Guide · Timers and smart plugs
Timer and smart plug load ratings
Why a '15 A' timer or smart plug can still be under-rated for a pump: how to read resistive, tungsten, ballast and horsepower ratings before plugging gear in.
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Short answer
The big number on a timer or smart plug is usually its resistive rating, which is the easy case. Motors and ballasts get lower ratings, and those are the ones that apply to pumps and some lights. TP-Link rates the Kasa EP25P4 at 15 A but only 1/6 HP for motors.[1] Find the rating that matches your load type, and keep pumps and filters off timers unless you have a reason.
Why one plug has several ratings
Different loads stress a switch differently.
- A resistive load, such as a heating element, draws a steady current. It is the gentlest case, so it gets the highest rating.
- A motor load, such as a pump, draws a surge when it starts. Ratings for motors are given in horsepower (HP) and are lower.
- Tungsten (incandescent) and ballast (some fluorescent) lighting also have their own ratings.
A load rating applies only to the load type it names. "15 A" on the box tells you nothing about motors unless the motor rating says so too.
Three published ratings, side by side
Here is what three makers state. We show them to explain the categories, not as picks.
| Rating | Kasa EP25P4 (smart plug)[1] | Intermatic TN311 (mechanical)[2] | BN-LINK U47 (mechanical)[3] |
|---|---|---|---|
| Voltage | 100-120 V, 50/60 Hz | 125 VAC, 60 Hz | 125 V, 60 Hz |
| Resistive | 15 A maximum (1.8 kW at 120 V) | 15 A | 15 A / 1875 W |
| Tungsten | Not listed | 1875 W | 8 A / 1000 W |
| Ballast | Not listed | 15 A magnetic ballast | 5 A |
| Motor | 1/6 HP | 1/3 HP | Not stated |
Two things stand out. The Kasa's 1/6 HP motor rating is far below its 15 A headline, so check any pump against the motor figure.[1] And BN-LINK states no motor rating and no safety certification mark for the U47, so its suitability for motors is unknown.[3]
How to check your load
- Find the load's rating plate. It gives watts or amps, and for motors sometimes horsepower.
- Pick the matching row. Lights with a ballast: ballast rating. Incandescent: tungsten. Pumps: motor rating.
- Stay well inside it. If the plate gives only watts and the timer gives only HP, or one figure is missing, treat it as not confirmed and ask the maker.
- Check the environment rating. TP-Link rates the EP25P4 for 0-40 C at 5-90% relative humidity, non-condensing,[1] so keep it away from splashes and lid condensation.
What to put on a timer
Lights are the usual job. A timer that fails, or a smart plug that loses its schedule, leaves the lights on or off for a while. A pump or filter switched off by mistake is a different risk, so keep life-support loads on their own outlet. We found no source that states how the EP25P4 behaves after a power outage, so plan as if a schedule might not resume by itself.
A smart plug with a published motor rating
If you want app schedules for lights, the Kasa EP25P4 publishes its voltage, its resistive and motor ratings and a UL certification on TP-Link's US page.[1] TP-Link gives no UL file number, and does not name the plug type in its text, so read the marks and plug on the unit itself.
Next
Questions
Can a 15 amp smart plug run an aquarium pump?
Check the motor rating, not the 15 A headline. TP-Link lists the Kasa EP25P4 at 15 A maximum but rates it for a 1/6 HP motor, which is far lower.[1]
What is a resistive load?
A load like a heating element that draws steady current with no startup surge. Timers often state their highest rating for resistive loads, so it is not the figure to use for motors.
What does a ballast rating mean on a timer?
It is the rating for lights that use a ballast, such as older fluorescent fixtures. Intermatic rates the TN311 at 15 A for magnetic ballast; BN-LINK rates the U47 at 5 A for ballast.[2][3]
Does a timer without a motor rating work for pumps?
We would not assume so. BN-LINK's U47 page states resistive, tungsten and ballast ratings but no motor rating, so its suitability for pumps is not stated.[3]
Should I put a filter on a timer?
Generally no: timers suit lights and other noncritical loads. A filter or air pump switched off by mistake is a welfare problem, not an inconvenience.