USB and Seat heater electrical mods: repurposing existing circuits

[txboiler]

Some background, I am starting the Ron Francis complete harness and Sniper EFI. I have used the sniper fuel pump relay and tied the blue fuel pump wire from the sniper and tied that into the tan fuel pump wire on the rear RF harness. I plan to connect the inertia switch in series with the ground wire coming from the fuel pump.

The following circuits are not currently being used on the RF harness:
1) heater
2) Fuel pump

I plan to install USB charges in the following areas: 1 ea under the dash, the cubby, and the trunk. I plan to connect this all on one circuit. I also plan to install “warm seat” seat heaters on both seats. I am planning to run the seat heaters off the heater circuit and the USB off the fuel circuit.

Will the 20 amp heater circuit be adequate for both the seater heaters to operate at the same time?

Do anyone see a problem with this plan or have other suggestions?

[Papa]

I'm using the heater circuit for my two seat heaters, and the radio circuit for my USB chargers. The fuel pump circuit relay is triggered with the 12v switched key on, so it would also work for either as long as you remember to complete the ground after moving the inertia switch.

[Sdonnel]

I've got mine running the same as you plan to. Everything works fine. The CFO even complains that her seat heater is TOO hot at times. You won't regret either of those additions.

Scott

[phileas_fogg]

Ditto on the 20A circuit being adequate for two seat heaters in operation at the same time. I seem to recall I put both of mine on a 10A fuse, and haven't popped one yet!


John

[txboiler]

Papa good point on the fuel pump being on the keyed and only work with the key on. Wouldn’t they same be true for the brown radio power wire? If I want the USB to be hot when the ignition is off should, I use the red radio memory wire.

Thanks again

[RJD]

Some background, I am starting the Ron Francis complete harness and Sniper EFI. I have used the sniper fuel pump relay and tied the blue fuel pump wire from the sniper and tied that into the tan fuel pump wire on the rear RF harness. I plan to connect the inertia switch in series with the ground wire coming from the fuel pump.

The following circuits are not currently being used on the RF harness:
1) heater
2) Fuel pump

I plan to install USB charges in the following areas: 1 ea under the dash, the cubby, and the trunk. I plan to connect this all on one circuit. I also plan to install “warm seat” seat heaters on both seats. I am planning to run the seat heaters off the heater circuit and the USB off the fuel circuit.

Will the 20 amp heater circuit be adequate for both the seater heaters to operate at the same time?

Do anyone see a problem with this plan or have other suggestions?

Did mine the same way. Removed the Ron Francis fuel pump relay and repurposed the circuit for the USB ports. Heater circuit powers the seat heaters with no issues.

[hineas]

I am long winded, so I apologize in advance for my long post.

We ran the seat heaters from the fuel pump circuit. I have used the heated seats for several hours straight without blowing a fuse. I just pulled up the Warm Seats specs and they say it only pulls 3.5 amps per set. The fuel pump circuit is a 15A circuit, which is more than adequate. 2 sets of seat heaters is only about 50% of the max load, so not an issue at all. There shouldn't be any issues using the 20A heater circuit since you would only be at 35% of the maximum load.

We installed our USB charger off of the radio power because we wanted it to be a keyed power source and it was available. The radio power has worked well for us. I had my phone plugged in for a 2.5 hour drive just this week and it was perfect. The radio circuit is only 10A and it also covers the gauges, so you have to be aware of that. From reading the specs on the Speed Hut gauges, each gauge pulls about 0.2 amps for a total of 1.4 A. Our USB charger has a max of 4.5 amp if both ports are used. This puts us at a total of almost 6A, which is only 60% of the maximum load.

I think using the heater and the fuel pump circuits is great and shouldn't cause issues for you.

Having said that, I would consider swapping which circuit you use. I like to keep my total load less than 80% of the maximum load. You mentioned you will be putting USB charges under the dash, in the cubby, and in the trunk. Many of the dual USB ports will pull a total of 4.8A if you are using both ports. If you have something plugged into all of the ports, that could be a total of 14.4 A draw. That is 96% of your maximum draw for a 15A circuit but only 72% of the 20A circuit. However, I doubt most people would have something plugged into all of the port so I would suspect in practice you would always be under 80% of a 15A circuit.

If you want to be extra cautious, you could use the heater circuit for the USB ports and the fuel pump circuit for the seat heaters. This way you would never be at risk of overloading the circuit. But like I said, in practice I doubt most people would have every port used.

You also mentioned the radio memory for the USB chargers so it would always be on. If memory serves, this is the same circuit as the headlights (looking at my fuse box) and is a 15A circuit. If you are using the halogen headlights, I believe a standard halogen headlight pulls almost 5 amps each. That means you have 10 amps when using the headlights and if you have 2 devices plugged into the USB charger you are at almost 100% of the max load. If you have more than 2 devices plugged in you will blow a fuse each time you turn on your headlights. If you are using the LED lights it is better as many of them only pull 1.6 amps on high beams. That means you would likely be using 3.2 amps with LED headlights so you could then use 4 USB ports and only be at 80% max load. It all depends on which lights you are using, but this could definitely be a potential source as well.

Another thing to consider if you use the radio memory is that many of the USB ports have a small LED that shows it is powered. This normally would be minimal current from your battery, but if you don't drive the car for a long stretch it could potentially drain the battery. If you end up using the radio memory, make sure you find a USB charger that doesn't have an LED light.

You will like having the USB chargers and the seat heaters. They are some of the best modifications we did to our car and I use them frequently!

[txboiler]

Hineas. Thanks for the detailed comments and background info on several of the circuits

[JeffP]

Having said that, I would consider swapping which circuit you use. I like to keep my total load less than 80% of the maximum load. You mentioned you will be putting USB charges under the dash, in the cubby, and in the trunk. Many of the dual USB ports will pull a total of 4.8A if you are using both ports. If you have something plugged into all of the ports, that could be a total of 14.4 A draw. That is 96% of your maximum draw for a 15A circuit but only 72% of the 20A circuit. However, I doubt most people would have something plugged into all of the port so I would suspect in practice you would always be under 80% of a 15A circuit.



That amperage seams excessive for a USB port. USB Ports typically advertise their amperage output to the device not the draw in which most of them are 4-5amps, but the USB spec is only 5v so thats 20-25W. A 10amp fuse at 12v is 120W. Most of the high powered 12v USB chargers on amazon pull around 30w. 10amp should easily handle 3.

My heater circuit is running my heater, so I tagged my extras on the Radio, Radio Mem, Electric choke.

[hineas]

That amperage seams excessive for a USB port. USB Ports typically advertise their amperage output to the device not the draw in which most of them are 4-5amps, but the USB spec is only 5v so thats 20-25W. A 10amp fuse at 12v is 120W. Most of the high powered 12v USB chargers on amazon pull around 30w. 10amp should easily handle 3.

My heater circuit is running my heater, so I tagged my extras on the Radio, Radio Mem, Electric choke.

Sweet! Thanks for the clarification. I assumed that the advertised amp was the draw, not the output. That absolutely changes the math! THANK YOU!

Let's see if I remember my physics lessons from undergrad. The dual USB charger I just looked up has a max output of 4.8 amps (2.4 amps per port). USB voltage is 5V. That means the power is 12 watts per port, or 24 watts total (similar to the 30W ones you referenced). That means 24 watts at 12V input would have a current of 2A total on the input side. So FULL capacity of 3 ports is 6A, well below a 10A circuit.

Or, as you mentioned, a 12V circuit at 10 amps is 120 watts. Depending on which device is installed, it will likely be less than 30 watts per dual charger. This is 90 watts total at max power draw, which is well within the capability of the circuit (depending on what other devices are installed on the circuit). Thank you for the correction! That's why I love this forum.