UNDERGROUND DUCTBANK
HEATING CONSIDERATIONS
A Practical Approach to Determining
UG Electrical Ductbank Ampacity
Mike Mosman, PE
CCG Facilities Integration Incorporated
March 2016
© CCG Facilities Integration Incorporated
Topics
2
ˆ
PHYSICS of HEAT in DUCTBANKS
ˆ
NEC and DUCTBANK AMPACITY
ˆ
TIPS for ACCURATE AMPACITY
CALCULATIONS
© CCG Facilities Integration Incorporated
Ductbanks Have Issues
3
ˆ
What’s wrong with this picture?
‡
ˆ
Maybe lots of things, maybe nothing.
One needs to know purpose and usage of ductbank before
design is deemed suitable.
© CCG Facilities Integration Incorporated
4
PART ONE
The Physics of Underground
Ductbank Heating and Ampacity
Calculations
Basic Thermodynamics
5
ˆ
Heat moves from hot
things to cold things.
ˆ
Heat flows through
liquids, solids and gasses
by various mechanisms.
ˆ
Heat transfer rate
depends on temperature
difference and thermal
resistance.
© CCG Facilities Integration Incorporated
Heat Generation
6
i = amps
Vin
Vout
Watts (W) = i2 x R = i x (Vin – Vout)
ˆ 1 Watt-second (W-s) = 1 Joule (J)
ˆ 1055.06 J = 1 BTU (British Thermal Unit)
ˆ Q = Heat, measured in Joules or BTU’s (and
sometimes calories)
ˆ
© CCG Facilities Integration Incorporated
Heat Flow
7
Q = Heat flow rate = ∂Q/∂t in J/s or BTU/h
ˆ Q = k x A x ∆T /L, with units in Watts (J/s) when:
ˆ
k = Thermal conductivity in W/ºC-cm
‡ ∆T = Thot – Tcold in ºC
‡ Dimensions are in centimeters (cm)
‡
ˆ
1/k = Thermal resistance, Rho (ºC-cm/W)
A = Area (cm2)
L = Length (cm)
Thot
Q
k
Tcold
© CCG Facilities Integration Incorporated
Special Considerations
8
ˆ
4y
y
Conductor impedance
increases with operating
temperature. (Be aware of
temperature correction factors.)
These facts can have
significant implications in
ductbank designs.
x
2x
Amps
Silver
Relative Impedance
ˆ
Conductor heat
dissipation is not
linear to the load.
It varies with the
square of the
current.
Watts
ˆ
Copper
Aluminum
Temperature (degrees C)
© CCG Facilities Integration Incorporated
Heat Transfer Mechanisms
9
© CCG Facilities Integration Incorporated
Representative Ductbank
10
AMBIENT
SLABB
NATIVE SOIL
BACKFILLB
ENCASEMENTB
DUCTB
INSULATION
CONDUCTOR
© CCG Facilities Integration Incorporated
Typical Ductbank Heat Flow
11
BACKFILL
FINISHED SURFACE
ENCASEMENT
NATIVE SOIL
ˆ
ˆ
ˆ
Encasement conducts heat from source (wires in duct).
Ultimately, almost all heat from encasement flows to surface.
Most heat flows path of least thermal resistance, which makes
backfill very important.
© CCG Facilities Integration Incorporated
Types of Heat Flow in Ductbanks
12
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
Radiation and conduction from surface to ambient
environment. (Lower ambient produces greater
heat flow.)
Conduction through slab or paving, if present.
(Often ignored.)
Conduction through backfill and native soil.
(Thermal resistivity, rho, of soil and backfill often
considered equivalent. Lower rho produces greater
heat flow.)
Conduction through encasement. (Thermal
resistivity, rho, of concrete often set at 55.
However, hardness and water content affect rho
values.)
Convection, radiation and conduction pass heat
from wire to duct. (Duct temperature assumed to
be that of cable surface.)
Conduction through wire insulation. (Includes
shields and outer coverings. Codes differ for LV
and MV cable types.)
© CCG Facilities Integration Incorporated
Equivalent Circuit
13
Conductor
Temperature
Wire
Surface
Temperature
Vc
Duct
Surface
Temperature
Encasement
Surface
Temperature
Vd
Ve
Vi
Slab
Underside
Temperature
Vs
Zb
Zi
Zd
Ze
Insulation
Duct
Encasement
Zs
Zn
Slab
Backfill/Native Soil
Current Source
Amps = Heat Generated
Ground = Ambient Temperature
Typical Equivalent Impedance
ˆ
ˆ
ˆ
Volts ≈ Temperature Above Ambient (∆T)
Impedance ≈ Thermal Resistivity (rho)
Amps ≈ Heat Flow (Q)
© CCG Facilities Integration Incorporated
Thermal Models
14
COFFE TEMP
VOLTS
TEMPERATURE
SW ON
SW OFF
GROUND POTENTIAL (AMBIENT)
AIR TEMP (AMBIENT)
TIME
TIME
AMP
SOURCE
A STEAMIN’ CUP’A JOE
LEFT ON THE TABLE.
SW
Z
V
VOLTS
SIMPLIFIED EQUIVALENT CIRCUIT.
© CCG Facilities Integration Incorporated
Temperature vs Time Domain
TEMPERATURE
15
ˆ
ˆ
ˆ
ˆ
Load On
Load Off
T2
T1
T0
t0
t1
t2
t3
TIME
This is a typical conductor temperature vs. time curve when a
load is turned on and off, and is constant while on.
T0 is ambient (or starting) temperature. T2 is maximum
conductor temp when thermal equilibrium is reached at t2.
Load is turned off at point of thermal equilibrium and cools to
ambient at t3. (t3 - t2 = t2 - t0)
t1 is the “time constant” of this curve type. (T1 = 63% of T2)
© CCG Facilities Integration Incorporated
16
PART TWO
The National Electrical Code and
Underground Ductbank Calculations
NEC Article 310
17
© CCG Facilities Integration Incorporated
NEC 310.15(A)(1)
18
ˆ
ˆ
Note – This is for low
voltage wires only.
Two methods of
calculating wire
ampacities is allowed:
‡
‡
ˆ
Tables in 310.15(B) which
are familiar to every
engineer, or
Under engineering
supervision per 310.15(C)
which is basically the
Neher-McGrath formulas.
Note the reference to
Annex B.
© CCG Facilities Integration Incorporated
NEC 310.15(A)(2)
19
There is an important Exception in 310.15(A)(2). It
will come in handy in all sorts of situations.
ˆ Note the reference to termination limitation. 90 {C
wire ampacity cannot be used with 75{C rated
terminations.
ˆ
© CCG Facilities Integration Incorporated
NEC 310.15(A)(3)
20
ˆ
ˆ
This paragraph in the code states that the manner of use of a
conductor has a bearing on the selection of its maximum
allowable ampacity.
It is incumbent on the Engineer to determine the purpose of
the conductors in UG ductbanks, and perform appropriate
ampacity calculations that find the most economical design
that results in safe operation.
© CCG Facilities Integration Incorporated
NEC 310.15(C)
21
ˆ
ˆ
ˆ
This is the basis for use of the Neher-McGrath.
All is fairly simple except for determining Rca.
Thus the popularity of ampacity software.
© CCG Facilities Integration Incorporated
NEC 310.60
22
ˆ
ˆ
This part of the code is for medium voltage cables.
It also allows “engineering supervision,” i.e. Neher-McGrath
calculations.
© CCG Facilities Integration Incorporated
NEC Annex B
23
ˆ
ˆ
ˆ
When you don’t have the software and want to do quick
calculations on simple ductbanks, Annex B is a good tool.
Annex B is information and not part of the required code.
It applies to low voltage wiring (up to 2000 volts) and is not
used for MV ductbanks.
© CCG Facilities Integration Incorporated
Table B.310.15(B)(2)(7)
24
ˆ
ˆ
ˆ
This table is used
more than all
others together.
It’s limited to just
three ductbank
configurations.
It uses “standard”
ductbank crosssections shown in
Figure B.310.15.
© CCG Facilities Integration Incorporated
Figure B.310.15(B)(2)(2)
25
ˆ
But what if your ductbank doesn’t look like these?
© CCG Facilities Integration Incorporated
B.310.15(B)(5)
26
AMPACITY = 2 x .88
AMAPACITY OF 1 DUCT
AMPACITY = 4 x .94
AMPACITY OF 1 DUCT IN
3-WAY DUCTBANK
ˆ
ˆ
What about a 5-way ductbank? Interpolation between the 4way (calculated) and the 6-way In chart is fairly accurate.
What about larger ductbanks?
© CCG Facilities Integration Incorporated
Figure B.310.15(B)(2)(3) INFO
27
ˆ
This figure give us a 9-way ductbank. Again, interpolation for
7-way and 8-way ductbanks is fairly accurate.
ˆ
Computers required beyond
this.
© CCG Facilities Integration Incorporated
Figure B.310.15(B)(2)(1)
28
ˆ
ˆ
ˆ
ˆ
If you know how to use this
chart, you’re already expert.
The bottom half allows you to
select a different Rho value and
load factor than those given in
the Tables.
The upper half is derived from
the amperages given in the
Tables, I1 being the larger
amperage and I2 being the
smaller amperage of the three
columns of amperages.
Note the dotted line is I1, the
larger amperage.
© CCG Facilities Integration Incorporated
B.310.15 (B)(3)(a)
29
25’
100’
ˆ
Does this mean if the ductbank was 400’ long a deeper part
could be 100’? No. It’s purpose is to avoid obstructions, not to
avoid ampacity adjustments.
‡
This applies to MV ductbanks as well.
© CCG Facilities Integration Incorporated
B.310.15.(B)(3)(b)
30
-30”
BAD
‡
GOOD
BETTER, BUT THE
NEC DOESN’T CARE.
This applies to MV ductbanks as well.
© CCG Facilities Integration Incorporated
Table B.310.15(B)(2)(11)
31
ˆ
ˆ
Account for the neutral wire if it’s current carrying.
Conductor count and ambient temp corrections must both be
applied. Why?
‡
‡
‡
Current carrying
neutral adds to Q, the
heat being generated.
Higher ambient temp
lowers ∆T which
reduces Q, the heat
flow.
More heat + less heat
flow = higher
conductor temps.
© CCG Facilities Integration Incorporated
32
PART THREE
Tips for Making Accurate and
Economical Underground Ductbank
Ampacity Calculations
Typical Service Entrance?
33
© CCG Facilities Integration Incorporated
Complex Ductbank Problem
34
© CCG Facilities Integration Incorporated
Is the Code Conservative?
35
ˆ
Read this excerpt from the NEC Handbook – I’ll wait.
© CCG Facilities Integration Incorporated
Rho Values
36
ˆ
The following (from Annex B) is a “suggestion” by the NEC,
but is it appropriate?
ˆ
It’s better to verify actual conditions to be found on site. Ask
for official reports.
© CCG Facilities Integration Incorporated
Concrete Rho Values (typical rpt.)
37
(Courtesy Near-Mcgrath.com)
ˆ
Note variation of rho values with concrete encasement
hardness and water content.
© CCG Facilities Integration Incorporated
Soil Rho Values (typical report)
38
Project
ˆ
Soil rho values are rarely consistent and depend heavily on
water content.
© CCG Facilities Integration Incorporated
A Word About Water Content
39
ˆ
ˆ
ˆ
ˆ
True: It never rains under a building.
False: That means it’s dry under there.
Q: Why do they put a vapor barrier under the most concrete
slabs on grade?
A: To keep the moisture from coming through the slab from
below.
Air-conditioned
building @ 70º
Parking lot @ 150º
Slab
Water evaporates
under hot paving
...and condenses
under cool slab.
© CCG Facilities Integration Incorporated
“Dry” vs “Wet” Soil
40
ˆ
ˆ
SATURATED SOIL
ˆ
ˆ
ˆ
“DRY” SOIL
Conduction of heat in soil occurs
at contact points between soil
particles.
Water in soil aids in conduction of
heat.
Saturated soils have all air gaps
filled with water.
As soil dries some water remains.
Due mainly to capillary action the
remaining water collects around
particle contact points.
Even a small amount of residual
water aids conduction at particle
contact points.
© CCG Facilities Integration Incorporated
Appropriate Ambient Temperature
41
Air-conditioned
building @ 70º
Parking lot @ 150º
Ambient 35ºC
(or higher?)
ˆ
ˆ
ˆ
Ambient
normal 20ºC
Heat generally flows toward surface.
Ductbanks under large buildings generally remain a stable
20ºC or close to the building’s interior temperature.
Ductbanks outdoors under heat-gathering surfaces will have
higher ambient temperatures.
© CCG Facilities Integration Incorporated
Temp Adjustments in Portions
42
ˆ
ˆ
Wire ampacity at termination must be based on 75ºC wire, not
90ºC, due to termination.
In portion of duct near heat source temperature deration must
be applied, but it may be applied to 90ºC wire rating instead of
75ºC wire rating.
75ºC TERMINATION
HEAT SOURCE
10’
90ºC WIRE
PORTION 1
PORTION 2
© CCG Facilities Integration Incorporated
Load Definition
43
NEC 100
NEC 210.19
NEC 220.60
ˆ
ˆ
These definitions imply a Load Factor effect on ampacity, but
Load Factor is not defined anywhere in the NEC.
Load Factor impact on conductor ampacity depends on what
time duration is used to define it. 3 hours? 24 hours? A
week? A year?
© CCG Facilities Integration Incorporated
Beware the PVC Duct Limitation
44
ˆ
ˆ
Most PVC conduits are UL listed for 90ºC max wires.
105ºC MV cables may be loaded only to 90ºC when in PVC.
© CCG Facilities Integration Incorporated
Good Ductbank Design Practices
45
Stack ducts 2-high maximum. All ducts get proximity
to the encasement surface to facilitate heat flow.
ˆ Uneven number of ducts? Leave the blank at the
bottom middle. That’s the hottest position.
ˆ
© CCG Facilities Integration Incorporated
Good Ductbank Design Practices
46
4’
OR
5’
ˆ
The code allows (in Annex B) that mutual heating of
ductbanks is negligible if edges of encasements are
4’ apart or nearest conduits are 5’ apart.
Only for ductbanks up to 2000 volts.
‡ This may conflict with many computerized programs.
‡
ˆ
This implies that feeders 5’ or more apart in a wide
ductbank will not mutually heat each other.
© CCG Facilities Integration Incorporated
Non-concurrent Redundancy
47
ˆ
B
A
B
B
A
B
A
When non-concurrent loads appear in same ductbank, the
code allows the calculation to be made with only the greater
load.
‡
‡
‡
‡
ˆ
A
Normal and emergency feeders to ATS’s.
UPS input and bypass feeders.
“A” and “B” circuits to double-corded computer equipment.
Utility and backup EG feeders.
Interleave “A” and “B” circuits for cooler ductbanks.
© CCG Facilities Integration Incorporated
Short Duration Loads
TEMPERATURE
48
ˆ
ˆ
T0
Time-domain curve
t0
t1
t2
TIME
Short duration or time-limited loads may benefit from a
temperature vs. time (time-domain) ampacity calculation.
‡
ˆ
T2
T1
Backup generator feeders, maintenance bypass feeders, etc.
T0 is ambient (or starting) temperature. T2 is maximum
conductor temp when thermal equilibrium is reached at t2.
If load is turned off before thermal equilibrium at t2, the
maximum conductor temperature T1 will be lower than T2.
© CCG Facilities Integration Incorporated
Ampacity Software
49
ˆ
When the design is complex, computerized ampacity
programs are a must. Common software programs are:
‡
ETAP (etap.com)
„
„
‡
AmpCalc (calcware.com)
„
„
„
‡
Single-purpose software.
Inexpensive and easy to use.
Does not perform time-domain calculations.
CymCap (cyme.com)
„
„
ˆ
Requires add-on package for underground cable thermal calculations.
Add-on includes time-domain (transient) calculations.
Single-purpose software.
Performs time-domain calculations.
All above programs based on Neher-McGrath equations.
© CCG Facilities Integration Incorporated
50
QUESTIONS & COMMENTS
Michael Mosman, PE
VP, CTO
CCG Facilities Integration Incorporated
Baltimore, MD
(410)525-0010
[email protected]