UNDERGROUND DUCTBANK HEATING CONSIDERATIONS Ductbank Heating Considerations.pdf · UNDERGROUND DUCTBANK HEATING CONSIDERATIONS A Practical Approach to Determining UG Electrical Ductbank

© 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

http://www.ccgfacilities.com/default.aspx

http://www.ccgfacilities.com/default.aspx


Topics

PHYSICS of HEAT in DUCTBANKS

NEC and DUCTBANK AMPACITY

TIPS for ACCURATE AMPACITY

CALCULATIONS

2


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


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.

5


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)


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


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.


Heat Transfer Mechanisms9


Representative Ductbank10

AMBIENT

SLABB

NATIVE SOIL

BACKFILLB

ENCASEMENTB

DUCTB

INSULATION

CONDUCTOR


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.

11

FINISHED SURFACE

NATIVE SOIL

ENCASEMENT

BACKFILL


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.)


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


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.


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)

15

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


NEC Article 31017


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.

18


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.

19


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.

20


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.

21


NEC 310.60

This part of the code is for medium voltage cables.

It also allows “engineering supervision,” i.e. Neher-McGrath

calculations.

22


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.

23


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.

24


Figure B.310.15(B)(2)(2)

But what if your ductbank doesn’t look like these?

25


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?

26

AMPACITY = 2 x .88

AMAPACITY OF 1 DUCT

AMPACITY = 4 x .94

AMPACITY OF 1 DUCT IN

3-WAY DUCTBANK


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.

27

Computers required beyond

this.


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.

28


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.

29

25’100’


B.310.15.(B)(3)(b)30

-30”

BAD GOOD BETTER, BUT THE

NEC DOESN’T CARE.

This applies to MV ductbanks as well.


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?

31

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


Typical Service Entrance?33


Complex Ductbank Problem34


Is the Code Conservative?

Read this excerpt from the NEC Handbook – I’ll wait.

35


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.


Concrete Rho Values (typical rpt.)

Note variation of rho values with concrete encasement

hardness and water content.

37

(Courtesy Near-Mcgrath.com)


Soil Rho Values (typical report)

Soil rho values are rarely consistent and depend heavily on

water content.

38

Project


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.

39

Parking lot @ 150º

Air-conditioned

building @ 70º

Slab

Water evaporates

under hot paving...and condenses

under cool slab.


“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.

40

SATURATED SOIL

“DRY” SOIL


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.

41

Parking lot @ 150º

Air-conditioned

building @ 70º

Ambient 35ºC

(or higher?)

Ambient

normal 20ºC


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.

42

75ºC TERMINATION

90ºC WIRE

HEAT SOURCE

10’

PORTION 1 PORTION 2


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?


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.


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.


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


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


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.

48

TIME

TE

MP

ER

AT

UR

E

t1 t2

T1

T2

Time-domain curve

T0t0


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

[email protected]

QUESTIONS & COMMENTS50

Documents

UNDERGROUND DUCTBANK HEATING CONSIDERATIONS Ductbank Heating Considerations.pdf · UNDERGROUND DUCTBANK HEATING CONSIDERATIONS A Practical Approach to Determining UG Electrical Ductbank