UNDERGROUND DUCTBANK HEATING CONSIDERATIONS

Transcription

1 UNDERGROUND DUCTBANK HEATING CONSIDERATIONS A Practical Approach to Determining UG Electrical Ductbank Ampacity Mike Mosman, PE CCG Facilities Integration Incorporated March 2016

2 Topics 2 PHYSICS of HEAT in DUCTBANKS NEC and DUCTBANK AMPACITY TIPS for ACCURATE AMPACITY CALCULATIONS

3 Ductbanks Have Issues 3 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.

4 4 PART ONE The Physics of Underground Ductbank Heating and Ampacity Calculations

5 Basic Thermodynamics 5 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.

6 Heat Generation 6 i = amps V in V out Watts (W) = i 2 x R = i x (V in V out ) 1 Watt-second (W-s) = 1 Joule (J) J = 1 BTU (British Thermal Unit) Q = Heat, measured in Joules or BTU s (and sometimes calories)

7 Heat Flow 7 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 = T hot T cold in ºC Dimensions are in centimeters (cm) 1/k = Thermal resistance, Rho (ºC-cm/W) L = Length (cm) A = Area (cm 2 ) Q T hot k T cold

8 Relative Impedance Watts Special Considerations 8 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.) 4y y x Amps Silver Copper Aluminum 2x These facts can have significant implications in ductbank designs. Temperature (degrees C)

9 Heat Transfer Mechanisms 9

10 Representative Ductbank 10 AMBIENT SLABB NATIVE SOIL BACKFILLB ENCASEMENTB DUCTB INSULATION CONDUCTOR

11 Typical Ductbank Heat Flow 11 BACKFILL FINISHED SURFACE ENCASEMENT NATIVE SOIL 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.

12 Types of Heat Flow in Ductbanks 12 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.)

13 Equivalent Circuit 13 Conductor Temperature Wire Surface Temperature Duct Surface Temperature Encasement Surface Temperature Slab Underside Temperature V c V i V d V e V s Z b Z i Z d Z e Z s Insulation Duct Encasement Slab Backfill/Native Soil Current Source Amps = Heat Generated Ground = Ambient Temperature Z n Typical Equivalent Impedance Volts Temperature Above Ambient ( T) Impedance Thermal Resistivity (rho) Amps Heat Flow (Q)

14 TEMPERATURE VOLTS Thermal Models 14 SW ON COFFE TEMP SW OFF AIR TEMP (AMBIENT) GROUND POTENTIAL (AMBIENT) TIME TIME AMP SOURCE SW Z V VOLTS A STEAMIN CUP A JOE LEFT ON THE TABLE. SIMPLIFIED EQUIVALENT CIRCUIT.

15 TEMPERATURE Temperature vs Time Domain 15 Load On Load Off T 2 T 1 T 0 t 1 t 2 t 3 t 0 TIME This is a typical conductor temperature vs. time curve when a load is turned on and off, and is constant while on. T 0 is ambient (or starting) temperature. T 2 is maximum conductor temp when thermal equilibrium is reached at t 2. Load is turned off at point of thermal equilibrium and cools to ambient at t 3. (t 3 - t 2 = t 2 - t 0 ) t 1 is the time constant of this curve type. (T 1 = 63% of T 2 )

16 16 PART TWO The National Electrical Code and Underground Ductbank Calculations

17 NEC Article

18 NEC (A)(1) 18 Note This is for low voltage wires only. Two methods of calculating wire ampacities is allowed: Tables in (B) which are familiar to every engineer, or Under engineering supervision per (C) which is basically the Neher-McGrath formulas. Note the reference to Annex B.

19 NEC (A)(2) 19 There is an important Exception in (A)(2). It will come in handy in all sorts of situations. Note the reference to termination limitation. 90 C wire ampacity cannot be used with 75 C rated terminations.

20 NEC (A)(3) 20 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.

21 NEC (C) 21 This is the basis for use of the Neher-McGrath. All is fairly simple except for determining R ca. Thus the popularity of ampacity software.

22 NEC This part of the code is for medium voltage cables. It also allows engineering supervision, i.e. Neher-McGrath calculations.

23 NEC Annex B 23 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.

24 Table B (B)(2)(7) 24 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

25 Figure B (B)(2)(2) 25 But what if your ductbank doesn t look like these?

26 B (B)(5) 26 AMPACITY = 2 x.88 AMAPACITY OF 1 DUCT 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? AMPACITY = 4 x.94 AMPACITY OF 1 DUCT IN 3-WAY DUCTBANK

27 27 Figure B (B)(2)(3) INFO This figure give us a 9-way ductbank. Again, interpolation for 7-way and 8-way ductbanks is fairly accurate. Computers required beyond this.

28 Figure B (B)(2)(1) 28 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, I 1 being the larger amperage and I 2 being the smaller amperage of the three columns of amperages. Note the dotted line is I 1, the larger amperage.

29 B (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.

30 B (B)(3)(b) BAD GOOD BETTER, BUT THE NEC DOESN T CARE. This applies to MV ductbanks as well.

31 31 Table B (B)(2)(11) 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.

32 32 PART THREE Tips for Making Accurate and Economical Underground Ductbank Ampacity Calculations

33 Typical Service Entrance? 33

34 Complex Ductbank Problem 34

35 35 Is the Code Conservative? Read this excerpt from the NEC Handbook I ll wait.

36 Rho Values 36 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.

37 Concrete Rho Values (typical rpt.) 37 (Courtesy Near-Mcgrath.com) Note variation of rho values with concrete encasement hardness and water content.

38 Soil Rho Values (typical report) 38 Project Soil rho values are rarely consistent and depend heavily on water content.

39 39 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. Parking 150 º Air-conditioned 70 º Slab Water evaporates under hot paving...and condenses under cool slab.

40 Dry vs Wet Soil 40 SATURATED SOIL 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.

41 Appropriate Ambient Temperature 41 Parking 150 º Air-conditioned 70 º Ambient 35ºC (or higher?) 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.

42 Temp Adjustments in Portions 42 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 10 HEAT SOURCE 90ºC WIRE PORTION 1 PORTION 2

43 Load Definition 43 NEC 100 NEC NEC 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?

44 Beware the PVC Duct Limitation 44 Most PVC conduits are UL listed for 90ºC max wires. 105ºC MV cables may be loaded only to 90ºC when in PVC.

45 Good Ductbank Design Practices 45 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.

46 Good Ductbank Design Practices 46 4 OR 5 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.

47 Non-concurrent Redundancy 47 A 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. 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.

48 TEMPERATURE Short Duration Loads 48 T 2 T 1 Time-domain curve T 0 t 0 t 1 t 2 TIME Short duration or time-limited loads may benefit from a temperature vs. time (time-domain) ampacity calculation. Backup generator feeders, maintenance bypass feeders, etc. T 0 is ambient (or starting) temperature. T 2 is maximum conductor temp when thermal equilibrium is reached at t 2. If load is turned off before thermal equilibrium at t 2, the maximum conductor temperature T 1 will be lower than T 2.

49 49 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.

50 50 QUESTIONS & COMMENTS Michael Mosman, PE VP, CTO CCG Facilities Integration Incorporated Baltimore, MD (410)