DOL Starter Component Calculator (IEC/NEC)

Calculate motor current, circuit breaker, thermal overload, and contactor sizes for DOL starters.

DOL Starter Component Calculator

Accurate sizing for Direct Online Starters

IEC NEC

Phase:

Motor Power (kW):

Voltage (V):

Power Factor (PF):

Service Factor (SF):

Results:

Motor Full Load Current (A): 0.00

Inv. Trip Breaker Size (SCPD) (A): 0.00

Magnetic Contactor Size (A): 0.00

OLR Recommended Size (A): 0.00

OLR Recommended Setting (A): 0.00

Making/Breaking Capacity (A): 0.00

IEC Validation Checklist

IEC Coordination

Ensure **Type 2 Coordination** between components. Check manufacturer's tables for your models and the available fault current (Icu).

Status: VERIFY MANUALLY

Inrush Current

Your upstream supply must handle the motor's starting inrush current without tripping.

Calculated Inrush: 0.00 A (8x FLC)

Conductor Sizing (IEC)

Select a cable with ampacity ≥ 125% of FLC *after* applying all derating factors (ambient, grouping, etc.).

Required Ampacity: ≥ 0.00 A

Voltage Drop

For long cable runs, calculate voltage drop. Aim for **≤ 3%** at the motor terminals during full load to prevent performance issues.

Status: VERIFY MANUALLY

Interrupting Rating (AIC)

The breaker must have an AIC rating **≥** the available short-circuit current at its location (e.g., 10kA, 22kA).

Status: VERIFY MANUALLY

Breaker Sizing (NEC 430.52)

The calculated value is the *maximum* size (250% FLC). A smaller standard size can be used if it allows the motor to start.

Max Breaker Size: 0.00 A

Conductor Sizing (NEC 430.22)

Conductors must have ampacity ≥ 125% of FLC. Use the 75°C column (NEC 110.14(C)) for terminations.

Required Ampacity: ≥ 0.00 A

Listing & Labeling

All components (breaker, contactor, OLR) must be **UL Listed** and suitable for use as a motor controller assembly.

Status: VERIFY MANUALLY

Star-Delta Starter Calculator

VFD Selection Calculator

How to Use the Calculator

Input Parameters

Select your standard (IEC or NEC).
Enter Motor Power in Kilowatts (kW).
Input the system Voltage in Volts (V).
Provide the Power Factor (PF) (e.g., 0.85).
Select the electrical system Phase.
If NEC, enter the motor's Service Factor (SF) (e.g., 1.15).

Calculation & Results

Click the "Calculate" button.
Results include FLC, Breaker (SCPD), Contactor, and Overload Relay (OLR) sizing and settings.
A **Validation Checklist** appears below the results with key safety checks.
Use "Reset" to clear all fields.

Important Notes

Ensure accurate input values.
Calculations are based on standard formulas.
**NEC Note:** This tool calculates FLC from kW/V/PF. Per NEC, breaker/conductor sizing should use FLC from Tables 430.248/250, and OLR sizing from nameplate FLA. This tool uses the calculated FLC as a proxy for both, which is a common simplification.
Always verify results with local codes and manufacturer data.

How to Size DOL Starter Components

What's in a DOL Starter?

For one motor, a DOL starter includes:

Magnetic Contactor
Thermal Overload Relay (OLR)
Short-Circuit Protective Device (SCPD: Fuses or Breaker)
Branch-Circuit Conductors (Cables)
Control items (Pushbuttons, control fuse, etc.)

Key Differences: IEC vs. NEC

IEC: Uses the motor **nameplate current (FLA)** directly for sizing all components.
NEC: It's a two-step process:
- Uses **NEC Table FLC** (e.g., 430.250) for sizing conductors and the breaker (SCPD).
- Uses the **motor nameplate FLA** for sizing the Overload Relay (OLR).

Step 1: Gather Motor Data

You need:

Power (kW or HP), Voltage, Phase
Nameplate Current (FLA)
Service Factor (SF) (e.g., 1.15)
NEC: Find Table FLC from NEC 430.250 (3-ph) or 430.248 (1-ph).
Typical DOL inrush is 6-8x FLC.

Step 2: Size the Contactor

Must be rated for motor starting (e.g., **AC-3** for IEC).
Rating should be ≥ motor FLC.
IEC Practice: AC-3 Rating ≥ 1.0 × Nameplate FLA. A margin of 1.15x-1.2x is common.
NEC Practice: Sized to handle the Table FLC and inrush. Using an AC-3 rating ≥ Table FLC is a reliable method.

Step 3: Size the Overload Relay (OLR)

This protects the motor from overheating.

IEC: Select a relay range that includes the nameplate FLC. **Set the OLR to 1.0 × Nameplate FLC.**
NEC (430.32): Based on **Nameplate FLA**.
- If SF ≥ 1.15: Max setting is **125% of FLA**.
- If SF < 1.15: Max setting is **115% of FLA**.
(You still start the setting at 100% of FLA and only increase if needed).

Step 4: Size the SCPD (Breaker)

This protects against short circuits.

IEC: Sized to coordinate with the contactor (Type 1 or Type 2 coordination). A common rule of thumb for inverse-trip breakers is ~250% of FLC.
NEC (430.52): Based on **Table FLC**.
- Inverse-Time Breaker: Max **250% of Table FLC**.
- Time-Delay Fuses: Max **175% of Table FLC**.
(You can go higher *only* if the motor fails to start).

Step 5: Size Conductors (Cables)

IEC: Ampacity ≥ 1.25 × Nameplate FLC, then apply derating factors for installation, grouping, and temperature.
NEC (430.22): Ampacity ≥ **125% of Table FLC**. Then apply derating and terminal temperature limits (e.g., 75°C).
Always check for voltage drop (aim for ≤3% on the branch circuit).

Step 6: Control & Accessories

Control MCB/Fuse: Sized for the control transformer or coil VA. (e.g., 2A).
Auxiliary Contact: 1 NO for seal-in (latching) circuit.
Pushbuttons: Start (NO), Stop (NC).
Enclosure: IP/NEMA rated for the environment.
Grounding: Must be per local code.

Quick Validation Checks

Can the supply handle the 6-8x FLC inrush current?
Is the SCPD's interrupting rating (kA) > available fault current?
Is Type 2 coordination required (for high uptime)?
Is the cable size sufficient after derating and for voltage drop?

Worked Example (IEC)

Motor Data: 15 kW, 400V, 50Hz, 3-Phase.
Nameplate FLA: 28 A

1. Contactor (AC-3):
Need ≥ 28 A.
Common practice: 1.15 × 28 A = 32.2 A.
Result: Select AC-3 32 A Frame.

2. Overload Relay (OLR):
Set to nameplate current.
Result: Select OLR with 24-32 A range, set at 28 A.

3. SCPD (Breaker):
Rule of thumb: 250% × 28 A = 70 A.
Practical pick for Type 2 coordination is often lower.
Result: 50-63 A MCCB (Verify with mfg. tables).

4. Conductors:
Min. ampacity: 1.25 × 28 A = 35 A.
Result: Select 6 mm² Cu (check local tables/derating).

Worked Example (NEC)

Motor Data: 20 HP, 460V, 60Hz, 3-Phase, SF 1.15.
NEC Table 430.250 FLC: 27 A
Nameplate FLA: 26 A (Assumed for OLR)

1. Conductors (NEC 430.22):
Based on **Table FLC**: 1.25 × 27 A = 33.75 A.
Result: Select 10 AWG Cu THHN (35 A @ 75°C).

2. SCPD (Breaker) (NEC 430.52):
Based on **Table FLC**: 250% × 27 A = 67.5 A.
Result: Select next standard size up: 70 A Breaker.

3. Overload Relay (OLR) (NEC 430.32):
Based on **Nameplate FLA (26 A)** & SF (1.15).
Max Setting: 125% × 26 A = 32.5 A.
Result: Select OLR covering 26A, set at 26 A (can increase to 32.5A if it trips).

4. Contactor:
Based on **Table FLC (27 A)**.
Result: Select AC-3 32 A Frame.

DOL Starter Rating Chart (Typical)

This chart shows typical component ratings for 3-phase, 400V/415V motors. These are for reference only. Always use the calculator or standards for your specific application.

Motor Power (kW)	Motor Power (HP)	Typical FLC (A)	Contactor (AC-3) (A)	OLR Range (A)	Breaker (MCCB) (A)
1.5	2	3.5	9	2.5 - 4	10
2.2	3	4.8	9	4 - 6	12
4	5.5	8.5	12	7 - 10	20
5.5	7.5	11.5	18	9 - 13	25
7.5	10	15.5	18	12 - 18	32
11	15	22	25	17 - 25	50
15	20	29	32	24 - 32	63
22	30	42	50	38 - 50	80
37	50	70	80	63 - 80	125

Frequently Asked Questions

How to calculate DOL starter current?

You don't calculate the "starter current" itself, but rather the motor's Full Load Current (FLC), which is used to size all the starter components. The formula depends on the phase. For a Three Phase motor, the formula is I = (kW × 1000) / (1.732 × Voltage × Power Factor). For a Single Phase motor, it is I = (kW × 1000) / (Voltage × Power Factor). This calculator computes this FLC for you in the results.

What are the components of a DOL starter?

A basic DOL starter consists of four main power components. These are the Short-Circuit Protective Device (SCPD), which is a breaker or fuses; the Magnetic Contactor, which turns the motor on and off; the Thermal Overload Relay (OLR), which protects against overheating; and the Conductors, which are the power cables. It also includes control elements like Start/Stop pushbuttons and a control circuit fuse/breaker.

How to select a DOL starter for a motor?

To select a DOL starter, you must size each component correctly based on the motor's Full Load Current (FLC) and the governing standard (IEC or NEC). First, you Calculate the FLC using the motor's data. Next, you Size the Contactor with a rating of at least 115%-125% of the FLC. Then, you Size the OLR with a setting range that includes the FLC (typically set at 100% FLC for IEC). After that, you Size the Breaker (SCPD) to handle the inrush current, often 250% of the FLC. Finally, you Size the Conductors to be rated for at least 125% of the FLC after deratings.

How to calculate OLR rating?

The OLR (Overload Relay) "rating" refers to its setting. The goal is to protect the motor's nameplate Full Load Current (FLC). For IEC standards, the OLR is typically set at 100% of the nameplate FLC, and you select a relay where this value is within its adjustable range. For NEC standards, you also start by setting the OLR to 100% of the FLC. However, NEC 430.32 allows you to increase this setting to a maximum of 115% (for SF < 1.15) or 125% (for SF ≥ 1.15) if nuisance tripping occurs.

What is the difference between IEC and NEC standards?

The biggest difference is the current value used for sizing. The IEC standard generally uses the motor nameplate FLC for sizing all components (breaker, contactor, OLR, cables). In contrast, the NEC standard uses two different current values. It uses the NEC Table FLC (from NEC 430.250) to size the breaker (SCPD) and conductors, but it uses the motor nameplate FLC to size the Overload Relay (OLR).

What is the purpose of the 'Service Factor' in the NEC calculation?

The Service Factor (SF) is a multiplier on the motor's nameplate that indicates how much overload the motor can handle for short periods, with a common SF being 1.15 (15% overload). In the NEC calculation, the SF is only used to determine the maximum possible setting for the Overload Relay (OLR). If the SF is 1.15 or higher, the OLR can be set up to 125% of the FLC. If it's lower, the max is 115%.

Why is the breaker (SCPD) sized so much larger than the motor's current?

A motor draws a very high inrush current when it starts, typically 6 to 8 times its normal FLC. This is also called "Locked Rotor Current" (LRC). The breaker (SCPD) must be sized large enough (e.g., 250% of FLC) to allow this normal inrush current to pass without tripping. The Overload Relay (OLR) protects against smaller, sustained overloads, while the breaker (SCPD) is designed to trip instantly on a dangerous short circuit.

What is Type 1 vs. Type 2 coordination?

This is an IEC concept that defines how a starter behaves after a short-circuit fault. With Type 1 coordination, the starter is allowed to be permanently damaged after a fault and may need to be replaced, though the fault is safely contained. With Type 2 coordination, the starter must be immediately serviceable after a fault, with no damage to the contactor or OLR. This is preferred for critical applications where downtime is costly. Achieving Type 2 coordination requires using specific, tested combinations of components from the manufacturer.

When should I use a DOL starter vs. a Star-Delta or VFD?

You should Use a DOL Starter for smaller motors (e.g., under 15kW/20HP) where the high inrush current (6-8x FLC) is acceptable for the electrical supply, as it's the simplest and cheapest method. Use a Star-Delta Starter for larger motors to reduce the inrush current; it starts the motor at a lower voltage, drawing about 1/3 of the normal inrush. Use a VFD (Variable Frequency Drive) when you need speed control, a very smooth start, or maximum energy efficiency. A VFD offers the lowest starting current but is the most expensive option.

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