UNDERGROUND DUCTBANK HEATING CONSIDERATIONS Ductbank Heating Considerations.pdf · UNDERGROUND...
Transcript of UNDERGROUND DUCTBANK HEATING CONSIDERATIONS Ductbank Heating Considerations.pdf · UNDERGROUND...
© CCG Facilities Integration Incorporated
March 2016
UNDERGROUND DUCTBANK
HEATING CONSIDERATIONS
A Practical Approach to Determining
UG Electrical Ductbank Ampacity
Mike Mosman, PE
CCG Facilities Integration Incorporated
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Topics
PHYSICS of HEAT in DUCTBANKS
NEC and DUCTBANK AMPACITY
TIPS for ACCURATE AMPACITY
CALCULATIONS
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Ductbanks Have Issues3
What’s wrong with this picture?
Maybe lots of things, maybe nothing.
One needs to know purpose and usage of ductbank before
design is deemed suitable.
The Physics of Underground
Ductbank Heating and Ampacity
Calculations
PART ONE4
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Basic Thermodynamics
Heat moves from hot
things to cold things.
Heat flows through
liquids, solids and gasses
by various mechanisms.
Heat transfer rate
depends on temperature
difference and thermal
resistance.
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Heat Generation6
i = amps
Vin Vout
Watts (W) = i2 x R = i x (Vin – Vout)
1 Watt-second (W-s) = 1 Joule (J)
1055.06 J = 1 BTU (British Thermal Unit)
Q = Heat, measured in Joules or BTU’s (and
sometimes calories)
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Heat Flow7
Q = Heat flow rate = ∂Q/∂t in J/s or BTU/h
Q = k x A x ∆T /L, with units in Watts (J/s) when:
k = Thermal conductivity in W/ºC-cm
∆T = Thot – Tcold in ºC
Dimensions are in centimeters (cm)
1/k = Thermal resistance, Rho (ºC-cm/W)
A = Area (cm2)
L = Length (cm)Q
kThot Tcold
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Special Considerations8
Wa
tts
Amps
Re
lative
Im
pe
da
nce
Temperature (degrees C)
Silver
Copper
Aluminum
x 2x
y
4y
Conductor heat
dissipation is not
linear to the load.
It varies with the
square of the
current.
Conductor impedance
increases with operating
temperature. (Be aware of
temperature correction factors.)
These facts can have
significant implications in
ductbank designs.
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Heat Transfer Mechanisms9
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Representative Ductbank10
AMBIENT
SLABB
NATIVE SOIL
BACKFILLB
ENCASEMENTB
DUCTB
INSULATION
CONDUCTOR
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Typical Ductbank Heat Flow
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.
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FINISHED SURFACE
NATIVE SOIL
ENCASEMENT
BACKFILL
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Types of Heat Flow in Ductbanks12
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.)
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Equivalent Circuit13
VsVeVdVi
Insulation
Current Source
Amps = Heat Generated
Duct Encasement Slab
Backfill/Native Soil
Ground = Ambient Temperature
Wire
Surface
Temperature
Duct
Surface
Temperature
Encasement
Surface
Temperature
Slab
Underside
Temperature
Volts ≈ Temperature Above Ambient (∆T)
Impedance ≈ Thermal Resistivity (rho)
Amps ≈ Heat Flow (Q)
Zi Zd Ze Zs
Zb
Zn
Vc
Conductor
Temperature
Typical Equivalent Impedance
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Thermal Models14
VO
LT
STIME
SW ON
GROUND POTENTIAL (AMBIENT)
VOLTS
AMP
SOURCE ZSWV
TE
MP
ER
AT
UR
E
TIME
COFFE TEMP
AIR TEMP (AMBIENT)
SW OFF
A STEAMIN’ CUP’A JOE
LEFT ON THE TABLE.
SIMPLIFIED EQUIVALENT CIRCUIT.
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Temperature vs Time Domain
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)
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TIME
TE
MP
ER
AT
UR
E
t1 t2
T1
T2
Load OffLoad On
T0t3t0
The National Electrical Code and
Underground Ductbank Calculations
PART TWO16
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NEC Article 31017
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NEC 310.15(A)(1)
Note – This is for low
voltage wires only.
Two methods of
calculating wire
ampacities is allowed:
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.
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NEC 310.15(A)(2)
There is an important Exception in 310.15(A)(2). It
will come in handy in all sorts of situations.
Note the reference to termination limitation. 90C
wire ampacity cannot be used with 75C rated
terminations.
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NEC 310.15(A)(3)
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.
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NEC 310.15(C)
This is the basis for use of the Neher-McGrath.
All is fairly simple except for determining Rca.
Thus the popularity of ampacity software.
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NEC 310.60
This part of the code is for medium voltage cables.
It also allows “engineering supervision,” i.e. Neher-McGrath
calculations.
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NEC Annex B
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.
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Table B.310.15(B)(2)(7)
This table is used
more than all
others together.
It’s limited to just
three ductbank
configurations.
It uses “standard”
ductbank cross-
sections shown in
Figure B.310.15.
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Figure B.310.15(B)(2)(2)
But what if your ductbank doesn’t look like these?
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B.310.15(B)(5)
What about a 5-way ductbank? Interpolation between the 4-
way (calculated) and the 6-way In chart is fairly accurate.
What about larger ductbanks?
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AMPACITY = 2 x .88
AMAPACITY OF 1 DUCT
AMPACITY = 4 x .94
AMPACITY OF 1 DUCT IN
3-WAY DUCTBANK
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Figure B.310.15(B)(2)(3) INFO
This figure give us a 9-way ductbank. Again, interpolation for
7-way and 8-way ductbanks is fairly accurate.
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Computers required beyond
this.
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Figure B.310.15(B)(2)(1)
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.
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B.310.15 (B)(3)(a)
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.
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25’100’
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B.310.15.(B)(3)(b)30
-30”
BAD GOOD BETTER, BUT THE
NEC DOESN’T CARE.
This applies to MV ductbanks as well.
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Table B.310.15(B)(2)(11)
Account for the neutral wire if it’s current carrying.
Conductor count and ambient temp corrections must both be
applied. Why?
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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.
Tips for Making Accurate and
Economical Underground Ductbank
Ampacity Calculations
PART THREE32
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Typical Service Entrance?33
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Complex Ductbank Problem34
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Is the Code Conservative?
Read this excerpt from the NEC Handbook – I’ll wait.
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Rho Values36
The following (from Annex B) is a “suggestion” by the NEC,
but is it appropriate?
It’s better to verify actual conditions to be found on site. Ask
for official reports.
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Concrete Rho Values (typical rpt.)
Note variation of rho values with concrete encasement
hardness and water content.
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(Courtesy Near-Mcgrath.com)
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Soil Rho Values (typical report)
Soil rho values are rarely consistent and depend heavily on
water content.
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Project
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A Word About Water Content
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.
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Parking lot @ 150º
Air-conditioned
building @ 70º
Slab
Water evaporates
under hot paving...and condenses
under cool slab.
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“Dry” vs “Wet” 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.
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SATURATED SOIL
“DRY” SOIL
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Appropriate Ambient Temperature
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.
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Parking lot @ 150º
Air-conditioned
building @ 70º
Ambient 35ºC
(or higher?)
Ambient
normal 20ºC
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Temp Adjustments in Portions
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.
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75ºC TERMINATION
90ºC WIRE
HEAT SOURCE
10’
PORTION 1 PORTION 2
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Load Definition43
NEC 100
NEC 220.60
NEC 210.19
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?
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Beware the PVC Duct Limitation44
Most PVC conduits are UL listed for 90ºC max wires.
105ºC MV cables may be loaded only to 90ºC when in PVC.
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Good Ductbank Design Practices45
Stack ducts 2-high maximum. All ducts get proximity
to the encasement surface to facilitate heat flow.
Uneven number of ducts? Leave the blank at the
bottom middle. That’s the hottest position.
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Good Ductbank Design Practices46
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.
This implies that feeders 5’ or more apart in a wide
ductbank will not mutually heat each other.
4’
5’
OR
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Non-concurrent Redundancy47
When non-concurrent loads appear in same ductbank, the
code allows the calculation to be made with only the greater
load.
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.
A BBA
AB B A
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Short Duration Loads
Short duration or time-limited loads may benefit from a
temperature vs. time (time-domain) ampacity calculation.
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.
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TIME
TE
MP
ER
AT
UR
E
t1 t2
T1
T2
Time-domain curve
T0t0
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Ampacity Software
When the design is complex, computerized ampacity
programs are a must. Common software programs are:
ETAP (etap.com)
Requires add-on package for underground cable thermal calculations.
Add-on includes time-domain (transient) calculations.
AmpCalc (calcware.com)
Single-purpose software.
Inexpensive and easy to use.
Does not perform time-domain calculations.
CymCap (cyme.com)
Single-purpose software.
Performs time-domain calculations.
All above programs based on Neher-McGrath equations.
49
Michael Mosman, PE
VP, CTO
CCG Facilities Integration Incorporated
Baltimore, MD
(410)525-0010
QUESTIONS & COMMENTS50