Question about contactors

[NormBrews]

While shopping for a contactor to switch on and off the 50 amp service inside my control panel, I notice they are amp rated for both inductive and resistive loads. Hopefully someone can explain this to me. I will be coming close to using 50 amps with a 4500 watt element and a 5500 watt element and do not want to buy one that is under rated.

 

[AllanMar]

To put it simply: Inductive loads are things like solenoids, motors, etc. Resistive loads are things like heaters (which will be the vast majority of your load).

 

[NormBrews]

so for instance this one would suffice
http://www.globalindustrial.com/p/e...ontactors-sub/packard-contactor-2-pole-40-120

 

[manoaction]

So... Just so we're clear. We should generally be buying things rated for resistive loads since the bulk of our amps are going to heater elements? Yes?

 

[DougK]

In short, inductive loads use magnetic fields, such as motors, relays, solenoids, etc. inductive loads can cause what is called blowback voltage, that is, a voltage surge that is created by a collapsing magnetic field.

Resistive loads do not use magnetic fields and have no risk of blowback.

This is why a contactor generally has a lower rating for inductive loads-built in blowback protection.

 

[DougK]

So... Just so we're clear. We should generally be buying things rated for resistive loads since the bulk of our amps are going to heater elements? Yes?

Yes. For the most part, only motors would provide an inductive load in our use.

Unless you are making a system with an inductive cook top or a huge motor, I don't see too many configurations exceeding an inductive load rating.

 

[aquenne]

Yes, you should be going by the resistive load values.

You can think of a induction motor as having a starting "inertia" it needs to over come, so it will draw more starting current. A resistive device draws its rated current, immediately.