Download USAmp for Windows User Manual

USAmp+ for Windows Users Manual
Page 62
_____________________________________________________________________________________
A.3
Equations for Ampacity & Temperature Rise for Multiple Circuits
1
1.0 For each loaded cable , an equation is written for either a)conductor temperature or b)
allowable conductor loss depending on whether:
a) conductor current I(n) is known or
b) the maximum conductor temperature Tmax is specified.
The equations are solved iteratively until all conductor temperatures and conductor loss values
converge.
1.1 The following equations apply to both steady-state and transient ratings.
Terms which are temperature dependent such as conductor a-c resistance and thermal
resistance between cable and duct wall are adjusted as required on each iteration.
o
For case a) the conductor temperature of cable n in C is:
T(n) = Wc (n) Rca(n) + dTd (n) +dTi(n) + dTø(n) + dTe(n) + Ta
(1a)
where:
2
W c (n) = I (n) * Rac (n) = conductor loss, In is in kiloamps and Rac (n) = conductor ac resistance in
microhms/ft. for cable n.
For case b) the conductor loss (per phase) is:
W c (n) = { T(n) - dTd (n) - dTi(n) - dTø(n) - dTe(n) - Ta } / Rca(n)
(1b)
where: T(n) = Tmax
½
I(n) = ( Wc (n)/ Rac (n) )
T(n) is first assumed = Tmax for each cable belonging to a circuit. Then the minimum value of
W c (n) and the corresponding I(n) used to compute the temperatures for the other cables of the
circuit.
For external heat sources the equations are:
Wy (n) = ( Ty (n)- dTi(n) - dTo(n) - dTe (n) - Ta ) /(Ry + Re(n,n))
(1c)
Ty (n) = dTi(n) + dTe(n) + dTo (n) + Wy (n) (Ry + Re(n,n)) + Ta
(1d)
where:
W y = watts flowing from external heat source
Ry = thermal resistance of pipe insulation or duct wall
Ty (n) = temperature of pipe for steam line or temperature of duct id for cable(s) in duct or earth
interface temperature for direct buried cable system.
1
in this context cable can be either a single isolated cable or group of cables in contact.
** © USi 1988 – 2002 All Rights Reserved **