This paper presents a thermal model that can be used to calculate both
the steady-state and transient electrical current carrying capacity (
ampacity) of bus bar. Using a thermal model, a computer program has be
en formulated that can calculate the ampacity of bus bar for any time-
varying current and any variation in environmental conditions. Relatio
ns for the convection heat transfer coefficient for bus bar with circu
lar, rectangular and angular cross-sections are presented. The steady
state values for ampacity predicted by the program are compared to acc
epted values. The transient capabilities of the program are illustrate
d by calculating the variation in bus bar temperature when it is subje
cted to a step change in current. The predicted temperature histories
of typical bus designs are compared to temperatures measured in the la
boratory using thermocouples attached to the bus material. The tempera
ture data is also used to estimate thermal time constants for common b
us cross-sections. An analytical expression for the time constant of t
he bus is derived and values determined from this analytical expressio
n are shown to be close to measured time constants. Analytically predi
cted and experimentally measured values both show that typical bus bar
designs have thermal time constants between 15 and 35 minutes. The co
ncept of a time constant is very valuable, because it can be used in a
simple temperature-time expression to estimate the thermal behavior o
f bus material without resorting to a complex mathematical model which
requires the solution of a non-linear differential equation.