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Motor Branch-Circuit and Voltage Drop Calculator
electrician2.com

 


(60 HZ Alternating Current induction type 1-phase & 3-phase, wound rotor, and under 600 volt motors )
Updated to 2011 NEC for ambient temperature  and ampacities 4/04/2011

Motor AC Voltage Horsepower  (Notes 5 - 9) Phase
(Notes 5 - 9)
Type of Motor
(Note 10)
Class of Service
(Notes 1 & 2)
Duty-Cycle
Service
(Notes 1 & 2)

Enter NamePlate Current
Required If Class of Service is other than Continuous. Optional otherwise.  Note 21


Conductor Material


Ambient

Temperature C


Number Current Carrying Conductors in Raceway or Cable or bundle

Service Factor is 1.15 or greater       

Special Conditions

Temp rise is not over 40 Degrees C. 
Select Conductor Insulation Temperature Note 22

Select Termination Temperature C.

Circuit conductors are bundled or in raceway 24 inches or less in length Conductors are in Type AC or MC cable that complies with Note 3.

Yes

Yes

No

No

CLICK BUTTON FOR OUTPUTS BELOW

Conductor Size

Motor branch-circuit short-circuit and ground-fault protection
(percentage of FLA from Tables 430.248 and 430.250)

1-phase and 3-phase motor outputs

Equipment Grounding Conductor Size (Note 23)

non time delay fuse (300% )

dual element time delay fuse (175%) instantaneous trip circuit breaker (800% or 1100% -design B) inverse time trip circuit breaker (250%)

 

Maximum Values see Note 11

Motor Disconnect Size (Note 17)
amperes minimum
or
hp rated switch (min)
a) 400% for fuses not greater than 600 amperes
b) 300% for fuses of 601 to 6000 ampere classification
Notes 12 & 16

 

225%
Note 13

 

1300%
1700% for Design B
Note 14

a) 400% for FLA of 100 amperes or less
b) 300% FLA is greater than 100 amperes
Note 15

Controller NEMA size
Notes 18, 19, & 20

Overload Protection
(
Note 21) normal max size
430.32

max relay size
430.34

Wound Rotor Motor outputs

non time delay fuse
( 150%)
dual element time delay fuse (150%) instantaneous trip circuit breaker (800%) inverse time trip circuit breaker (150%)

Data

Conductor Derated
ampacity
Terminal
Ampacity
AC or MC Cable Derating Factor
(if applicable)
Ambient
Correction
Factor


 

Total Load


Terminal Load

(total load)
Full Load
amperes (FLA)

Conductor Derating Factor for more than three current carrying conductors

Conductor ampacity


Conductor circular mil area

Total conductor ampacity Derating Factor
  Motor Full Load Amperes
Current Multiplier (for continuous duty Class of Service)
Nameplate current multiplier (for other than continuous duty Class of Service )
see Note 4


 

Voltage Drop for above Conductors

Enter Distance, Then Click Calculate Button
(If available, this calculator uses nameplate FLA as a load.  Otherwise the Motor FLA from the NEC Tables is used.)


Enter Distance in feet from supply to load (one way)

 

Input from above program:
Material Volts/Phase conductor Size CMA of conductor Load (FLA or Nameplate) k (circular mil-ohm ft.
Voltage drop
Voltage at load end of circuit
Per Cent voltage drop

 
 

Copyright Notice

Notes

1. Any motor application shall be considered as continuous duty unless the nature of the apparatus it drives is such that the motor will not operate continuously with load under any condition of use.
2. 
See Table 430.22(E) for more information on Classification of Service. 
3.  310.15(B)(2)(3)(a)Exception No, 5  Circuit is Type AC cable or to Type MC cable without an overall outer jacket under the following conditions:(1) Each cable has not more than three current-carrying conductors. (2) The conductors are 12 AWG copper.(3) Not more than 20 current-carrying conductors are bundled, stacked, or supported on “bridle rings. (Load must be 18.6 amperes or less - ambient multiplier 1.04 x .6 x 30))
4.  Table 430.22(E) lists Duty-Cycle Service descriptions.
5.  Single phase motor horsepower shall be from 1/6 hp to 10 hp and 115 volts to 230 volts.
6.  Three phase horsepower shall be from 1/2 hp to 500 hp.
7.  Three phase 115 volt motors shall be 1/2 hp to 2 hp.
8.  Three phase Motors from 3 hp to 200 hp shall be 200 volts to 575 volts.
9.  Three phase Motors from 250 hp to 450 hp shall be  460 volts or 575 volts.
10.  Single Phase Motors cannot be design B or wound rotor types.
11. 430.52(C)(1) Exception No. 2 Where the rating specified in Table 430.52, as modified by Exception No. 1, is not suffıcient for the starting current of the motor: notes 12 thru 16 apply.
12.
  The rating of a nontime-delay fuse not exceeding 600 amperes or a time-delay Class CC fuse shall be permitted to be increased but shall in no case exceed 400 percent of the full-load current.  See Note 15 below.
13.  The rating of a time-delay (dual-element) fuse shall be permitted to be increased but shall in no case exceed 225 percent of the full-load current.
14.  Where the setting specified in Table 430.52 is not suffıcient for the starting current of the motor, the setting of an instantaneous trip circuit breaker shall be permitted to be increased but shall in no case exceed 1300 percent of the motor full-load current for other than Design B energy-effıcient motors and no more than 1700 percent of full-load motor current for Design B energy-effıcient motors. Trip settings above 800 percent for other than Design B energy-effıcient motors and above 1100 percent for Design B energy-effıcient motors shall be permitted where the need has been demonstrated by engineering evaluation.
15.  The rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes.
16.
The rating of a fuse of 601–6000 ampere classification shall be permitted to be increased but shall in no case exceed 300 percent of the full-load current.
17.
The disconnecting means for motor circuits rated 600 volts, nominal, or less shall have an ampere rating not less than 115 percent of the full-load current rating of the motor, or be a listed nonfused motor-circuit switch having a horsepower rating not less than the motor horsepower.
18.
The controller shall be a horsepower rated switch, general use switch for motors up to 2 hp and 300 volts, or an inverse time circuit breaker.
19.
  Motors  1/8 hp and less do not require a separate controller.  The
branch-circuit protective device shall be permitted to serve as the controller.
20.
For portable motors rated at 1⁄3 hp or less, the controller shall be permitted to be an attachment plug and receptacle.
21.
  Nameplate full load amperes is used to calculate the overload protection size.  The overload protection size will result in an output of "N/A" if the nameplate full load current is not entered.  The nameplate full load current value is required for other than continuous class of service since this value is used to calculate the branch-circuit ground-fault and short-circuit protection device values.
22.
NM cable shall have a 90 degree C insulation (used for derating) but shall be used at the 60 degree C ampacity (334.80).  (Use 90 degree C insulation and 60 degree C termination ampacity.)
23.
 If the motor circuit overcurrent device consists of an instantaneous trip circuit breaker or a motor short-circuit protector, as allowed in 430.52, the equipment grounding conductor size shall be permitted to be based on the rating of the motor overload protective device using Table 250.122.  If the nameplate full load current value is not entered this determination cannot be made.  In this case "N/A" shall appear in the output box for the equipment grounding conductor.