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

Start with full-load current…

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.

How do you know which current to set on a motor overload relay
How do you know which current to set on a motor overload relay (photo credit: Edvard CSANYI, EEP)

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

Motor thermal overload relay
Figure 1 – Motor thermal overload relay

Calculation example

When we know the precise voltage for the installation, the full-load current can be calculated at 254 Δ/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 ∆/380 – 480 Y V
  • In = 5.70 – 5.00/3.30 – 2.90 A
The full-load current at a given voltage indicated on the nameplate is normative for setting the overload relay
Figure 1 – The full-load current at a given voltage indicated on the nameplate is normative for setting the overload relay

60 Hz data calculation:

  • Ua = actual voltage 254 ∆/440 Y V (actual voltage)
  • Umin = 220 ∆/380 Y V (Minimum values in the voltage range)
  • Umax = 277 ∆/480 Y V (Maximum values in the voltage range)
The voltage ratio is determined by the following equations:

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

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

So, UΔ = UY


Calculation of the actual full-load current (I)

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

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

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

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

The values for the full-load current correspond to the permissible full-load current of the motor at 254 ∆/440 Y V, 60 Hz.

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

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

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

Reference // The motor book by Grundfos

Premium Membership

Premium membership gives you an access to specialized technical articles and extra premium content (electrical guides and software).
Get Premium Now ⚡

About Author

author-pic

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 fascilities. Professional in AutoCAD programming. Present on

2 Comments


  1. Umar
    Sep 20, 2018

    I need more diagrams about motor connections


  2. Glen Sullivan
    Sep 20, 2018

    Thank you for a good explanation. I have a couple of people I work with that believe if you have a fast acting overtemp relay, you dont need overloads. If it gets hot, it’s overloaded. Not very fast enough for lost phases or locked rotors. I am not sure if it’s even legal as the standard calls for overload fitted to motors over 370w. Has anyone here got a correct answer?

Leave a Comment

Tell us what you're thinking... we care about your opinion!

Premium Membership

Get access to premium electrical guides, technical articles and much more!