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# How to know if you set the correct current on a motor thermal overload relay

Home / Technical Articles / How to know if you set the correct current on a motor thermal overload relay

The full-load current at a given voltage indicated on the nameplate is normative for setting the overload relay. Because of the variable voltages around the world, motors for pumps are made to be used at both 50 Hz and 60 Hz in a wide voltage range.

Therefore, a current range is indicated on the motorâ€™s nameplate. The exact current capacity can be calculated when we know the voltage.

### Calculation example

When we know the precise voltage for the installation, the full-load current can be calculated at 254 Î”/440 Y V, 60 Hz. The data is indicated on the nameplate as shown on the illustration on below:

• f = 60 Hz
• U = 220-277 âˆ†/380 – 480 Y V
• In = 5.70 – 5.00/3.30 – 2.90 A

60 Hz data calculation:

• UaÂ = actual voltage 254 âˆ†/440 Y V (actual voltage)
• Umin = 220 âˆ†/380 Y V (Minimum values in the voltage range)
• UmaxÂ = 277 âˆ†/480 Y V (Maximum values in the voltage range)
The voltage ratio is determined by the followingÂ equations:

UÎ” = (UA – Umin) / (Umax – Umin)
which is in this case: UÎ” = (254 – 220) / (227 – 220) = 0.6

UY =Â (UA – Umin) / (Umax – Umin)
which is in this case: UY = (440-380) / (480-380) = 0.6

So,Â UÎ” =Â UY

#### Calculation of the actual full-load current (I)

Imin = 570/3.30 A
(Current values for Delta and Star at minimumÂ voltages)

Imax = 500/2.90 A
(Current values for Delta and Star at maximumÂ voltages)

Now, it is possible to calculate the full-loadÂ current by means of the first formula:

• I for Delta values:Â 5.70 + (5.00 – 5.70) Ã— 0.6 = 5.28 = 5.30 A
• I for Star values:Â 3.30 + (2.90 – 3.30) Ã— 0.6 = 3.06 = 3.10 A

The values for the full-load current correspondÂ to the permissible full-load current of the motorÂ at 254 âˆ†/440 Y V, 60 Hz.

Rule-of-thumb: The external motor overload relay is always set to the nominalÂ current shown on the nameplate.

However if motors are designed with a serviceÂ factor, which is then shown on the nameplateÂ eg. 1.15, the set current for the overload relayÂ can be raised by 15% compared to full-load current or to the service factor amps (SFA) which isÂ normally indicated on the nameplate.

If the motor is connected in star = 440 V 60 HzÂ the overload relay then has to be set to 3.1 A.

Reference // The motor book by Grundfos

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### Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

1. Meta
Nov 22, 2021

Nice article

2. Lydia Mbithi
Oct 22, 2021

5.7+(5.00-5.70)*0.6=5.28
Where has this 0.6 come from and what is it?

3. Lascelles Dacres
Oct 03, 2021

Good day
I am happy to know I can get assistance on your site.
I am doing a family portable water project….with a 230v 10 hp 50 amp motor for the pump.
Can you tell me the best amp setting for the contactor overload.

Regards
L Dacres