10
EXCERPTS FROM NEC 2005 HANDBOOK - BASED ON NFPA 70
Partial Table 310.16 Allowable Ampacities of Insulated Conductors (0-2000 Volts)
- Not more then 3 current carrying conductors in cable or raceway
- Based on Ambient Temp of 30˚C
Copper Wire Only
How to Estimate Unit Supply Wire Requirements:
1.) Determine Nominal Input Voltage
2) Estimate Length of Wire from Breaker to Unit Disconnect
3) Find Unit MCA for desired confi guration
4) Using the minimum wire size based on the unit MCA, calculate
the % Voltage Drop to determine the initial wire gauge. The
%VD should be 3% or less at the unit
5) Verify Unit Voltage is within the Appliances min/max limits
6) Verify that the Wire ampacity (corrected for temperature &
number of conductors) is above the Appliances MCA
7) Based on fi nal wire gauge selection, determine conduit size.
Refer to NEC for details and allowable fi ll rate
8) Based on Appliance MOP and fi nal wire selection, make
selection of Appliance branch circuit protection
Size
Temperature Rating of
Conductor
Size
AWG or
kcmil
60˚C 75˚C 90˚C
AWG or
kcmil
14 20 20 25 14
12 25 25 30 12
10 30 35 40 10
84050558
65565756
47085954
3 85 100 110 3
2 95 115 130 2
1 110 130 150 1
1/0 125 150 170 1/0
2/0 145 175 195 2/0
3/0 165 200 225 3/0
Amb. Temperature Correction Factors
Amb.
Temp
(˚C)
Temperature Rating
of Conductor
Amb.
Temp
(˚F)
60˚C 75˚C 90˚C
21-25 1.08 1.05 1.04 70-77
26-30 1.00 1.00 1.00 78-86
31-35 0.91 0.94 0.96 87-95
36-40 0.82 0.88 0.91 96-104
41-45 0.71 0.82 0.87 105-113
46-50 0.58 0.75 0.82 114-122
51-55 0.41 0.67 0.76 123-131
# of Current-Carrying
Conductors
Amount to adjust values in
Table 310.16 after correction
for Ambient Temperature (if
necessary)
4 to 6
7 to 9
80%
70%
Partial Table 9 Effective Impedance for 600V Cables, 3Ø, 60 Hz, 75˚C - 3 single current-carrying conductors in conduit
Uncoated Copper Wire Only
Effective Impedance (Z) at 0.85 PF
to Neutral / 1000 ft
AWG or
kcmil
PVC
Conduit
Aluminum
Conduit
Steel
Conduit
14 2.7 2.7 2.7
12 1.7 1.7 1.7
10 1.1 1.1 1.1
8 0.69 0.69 0.70
6 0.44 0.45 0.45
4 0.29 0.29 0.30
3 0.23 0.24 0.24
2 0.19 0.19 0.20
1 0.16 0.16 0.16
1/0 0.13 0.13 0.13
2/0 0.11 0.11 0.11
3/0 0.088 0.092 0.094
The above tables and recommended procedures are provided for reference only. For complete details refer to the current version of
the National Electric Code, the applicable local codes & ordinances, or consult a qualifi ed professional.
100%
3
1000
9
tageNomLineVol
pVoltageDro
pVoltageDro
pVoltageDropVoltageDro
UnitMCA
ft
gthCircuitLen
ValueTablepVoltageDro
Lin etoLin e
NeutraultoLin eLin etoLin e
NeutraultoLin e
Figure 9. Excerpts from NEC 2005 Handbook
RECOMMENDED SIZING OF 3 PHASE SUPPLY WIRES
(12 pages)
(24 pages)
(60 pages)
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