ECN Forum Forums General Topics of the Trade General Discussion Area contact ratings

Can someone help me decipher the information provided by manufacturers regarding contact ratings. Here is an example from a standard time clock:

At 120V/1 phase/60Hz ...
30 amp inductive/resistive
1 HP (N.E.C. shows 16A FLA)
5 amp tungsten
20 amp ballast

What is the application of the different ratings?

All contacts realy have three ampere ratings; make, break, and continuous. In most cases the only one publishedis the continuous rating. These ratings need to be matched to the load currents. There is also consideration for the length of time the inrush can be tolerated.

All loads have three amp levels; inrush (turn on), turn off, and continuous. Again the only one published (except in vacuum cleaner advertisments)is the continuous amps.
The the load amps change drastically based on the inductive, capacitive, and resistive components of the load. For example (these are typical):

Motor loads: Inrush 6x; Continuous 1x; Turn Off 6x (UL requires the switch to break the starting current).
Tungsten Lamps: Inrush 14x; Continuous 1x; Turn Off 1x.
Ballasts: Inrush 4x; Continuous 1x; Turn off 1x.
Resistance Heaters: Inrush 1x; Continuous 1x; Turn off 1x.

So it appears that your switch has a make rating of about 3x (90A).

[This message has been edited by JBD (edited 07-21-2004).]

JBD,
Thanks! I learned something today!
Just a couple more quick questions if I could:
I'm assuming that you got the typical numbers from experience; however, are you aware of a handy reference that has this info in it (i.e. typical inrush and
outrush multipliers)?
Also, using the info given and your comments the inrush would be:
1) 30A inductive/resistive
1x or 30A
2) 1HP (760 W)/120 = 6.3A motor
6x or 6*6.3= 37.8
3) 5A tungsten
14x or 14*5= 70
4) 20A ballast
4x or 4*20 = 80

Total inrush current (30+37.8+70+80) 217.8, correct?

The continous current would then be right off the plate or (30+6.3+5+20) 61.3, correct?

So when you say at the end of your comments above that the switch rating must be about 3x, given the calculations above this makes sense in that 217.8/61.3 = 3.5

If I've got my stuff straight to this point, my only remaining question would be why do you mention 90A at the end of your response?

Thanks again!

I mentioned 90A to generate more discussion, it is an estimate of the maximum inrush. In reality it is an irrelevant number because it is not from the manufacturer.

Your switch is rated for 30A resistive OR 1HP OR 5A tungsten OR 20A ballast. You do not add all of the ratings together.

1HP does not equal 746W of input power. Never ever do this calculation.

The formula is 1HP mechanical output power = 746W electrical OUTPUT power. You must then divide the 746W by the efficiency and power factor of the motor to determine input power -> Watts in = HP x 746 x P.F. x Eff.

Err, Watts in = HP x 746 / Eff.

VA in = HP x 746 / (Eff x PF).

My Bad.

Error acknowledged.

This is very helpful. One additional question ... what type of loads generally fall in the catagory of 'tungsten'?

"Tungsten" is plain-old incandescent or quartz-halogen lighting, typically with very high inrush current from cold-to-hot filaments.




[This message has been edited by Bjarney (edited 07-21-2004).]

Generally, IME, 1HP (for cpacitor start induction motors), is 14A or so FLA.

Let's also include the voltage of the motor, classicsat