p>10-16 wire ampacity tables do not take into account
voltage drop,      only thermal limit. Overhead
triplex, use 310-16 for wire ampacity.

Rule of Thumb

If circuit is loaded
up to ampacity and distance to load is around 100 feet, then voltage
drop will govern and will require larger wire size. If more than 3 phase
wires are in one conduit, then wires must be derated according to Note
8, after Table 310-19 as follows:

TABLE 1

Number of Conductors Percent of Values in 310-16

*1-3      80

*4-6      80

70

25-42      60

43 and above     50

*If circuit is loaded
up to 80% ampacity, then 6 phase wires can be put in one conduit. Neutral
conductors supplying balanced loads do not have to be counted.

Note: Minimum Circuit size is #12 with 20 Amp CB.

Circuit Breaker Size

  Maximum loading of any circuit breaker is 80% of rating for
non-motor loads with exceptions as noted below. Breaker cannot be larger
than ampacity of wire, except for motors and a few othe</span><span class="Normal--Char" style=" font-size: 12pt;
">r loads.

TABLE 2

Load Maximum Circuit Breaker

Size % of Current

Resistance Loads, Heat, Stoves, Toasters, Water Heater     
125%

Lighting            125%

Motors except Hermetically Sealed 00-250% NEC 430-152

Motors Hermetically Sealed, Air conditioners       175%

And heat pumps            

Welders            200%

MCP Breakers for Motors          125% or next larger size

Standard breaker sizes are listed Page 70-74, 240-6 NEC

Conduit Sizing

1. After circuits
have been derated to get proper wire size use Table 3A if wires are
all same size. Use Page 70-57"9, or Tables 4 and 5 if wires are
all different size. Ground wires should be run with all branch circuits.

Example:

Load Furnace
15 KW, I phase, 230V, find current, KVA, wire size, circuit breaker
size, conduit size.

I = _KW = 15KW = 65.2 Amp KV     .23KV

KW = KVA, P.F. = 1.0 KVA
= 15

Wire Size

Wire Size = Load = ampacity = 65.2 Amp = 81.5 Amp

                                                  
.8

                         

                            
Use NEC 310-16, 75°C, THW, wire

#4 wire has 85 amps
 Use #4  2 #4 and, 1 #4 ground Circuit Breaker Sizing

CB = Load = 65.2 Amp = 81.5 Amp

           .8            
.8

Next higher CB size
is 90 Amp.

NEC 240-6 and 240-3 exception 1, conduit-size, 3 #4 in conduit Table
3A =

1" conduit which
allows 3 #4 maximum in a 1" conduit

Example:

Load: Lighting circuit
9-4 tube 4', 50VA/tube 115V fluorescent circuits. find I, circuit breaker,
wire.

(9 fixtures) (4 tubes/fixture) (50V A/tube) = 1800 VA = 1.8 KVA

I = KVA = 1.8 KVA = 15.6 Amp

KV .115 KV

                           Wire Size = Load = 15.6A = 19.5 amps.

                                                  
.8          .8

 

NEC 310-16, THW, 75°C #12 = 20 Amp

Circuit breaker
size = Load = 15.6 = 19.5

                                     
.8         .8

USE 20 Amp breaker

#12 in conduit Table 3A, 1/2" conduit will take up to 4 #12 THW

Example:

    Load: Motor 3 HP, 1 phase, 230V run ground wire with all
motors

I = 17A NEC 430=148

  Wire Size = 17A = 21.5A

                      
.8

                                  
NEC 310-16 #10 is next size wire good for 30 Amp CB = (I)(2 to

2.5) NEC 430-152

                                  CB Size = (17 A) (2) = 34A USE 35A circuit breaker.

                                 Conduit Size

#10, (2 power,1 ground), Table 3A, THW, 1/2" conduit will take
4 #10

                   Example

                  
Load: Unit Heater 10 KW, 230V, 3 phase, find I, wire size, circuit breaker
size

                  
Conduit Size

I =   10 KW =   25.1 Amp 

    
.23 KV 3

                  
Wire Size- 25.1A = 31.3 Amp

                    
.8

                   
NEC 310-16 Use #8 good for 45 amp,
3 #8 + 1 #8 ground in 1" RGC Table 3A

                   
Circuit Breaker = 25.1A = 31.3 Amp so use 35A

                                                    
.8

       Conduit Size if different size wires are used

If different size wires are used in one conduit, the total area filled
cannot ex</span><span class="Normal--Char" style=" font-size: 12pt;">ceed
the percentage area given in Table 4, page 70-637 NEC. Use Table 5,
Page 70-638 to get cross sectional area, then add all areas of all wires
and pick the next largest conduit size