Motor Starters Part 2: Selecting and Sizing DOL Parts
This article instructs readers on how to size and select Direct OnLine motor starter parts like circuit breakers, contactors, and thermal overload relays.
Part 1 of this series took a broad look at the basics of Direct OnLine (DOL) motor starters. The DOL starter has many important parts–the circuit breaker, the contactor, and the thermal overload relay.
The three parts provide overvoltage protection. Due to their use as protective devices, the components should be selected and sized according to function. The process of sizing them requires an engineer or technician to be equipped with the necessary knowledge to calculate the parameters of these components. This article will examine how to size and select the circuit breaker, the magnetic contactor, and the thermal overload relay for the DOL motor starter.
DOL Motor Starter Circuit Breaker Selection and Sizing
From the main, the circuit breaker is the first component that current passes across before reaching the magnetic contactor. The circuit breaker is connected between the main supply and the magnetic contactors. For sizing and selection of the circuit breakers, the following steps are very significant:
 Step 1: From the motor nameplate, read and record the size of the motor. This motor size is always in horsepower (HP).
 Step 2: Determine the motor’s Full Load Current (FLC).
 Step 3: Factor in safety by adding 50% to the FLC calculated in step 2 above.
 Step 4: Select the bestsuited circuit breaker as per available market ratings.
Step 1: Motor Size as per the nameplate
Motor Size = 6HP
1HP = 746W
Converting motor size to watts:
Motor Size = 6x746 = 4476W
Step 2: Determine the motor’s Full Load Current (FLC)
Power(P) = 4476W
The formula for calculating power in a threephase is determined by:
\[P=\sqrt{3}VIcos\empty\]
Where
cos∅ = power factor (PF) = 0.8
V = 3  Phase voltage = 415V
I = Full Load Current of the motor
Therefore, the motor’s Full Load Current can be determined by:
\[I=\frac{P}{\sqrt{3}V\times PF}=\frac{4476}{\sqrt{3}\times415\times0.8}=7.78A\]
The motor FLC is 7.78 A.
Step 3: Factor in safety by adding 50% to FLC calculated in step 2
Increase the calculated FLC by 50% or, in other words, multiply it by a factor of 1.5.
Circuit Breaker Rating = FLC×1.5 = 7.78×1.5 = 11.67A
Therefore, the calculated circuit breaker rating is 11.67 A.
Step 4: Select the bestsuited circuit breaker by rating
The table below represents all standard circuit breakers available in the market.
Table 1. Table of Standard Breaker Ratings
CIRCUIT BREAKER SELECTION TABLE(A) 

6 
10 
16 
20 
25 
32 
40 
50 
63 
80 
100 
125 
160 
180 
200 
250 
300 
350 
400 
630 
800 
1000 
1500 
1600 
2000 
2500 
3000 
3500 
4000 
4500 
5000 

CURRENT RANGE(A) 
BREAKER TYPE 
6 – 63 
MCB 
64 – 800 
MCCB 
Above 800 for LT Side 
ACB 
The circuit breaker’s calculated current rating is 11.63 A which falls between 10 A and 16 A in the table above. Therefore, the proposed breaker will have a 16 A current rating. 16 A lies between 6 and 63 A, so the circuit breaker will be MCB.
Hence, the proposed circuit breaker is 16 A TP MCB.
DOL Motor Starter Contactor Selection and Sizing
The contactor is connected between the circuit breaker and the thermal overload protection relay. The sizing and selection process for the contactor is similar to that of the circuit breaker in calculating the Full Load Current and the calculated current ratings. The difference is when it comes to the selection table for the components that are available on the market.
From the sizing and selection of the circuit breaker, we found that:
Motor Size = 6HP
Power(P) = 4476W
The motor FLC = 7.78A
Circuit Breaker Rating = FLC×1.5 = 7.78×1.5 = 11.67A
The circuit breaker rating is similar to the contactor rating. Select the contactor rating according to Table 2 below:
Table 2. Table of the Market Contactor Ratings

THE CONTACTOR RATINGS 

9 
12 
18 
22 
25 
32 
38 
40 
45 
50 
65 
70 
80 
95 
115 
150 
185 
200 
225 
265 
300 
330 
400 
460 
500 
580 
630 
780 
800 
1215 
2250 

The contactor size calculated current rating is 11.63 A, which falls between 9 A and 12 A in Table 2. Therefore, the proposed contactor will be a 12 A current rating.
Selecting and Sizing Thermal Overload Relay for the DOL Starter
A thermal overload relay is connected between the motor and the contactor. It protects the motor against overvoltage. The following steps are used in the selection and sizing of the thermal overload relay.
 Step 1: Take the motor size from the motor nameplate.
 Step 2: Compute the Full Load Current of the motor.
 Step 3: Calculate the lower limit of the thermal overload relay by subtracting 20% from the calculated FLC.
 Step 4: Calculate the higher range limit of the thermal overload relay by adding 20% to the calculated FLC.
 Step 5: Select the thermal overload relay from the sizes readily available in the market.
Step 1: Motor size from the nameplate
Motor size = 5HP
Motor size in watts = 746×6 = 4476W
This is the power needed to start the motor.
Step 2: Calculating the FLC
Power(P) = 4476W
The formula for threephase power:
\[P=\sqrt{3}VIcos\empty\times Efficiency\]
Efficiency = 90%, V = 415V, COS∅ = PF = 0.8 and I = ?
Rearranging the formula above:
\[I=\frac{4476}{\sqrt{3}\times414\times0.8\times0.90}=8.65A\]
FLC = 8.65A
Tolerance
Tolerance will help solve for step 3 and step 4. The thermal overload relay is represented by a tolerance of 20% from the FLC.
Therefore,
Tolerance = 8.65×20% = 1.73A
Step 3: Calculate the lower limit of the thermal overload relay by subtracting 20% from the FLC
Lower limit = FLC  Tolerance = 8.65  1.73 = 6.92A
Step 4: Calculate the higher range limit of the thermal overload relay by adding 20% to the FLC
Higher Range = FLC + Tolerance = 8.65 + 1.73 = 10.38A
Therefore, the range is 6.92 A to 10.38 A.
Step 5: Select the thermal overload relay from the sizes readily available in the market
Table 3 represents all the thermal overload protection relays available in the market.
Table 3. Thermal Overload Relay Standard Sizes

THERMAL OVERLOAD RELAY SIZE RATINGS 

0.10.16 
0.160.25 
0.40.63 
0.631 
11.6 
1.252 
1.62.5 
2.54 
46 
5.59 
710 
913 
1218 
4865 
2332 
2836 
3040 
3750 
5570 
6380 
8093 
80125 
100160 
125200 
160250 
200315 
250400 
315500 
400630 
0.250.4 
From Table 3, the thermal overload relay current rating calculated range falls between 913, and this is the proposed thermal overload relay for the DOL starter.
Part 3 of this series will focus on StarDelta motor starters.
Featured image used courtesy of Adobe Stock