Motor Starting and Running Currents and Rating Guide

A word of caution: The following article is based on National Electrical Manufacturers' Association (NEMA) tables, standards and nomenclature. This is somewhat different from Indian and European practice. The class designations are applicable only to NEMA compatible motors which are in use in the US only. However, the logic and pattern of calculations are the same everywhere. Hence the reader is cautioned to follow only the logical sequence of the calculations.

Motor Starting Current

When typical induction motors become energized, a much larger amount of current than normal operating current rushes into the motor to set up the magnetic field surrounding the motor and to overcome the lack of angular momentum of the motor and its load. As the motor increases to slip speed, the current drawn subsides to match (1) the current required at the supplied voltage to supply the load and (2) losses to windage and friction in the motor and in the load and transmission system. A motor operating at slip speed and supplying nameplate horsepower as the load should draw the current printed on the nameplate, and that current should satisfy the equation

Horsepower = (voltage X current X power factor X motor efficiency X √3) / 746

Typical induction mo

tors exhibit a starting power factor of 10 to 20 percent and a full-load running power factor of 80 to 90 percent. Smaller typical induction motors exhibit an operating full-load efficiency of approximately 92 percent, whereas large typical induction motors exhibit an operating full-load efficiency of approximately 97.5 percent.

Since many types of induction motors are made, the inrush current from an individual motor is important in designing the electrical power supply system for that motor. For this purpose, the nameplate on every motor contains a code letter indicating the kilovoltampere/horsepower starting load rating of the motor. A table of these code letters and their meanings in approximate kVA and horsepower is shown in the following table.

Code Letter on motor name plate	kVA per HP with locked rotor
	Minimum	Mean	Maximum
A	0	1.57	3.14
B	3.15	3.345	3.54
C	3.55	3.77	3.99
D	4	4.245	4.9
E	4.5	4.745	4.99
F	5	5.295	5.59
G	5.6	5.945	6.29
H	6.3	6.695	7.09
J	7.1	7.545	7.99
K	8	8.495	8.9
L	9	9.495	9.9
M	10	10.595	11.19
N	11.2	11.845	12.49
P	12.5	13.245	13.99
R	14	14.995	15.99
S	16	16.995	17.99
T	18	18.995	19.99
U	20	29.2	22.39
V	22.4	No Limit	No Limit

Using these values, the inrush current for a specific motor can be calculated as

I_inrush=(code letter value X horse power x 1000) /( √3 X Voltage)

An example of this calculation for a 50-hp code letter G motor operating at 460 V is shown below

Because of the items listed above, motors that produce constant kVA loads make demands on the electrical power system that are extraordinary compared with the demands of constant kilowatt loads. To start them, the overcurrent protection system must permit the starting current, also called the locked-rotor current, to flow during the normal starting period, and then the motor-running overcurrent must be limited to approximately the nameplate full-load ampere rating. If the duration of the locked-rotor current is too long, the motor will overheat due to I2R heat buildup, and if the long-time ampere draw of the motor is too high, the motor also will overheat due to I2R heating. The National Electrical Code provides limitations on both inrush current and running current, as well as providing a methodology to determine motor disconnect switch ampere and horsepower ratings.

Table 430-152 of the National Electrical Code provides the maximum setting of overcurrent devices upstream of the motor branch circuit, and portions of this table are replicated below

% of Full load current
Motor type	Single element fuse	Dual-element time delay fuse	Inverse time breaker	Instantaneous & Magnetic trip breaker
Single phase motor	300	175	250	800
Three phase squirrel cage motor	300	175	250	800
Design E three phase squirrel cage	300	175	250	1100
Synchronous	300	175	250	800
Wound rotor	150	150	150	800
Direct current	150	150	150	250
For example, a 50 hp, Design B, 460V 3 phase motor has a full load current of 65A at 460V. The maximum rating of an inverse time breaker protecting the motor branch circuit would be 65A x 250%, or 162.5A. The next higher standard rating is 175A (US), so 175A is the maximum rating that can be used to protect the motor circuit.

 

Motor Running Current                                            

The following figures illustrate the calculations required by specific types of motors in the design of electric circuits to permit these loads to start and to continue to protect them during operation.

Table of full-load currents for three-phase ac induction motors (A part of table 430-150 of NEC).

HP	208 V	230 V	460 V	575 V
0.5	2.5	2.2	1.1	0.9
0.75	3.5	3.2	1.6	1.3
1	4.6	4.2	2.1	1.7
1.5	6.6	6	3	2.4
2	7.5	6.8	3.4	2.7
3	10.6	9.6	4.8	3.9
5	16.7	15.2	7.6	6.1
10	30.8	28	14	11
15	46.2	42	21	17
20	59.4	54	27	22
25	74.8	68	34	27
30	88	80	40	32
40	114	104	52	41
50	143	130	65	52
60	169	154	77	62
75	211	192	96	77
100	273	248	124	99
125	343	312	156	125
150	396	360	180	144
200	528	480	240	192

 

Calculating Motor Branch-Circuit Overcurrent Protection and Wire Size

                              

Article 430-52 of the National Electrical Code specifies that the minimum motor branch-circuit size must be rated at 125 percent of the motor full-load current found in Table 430-150 for motors that operate continuously, and Section 430-32 requires that the long-time overload trip rating not be greater than 115 percent of the motor nameplate current unless the motor is marked otherwise. Note that the values of branch-circuit overcurrent trip (the long-time portion of a thermal-magnetic trip circuit breaker and the fuse melt-out curve ampacity) are changed by Table 430-22b for motors that do not operate continuously.

This is illustrated with a sample problem. Consider the circuit shown.

A 40 HP, 460 V, 3 phase, Code letter G, Service factor of 1.0 is planned for operation from a 460 V, 3 phase system. The name plate ampere is 50A. The motor is rated for continuous duty and the load is continuous. Solve for minimum sizes of branch circuit elements?

1. Take motor full load current from table 430-150 as 52A which is higher than name plate value.

2. Determine wire size: 125% of 52A = 65A.

3. Determine inverse time breaker setting: 250% of 52A = 130A, next standard rating is 150A.

4. Determine the rating of thermal overloads: 115% of 50A (name plate current) = 57.5 A

5. Determine disconnect switch ampere rating: 115% of 52A = 59.8 A

6. Determine controller HP rating: 40 HP (same as motor nameplate HP)

The completed circuit will look like this.

NEC Torque classes and characteristics

    

Design Letter	Starting current (%FLC)	Relative Efficiency	Slip in % rpm	Starting torque (%FLT)	Stalling torque (%FLT)
A	Depends upon name plate code letter Normally 630-1000%	High	3%	120-250%	200-275%
B	Normally 600-700%	High	1.5-3%	120-250%	200-275%
C	Normally 600-700%	High	1.5-3%	200-250%	190-225%
D	Normally 600-700%	Medium	5-8%	275%	275%

Excerpts from EC&M's Electrical Calculations Handbook, by John M Paschal, Jr: Published by McGraw-Hill 2001.