I am looking at the spec sheet for an Intermatic STW700W in-wall timer.

It says:

Switch Ratings:
General Purpose:         15 A, 120 VAC, 50/60 Hz
Tungsten:                 8 A, 120 VAC, 50/60 Hz
Inductive:               15 A, 120 VAC, 50/60 Hz
Electronic Ballast//LED:  5 A, 120 VAC, 50/60 Hz
LED Load:               600 W
Motor:                    1 HP, 120 VAC

I would understand if it had different load capacities for different types of lighting if it also had dimming capabilities, since dimming works differently for different types of fixtures.

Why should the type of load matter if all it is doing is turning the load on/off?

As an extension of the question, what kind of loads could the spec sheet mean when it says "general purpose"?

(In general, this spec sheet doesn't make much sense to me. For example, it says it can handle inductive loads up to 15A, but it also says it can handle motors up to 1 HP. 1 HP is approximately 750 watts which at 120V is just under 7A, less than half of the 15A rating. Why should a motor load be any different than any other inductive load?)

  • Each of those has potential spikes in current associated with their use. Motors can draw much more than their nominal ratings at startup, for example. I'll let someone more knowledgeable spell out the nuances of that. – isherwood Aug 20 '20 at 16:24

This product is marketed primarily for lighting application, and so, its rating focuses on lighting applications.

  • Tungsten refers to incandescent, halogen and other lights in that family.
  • Inductive refers to HID (sodium, mercury, metal halide) and old-school magnetic ballasts for fluorescent.
  • Electronic/LED refers to modern electronic ballasts for fluorescent/LED.
  • The "LED Load" figure is just to restate the rating in commonly used terms.

The reason is because these different loads have different electrical characteristics, and that affects the contactor make/break.

A plain resistive load follows Ohm's Law steadily during both make and break events -- a 12 ohm resistor will draw 10 amps on switch make, and 10 amps on switch break. The "general" rating applies here.

Tungsten (incandescent, halogen etc.) have a much lower resistance quiescent than they do upon reaching operating temperature. When you drive them constant-voltage, that results in "inrush current" which quickly brings them up to temp. It is quite a current spike, and that means relay contacts have to contend with it on "make". As such, relays are derated for tungsten loads. Breaking an incandescent is just like a resistor.

Electronic ballasts for both fluorescent and LED are wildly variant. Many have have power supply capacitors or chokes on the DC side which on initial turn-on will drink up current very aggressively. This works out to be an inrush current similar or even worse than incandescents. But again little trouble on break.

"Inductive" means Old Fluorescent, and HID (Low pressure sodium, high pressure sodium, mercury vapor and metal halide) These contain bulbs which, upon striking the arc, act like a dead short. These days you drive that with a switching power supply in CC mode, but back in the day, you used a transformer wound in constant-current mode.

This transformer is a large inductor, which stores energy like a capacitor. Just as capacitors use their energy to combat changes in voltage, inductors use their energy to combat changes in current. An inductor does that by increasing the voltage - to infinity, or to the point where insulation breakdown occurs, whichever occurs first.

This means that HID loads (or as they call them "inductive") are pretty docile on make -- but on break, they do not want current flow to stop, and will attempt to drive voltage to infinity to preserve current flow. This high voltage will force current to leap across the relay contacts. This is often called an inductive "kick" - and obviously it causes relays to be derated.

Motors are inherently inductive machines, with the same problems with inductive kick. Since motors are all inductor, it can be worse. However it looks like the 1HP motor rating is merely the inductive rating restated in horsepower (equaling 1.287 horsepower) and rounded down to the next common motor size.

Motors get you another way too: they also have very low resistance on initial startup; they need to be spinning to provide enough "back EMF" to limit current to reasonable values. This is called the "Locked Rotor Amperage" and again, relays must contend with this on make.

  • All the intermatics I have installed break out electronic or magnetic ballast as a ballast once it strikes it is a discharge or arc lamp the ballast is used to provide the proper voltage for the lamp and as a current limiter not as a storage device. This current limit is why ballasts must be matched to the lamp for proper performance. The reason motors are sized differently is the NEC requires standard snap switches to have 2x the motor fla rating so standard non motor switches only have 1/2 the rating at best. It’s not a kick back but the locked rotor value on startup. – Ed Beal Aug 20 '20 at 17:39
  • @EdBeal oh, motors have inductive kick on interrupt, oh, boy howdy! – Harper - Reinstate Monica Aug 20 '20 at 18:09
  • Kick back is when the field is released and the breakdown voltage is what is needed here not startup , it’s simple to prove. Take an inductor and put it on a DC Supply it will charge go to that voltage and stay there the kick back or counter emf comes when the voltage is removed and ghe inductor tries to maintain the voltage can spike into the thousands of volts that is kick back high voltage not much current this is how automotive systems get highvoltage from breaking the voltage to the coil. But switches have no rating other than breakdown values because the inrush is hundreds Of times CEMF – Ed Beal Aug 20 '20 at 18:40
  • Additionally the electronic and tungsten loads are derated due to the arcing characteristics experienced when separating contacts under higher frequencies loads. – NoSparksPlease Aug 20 '20 at 19:39

In addition to the excellent explanation by Harper of why different loads function differently, there is another concern with typical small timers: electronic switching. In the old days, a timer was simply a relay or switch connected to a clock circuit. Sometimes even a switch physically moved by a rotating mechanism that is really a simple analog clock.

Most modern timers, especially at the smaller sizes, don't use a traditional relay but instead switch purely electronically. This has advantages in terms of size, weight and (lack of) moving parts. However, electronic switches can, in some ways, be even more susceptible to the differences in inductance, startup surges, etc. between different types of loads. With a simple relay, if you overload it, you will burn up a wire somewhere. With electronic switching, you are likely to produce magic smoke.

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