How many watts on one 15amp DC circuit?

[Jey]

Hey guys,

I have a free dc circuit (15amp fuse) after getting rid of the propane furnace.


I?m wanting to run led lights in the pantry and one under the kitchen overhead cabinet to give some light on the counter top. But I also want to have a 12v usb outlet like this


MICTUNING Pre-wired 12V Dual USB Breaker Switch Panel - Socket Charger 2.1A 1A with LED Indicator and Cigarette Lighter Socket for Marine Boat Car RV Vehicle Truck https://www.amazon.com/dp/B07GFMBD2K/ref=cm_sw_r_cp_api_i_bwpFDbMS0MRRH


I?m trying to research here to understand my limits on that one circuit using the 15amp fuse installed as my basis.


I found this page https://www.inchcalculator.com/watts-to-amps-calculator/#idx_equivalent_watts_and_amps_at_12v_dc


Using the 15amp fuse as a base at 12v my maximum use on that circuit should be under 180watts, Is this true?

I think an iPhone pulls under 20w when charging and I?m sure the usb outlet itself has a draw. The lights I have are 12-20watts each and there would be 4 of them.


Is this safe?

 

[darsben]

Volts times amps equals watts
BUT THE WIRE SIZE IS THE MOST LIMITING FACTOR
So what size wire do you have to work with

 

[Back2PA]

You want to leave yourself some margin so you're not blowing fuses. Assuming a 13 amp draw on a 12.6 amp circuit (fully charged 12V battery = 12.6 volts) gives you 164 watts to play with

 

[Jey]

Volts times amps equals watts
BUT THE WIRE SIZE IS THE MOST LIMITING FACTOR
So what size wire do you have to work with

There are pigtail wires coming straight from the breaker panel about 12? long and I believe they are 10 gauge. I would be connecting to that pigtail so I could run whatever size needed. Keeping in mind the lights have like 14 gauge wires coming out of them and I believe the usb outlet has 12ga

 

[darsben]

If  ten gauge or even 12 gauge then wire size is not a factor you need to worry about

 

[SLOweather]

Check the watts on those LED lights again, or post a make and model. True 12-20 watt LEDs would equal about 90- 150 watt incandescents in light output.

OTOH, if the are 12-20 watt incandescent EQUIVALENT lamps, then they are about 1.6 watt- 2.7 watt lamps, and draw 0.13 -0.23 amps.

 

[Isaac-1]

It is a good idea to include a safety factor to prevent blowing fuses, a good rule of thumb is to design around 80% circuit capacity, of in this case say 12 amps at 12.0 V (50% discharged battery) = 144 watts.

Having said that you should be fine for you application as long as don't go overboard on plugging stuff into that 12V lighter socket.  Though personally I would use one of these sockets with dual 2.1 amp (at 5V) USB sockets https://www.amazon.com/MICTUNING-Charger-Compatible-Motorcycle-Marine/dp/B076D8CQYV/ , not the 1A / 2.1 amp combo like you have listed or one of these that support the higher output QC3.0 standard https://www.amazon.com/Charger-CHGeek-Waterproof-Voltmeter-Motorcycle/dp/B07GKJ4ZLC

 

[Gary RV_Wizard]

I think you need to look again at those fixture pigtails - I think you have grossly overestimated them.  I'm having a hard time imagining any residential light fixture with a 14 gauge pigtail - that would be a huge overkill even if incandescent.  Led light fixtures often have 20 ga or 18ga and USB gadgets use so little power than wiring is typically 22-26 ga.  5A @ 12v-20v is a common spec for a USB-C hub.
In any case, you need have no worries about that 15A main branch.

 

[Henry J Fate]

125% is a standard formula used in circuit design. Ex: If all your devices total 10 amps, it's good to add another 25% which would put the total at 12.5 amps. Most of the allowance is for thermal variances.

Also remember that the voltage should be considered at 13-14 volts. Voltage is always higher than 12 especially with a converter/inverter.

An easy way to remember the math to convert electrical specs is if the wattage equals the voltage, the amperage is 1 amp. Ex: 1 amp @ 120 volts = 120 watts.

Some appliances will use more energy at start up and then settle into a constant amp/wattage usage.