from an NEC listing
is shown for example:
COPPER CONDUCTOR AMPACITIES, IN FREE AIR AT 30 DEGREES C
========================================================
INSULATION RUW, T THW, THWN FEP, FEPB
TYPE: TW RUH THHN, XHHW
========================================================
Size Current Rating Current Rating Current Rating
AWG @ 60 degrees C @ 75 degrees C @ 90 degrees C
========================================================
20 -------- *9 ----------------------------- *12.5
18 -------- *13 ------------------------------ 18
16 -------- *18 ------------------------------ 24
14 --------- 25 ------------- 30 ------------- 35
12 --------- 30 ------------- 35 ------------- 40
10 --------- 40 ------------- 50 ------------- 55
8 ---------- 60 ------------- 70 ------------- 80
6 ---------- 80 ------------- 95 ------------ 105
4 --------- 105 ------------ 125 ------------ 140
2 --------- 140 ------------ 170 ------------ 190
1 --------- 165 ------------ 195 ------------ 220
1/0 ------- 195 ------------ 230 ------------ 260
2/0 ------- 225 ------------ 265 ------------ 300
3/0 ------- 260 ------------ 310 ------------ 350
4/0 ------- 300 ------------ 360 ------------ 405
* = estimated values; normally, these small wire sizes
are not manufactured with these insulation types
Notice the substantial ampacity differences
between same-size wires with different types of insulation.
This is due, again, to the thermal limits of each type of
insulation material.
These ampacity ratings are given for copper
conductors in "free air" (maximum typical air circulation),
as opposed to wires placed in conduit or wire trays. As you
will notice, the table fails to specify ampacities for small
wire sizes. This is because the NEC concerns itself
primarily with power wiring (large currents, big wires)
rather than with wires common to low-current electronic
work.
There is meaning in the letter sequences
used to identify conductor types, and these letters usually
refer to properties of the conductor's insulating layer(s).
Some of these letters symbolize individual properties of the
wire while others are simply abbreviations. For example, the
letter "T" by itself means "thermoplastic" as an insulation
material, as in "TW" or "THHN." However, the three-letter
combination "MTW" is an abbreviation for Machine Tool
Wire, a type of wire whose insulation is made to be
flexible for use in machines experiencing significant motion
or vibration.
INSULATION MATERIAL
===================
C = Cotton
FEP = Fluorinated Ethylene Propylene
MI = Mineral (magnesium oxide)
PFA = Perfluoroalkoxy
R = Rubber (sometimes Neoprene)
S = Silicone "rubber"
SA = Silicone-asbestos
T = Thermoplastic
TA = Thermoplastic-asbestos
TFE = Polytetrafluoroethylene ("Teflon")
X = Cross-linked synthetic polymer
Z = Modified ethylene tetrafluoroethylene
HEAT RATING
===========
H = 75 degrees Celsius
HH = 90 degrees Celsius
OUTER COVERING ("JACKET")
=========================
N = Nylon
SPECIAL SERVICE CONDITIONS
==========================
U = Underground
W = Wet
-2 = 90 degrees Celsius and wet
Therefore, a "THWN" conductor has Thermoplastic
insulation, is Heat resistant to 75o
Celsius, is rated for Wet conditions, and comes with
a Nylon outer jacketing.
Letter codes like these are only used for
general-purpose wires such as those used in households and
businesses. For high-power applications and/or severe
service conditions, the complexity of conductor technology
defies classification according to a few letter codes.
Overhead power line conductors are typically bare metal,
suspended from towers by glass, porcelain, or ceramic mounts
known as insulators. Even so, the actual construction of the
wire to withstand physical forces both static (dead weight)
and dynamic (wind) loading can be complex, with multiple
layers and different types of metals wound together to form
a single conductor. Large, underground power conductors are
sometimes insulated by paper, then enclosed in a steel pipe
filled with pressurized nitrogen or oil to prevent water
intrusion. Such conductors require support equipment to
maintain fluid pressure throughout the pipe.
Other insulating materials find use in
small-scale applications. For instance, the small-diameter
wire used to make electromagnets (coils producing a magnetic
field from the flow of electrons) are often insulated with a
thin layer of enamel. The enamel is an excellent insulating
material and is very thin, allowing many "turns" of wire to
be wound in a small space.
-
REVIEW:
-
Wire resistance creates heat in operating
circuits. This heat is a potential fire ignition hazard.
-
Skinny wires have a lower allowable
current ("ampacity") than fat wires, due to their greater
resistance per unit length, and consequently greater heat
generation per unit current.
-
The National Electrical Code (NEC)
specifies ampacities for power wiring based on allowable
insulation temperature and wire application.
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