Difference Between Rated Current and Nominal Current

Electrical Technology

Difference Between Rated Current, Nominal Current, Max Current and Relation with Full-Load Ampere (FLA) and Full-Load Current (FLC)

The terms rated current and nominal current sound similar and are often used interchangeably, but they have slightly different meanings depending on standards, design, or specific applications in electric circuits and devices.

To make things simple and easy to understand, even for a layperson, let’s look at how nominal current and rated current are applied in motors, transformers, circuit breakers, and other electrical devices.

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Rated Current

Definition: The rated current is the maximum continuous current that a device, conductor, or equipment can safely carry continuously under specified operating conditions without overheating or degrading.

The value of rated current is given and tested by the manufacturers, based on design, insulation class, temperature rise, and safety standards. This value represents the safe and continuous operating current under normal conditions for motors, transformers, circuit breakers, or wire/cables etc.

In most cases, the current should not exceed the rated value by more than 10%. Otherwise, it will “overload” the branch circuit and branch circuit conductors used for an appliance.

For Example:

A 10A circuit breaker has a rated current of 10A. It can carry 10A indefinitely under standard conditions.
A motor with a rated current of 15A. It is designed to safely handle up to 15A continuously without issues.

In motor circuits, rated current is refer to the Full Load Amperes “FLA” or maximum current (I_max) which is the value of current printed on the motor’s nameplate. It means, the rated current is the actual maximum current drawn by an electric motor when operating at rated voltage and frequency while providing full-rated horsepower (H.P). on the other hand, nominal current is the current drawn by the motor under normal operating conditions.

Good to Know:

Rated current I_r is the maximum safe continuous limit of an electrical device.
In IEC 60364, the rated current is represented by “I_b” while the FLA is denoted by “I_N“.
According to NEC 430.6(A)(2), rated current or FLA is the “Nameplate Value” marked on the motor’s nameplate.
According to IEEE, rated current or value is the “rating” of the device nameplate i.e. 5A, 120/240V, 50/60Hz.
Max current “I_max” is the value of maximum current a device can safely handle before damaging the component if exceeded.
Full load current (FLC) is the standardized value of current drawn by a motor of a given horsepower, voltage, and type. It is based on NEC (Table 430.247 to 430.250) and UL 508A (Table 50.1 and 50.2). The value of FLC is not calculated, but rather assumed values and refer as “Table Values” in NEC.

Nominal Current

Definition: The nominal current is a reference value or a designated current that a conductor or an electrical device is expected to carry under normal operating conditions (IEEE. 725.2). It is often used as a general or approximate value by equipment design for selection or comparison purposes.

As the name suggests, nominal current (I_NOM) is more like an expected or average working current rather than the maximum safe limit (rated current). It is used for system design, such as for selecting fuses, circuit breakers, or cables, to match the expected load.

In the case of transformers, the terms nominal current and rated current are generally used in the same context. However, in the case of motors, the nominal current refers to the current drawn by the motor under normal operating conditions, while the rated current indicates the maximum current the motor can safely handle.

The value of nominal current is generally derived from the device’s nominal power rating and supply voltage. Think of a a circuit breaker which have a nominal current of 16A. It indicate it’s designed for a circuit where the typical load is around 16A, but with designed tolerance in %, its rated current might be higher to account for temporary overloads.

For example:

A 2 kW motor at 230V, cosϕ = 0.85, will have a nominal current of about 10A (calculated).
A 100W lamp operated at 230V will have a nominal current = 0.43A.

Good to Know:

The nominal current is a typical approximately working current in normal use.
In case of transformers, the nominal current and rated current is taken in the same context.
In case of motors, nominal current is the current drawn by motor while rated current is the maximum current a motor can handle.
The nominal current may be slightly lower than the rated current. This is because it might not account for extreme conditions or safety margins.

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Calculation of Rated Current & Nominal Current

If the exact values are unknown from the table and nameplate rating, the following formulas can be used to determine the approximate value of rated current, nominal current, full-load-ampere (FLA) or full-load current (FLC).

Single-Phase Motors

When the 1-phase motor’s rating in given in HP:

Rated Current = P_HP × 746 / (V × Cosϕ × ɳ)

When the 1-phase motor’s rating in given in kW:

Rated Current = P_kW × 1000 / (V × Cosϕ × ɳ)

Three-Phase Motors

When the 1-phase motor’s rating in given in HP:

Rated Current = P_HP × 746 / (√3 × V × Cosϕ × ɳ)

When the 1-phase motor’s rating in given in kW:

Rated Current = P_kW × 1000 / (√3 × V × Cosϕ × ɳ)

Where;

P = Motor rating in HP or kW
V = Line Voltage
P.F = Power Factor (Cos ϕ)
ɳ = Motor efficiency

Related Post: Difference Between Current and Voltage

The following table shows the approximate rated current of single-phase and three-phase induction motors.

Rated Currents of 1-Phase & 3-Phase Motors
HP	kW	Single – Phase			Three – Phase
HP	kW	110V	220V	240V	240V	380V	415V
¹⁄₁₂	0.07	2.4	1.2	1.1	– – –	– – –	– – –
⅛	0.09	3.3	1.6	1.5	0.6	0.4	0.4
⅙	0.12	3.8	1.9	1.7	0.9	0.5	0.5
¼	0.18	4.5	2.3	2.1	1.2	0.8	0.7
⅓	0.25	5.8	2.9	2.6	1.5	0.9	0.9
½	0.37	7.9	3.9	3.6	2.3	1.4	1.3
¾	0.56	11	5.5	5	2.8	1.8	1.6
1	0.75	15	7.3	6.7	3.2	2	1.8
1½	1.1	21	10	9	4.5	2.8	2.6
2	1.5	26	13	12	5.8	3.7	3.4
3	2.2	37	19	17	8.7	5.5	5
4	3.0	49	24	22	11	7	6.5
5	3.7	54	27	25	13	8	8
5½	4.1	60	30	27	14	9	8
7½	5.6	85	41	38	19	12	11
10	7.5	110	55	50	25	16	15
12½	9.3	– – –	– – –	– – –	32	20	18
13½	100	– – –	– – –	– – –	34	22	20
15	11	– – –	– – –	– – –	37	23	22
20	15	– – –	– – –	– – –	50	31	28
25	18	– – –	– – –	– – –	62	39	36
30	22	– – –	– – –	– – –	70	43	39
40	30	– – –	– – –	– – –	91	57	52
50	37	– – –	– – –	– – –	119	75	69
60	45	– – –	– – –	– – –	136	86	79
75	55	– – –	– – –	– – –	166	105	96
100	75	– – –	– – –	– – –	219	138	125
125	90	– – –	– – –	– – –	269	170	156
150	110	– – –	– – –	– – –	325	205	189
175	130	– – –	– – –	– – –	389	245	224
200	150	– – –	– – –	– – –	440	278	255
The Table is based on Approximately 1450-1500 RPM with Average Efficiency & Power Factor. Approximate Values of Rated Current for Single-Phase & Three-Phase Motors. See “Note” Below. A Reference Table is Provided Below in Image Format for Download.

Note:

The values of rated current for single-phase and three-phase motors given in the table above are approximate reference values. The actual current varies depending on the voltage, power factor, and motor efficiency. The actual current, also known as the full-load current (FLC), can be determined from the motor’s nameplate rating or by using IEC/NEC tables.
The data may vary from different Motors manufactures. The current data is obtained from RECON Electrical and R&M Electrical Group

Table in Image Format – Click or open in a new tab to enlarge

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Comparison Between Nominal and Rated Current

Aspect	Rated Current	Nominal Current
Definition	The maximum continuous current a device can safely carry under normal conditions.	The expected operating current under nominal voltage, power, and load conditions.
Given By	Manufacturer on the device nameplate (based on design limits, insulation, standards).	Calculated from nominal voltage, power, and power factor (or sometimes stated on nameplate).
Purpose	Safety/design limit → tells how much current it can withstand safely.	Normal working condition → tells how much current it will actually draw.
Nature	A limit value (not to be exceeded in continuous use).	A working value (usually below rated current).
Depends On	Temperature rise, insulation, material, ambient conditions.	Rated power, supply voltage, power factor, efficiency.
Examples:	Motor Rated current = 15 A (manufacturer’s safe continuous current).	Motor Nominal current = 13 A (actual current at rated load & voltage).
	Circuit Breaker Rated current = 20 A (safe continuous capacity).	Circuit Breaker Nominal current = load current (e.g., 16 A lighting circuit).
	Transformer Rated current = 100 A (safe continuous current at rated kVA).	Transformer Nominal current = ~95 A (actual load current at typical power factor).
	Lamp Rated current = 0.5 A (max safe at rated voltage).	Lamp Nominal current = 0.43 A (calculated from P = VI).

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