Max. amps and hp rating for electrical switch

[bjbdts]

Hi, everyone:
I need a switch to turn on/off a 2500W power supply(single phase) from wall outlet (220Vac). When I look at the specs of related product I get a little confusion: Why the manufactures give Max. amps at 240VAC/600VAC, but at the same time MAX. horse power. By just looking at the Amps spec, it's sufficient for my application, for example 30Amps @240VAC. But then the MAx hp is lower than my application,for example, 3hp. 3hp translated to 746Wx3=2238Watts which is lower than 2500W. Can somebody here help me to clarify this issue?

Thanks in advance

bj

 

[davidbeach]

Motors are a much more difficult load to interrupt than resistors. Many devices have different ratings for different types of loads and motors/inductive loads generally have the lowest rating.

 

[bjbdts]

Thanks, davidbeach. Actually I'm not using moters, I only want to turn on/off a 2500W DC power supply( it doesn't have a switch itself). In that sense maybe I don't need to worry about the hp issue?

bj

 

[itsmoked]

No. But!
As davidbeach was pointing out switches have different ratings for different types of loads. The standard value they provide, say, 10A @ 240V would be for a purely resistive load that has no inrush associated with it. It also means it would interrupt that 'easy' load perhaps 100,000 times before failing.

Now if you instead use it on tungsten filament light bulbs which have a high inrush the switch might work fine but only for 20,000 cycles.

Now use it on a motor that has a large inrush and a large inductance that defies easy interruption and that switch may provide only 20 cycles or may just explode. To avoid that the switch makers deal with that nasty motor load by derating the power allowed for motor starting.

Now with your device you have to think about how far from a straight forward resistive load it looks to a switch. I suggest that a typical supply might have a very large inrush thru rectifiers to initially charge a capacitor. This would impact the switch in a negative way. So if your supply is 2.5kW and demands a continuous 30A to operate you might want to go with a larger switch say a 40A switch or you will experience premature failure, maybe 300 switch cycles.

If your supply is a sophisticated supply with PF correction and softstart then you can likely use just a 30A switch with fine lifetime performance.

If not, you might want to consider instead switching a small inexpensive switch that switches a large relay or small contactor to provide what you need. You ca parallel relay contacts to boost lifetime and reduce cost.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[bjbdts]

Thanks itsmoked for the detailed explanation. My power supply needs about 10-15A for operating. It has a 25A peak inrush current limiting (Now this spec makes sense to me). So I think it will be sufficient to buy a 30A switch.

bj

 

[itsmoked]

I concur!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

A quick and dirty "rule of thumb" I just came across is that for loads such as power supplies where there is capacitor charging involved, take the unit max. amps and multiply that by 1.43, then use that as if it were an inductive load rating (AC-3) for a switching device. The reasoning explained to me is that capacitor switching can involve both the charging AND the possibility that the capacitor is already holding a charge when you begin to close the contacts, which increases the contact stress beyond that of an inductive load inrush current. So for your 240V 2500W PSU with 15A max continuous, use a switch with an AC-3 (inductive current) rating of no less than 21.5A. that would be a switch with greater than a 30A AC-1 rating in most cases.

UNLESS... there is a current limiting (a.k.a. soft start) circuit in the power supply, such as a pre-charge resistor. If that is the case, then the capacitor charging current and residual charge issue become less relevant. So in that case, just derate the thermal (resistive) current rating ( a.k.a AC-1) of the switch by 30%, which means your 30A switch would be fine.