Wiring Info/Help/Understanding

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GordDavey

GordDavey

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"1000 watt heating pads at 8.3 amps each"

Not possible on 12 volts. P=VxA so 8.3A x 14 v = 116.2 watts
Are those being fed 120V or are they rated for 120V?

"So, 3 amps for one, means to me 6 amps for 2, I would think it would require a bit more power in than you get out thanks to converting down to 5v, so 6 amps min, even though there is no startup load, would it still be a good idea to use the 125% as a safety net... especially when the input is unknown? If so, then 6 becomes 7.5 amps"

Are you going to run 2 of the dual usb adapters? Or are you just considering the one dual unit?
They are rated at an output of 3A @ 5 V EACH port, but the input isn't 6A due to the input being 12/14V. That why you go for the wattage and convert from there.
So if you run 2 separate adapters @ 36 watts (still a guess that there is 6 watt loss) that will equal 72 watts of power needed at 12-14V. P=V*I so P/V=I so 72/12=6A input just like you figured :) I would 10A fuse it and call it a day.

Many things will actually have the input voltage and current needed as well as the output voltage and current when converting. On most things it is printed on them if not in the specs.
Pacofortacos,

Thanks. I have two of these dual usb’s and the only power use indication is they say 36watts output 12v/24v. No other specs and nothing I’m the box or even printed on them. So it’s a bit of a guess. The choice I have is to run one by replacing an existing power outlet and one on a new wire. Or I can run one wire to power them both. Understanding how to choose the right wire and how to fuse them properly is what I’m trying to learn. That way I can make the right decisions later.

The 1000watts for the heating pads is the value KC said in his post. I should look up what I bought and try to get something more accurate. Both pads come with wiring that has a 10amp fuse on the main wire and a 5amp fuse for the On wire, relay and switches. At a guess the 8.3 amps KC said is I would imagine pretty close to wait they would draw. So again understanding which wire and how to fuse it all properly is what I’m trying to learn that way I can make the right decisions when I want to do something else. Like bed lighting or adding a light to the lower glove box etc.

Between you and KC I understand now how to work out how much power each unit draws and pick a wire based on that. I think there is no difference if doing 2 units as you just double the values and choose the correct wire for that. Having a chart that shows the ampacity down to 22ga really helps.

I still need to understand how to fuse it properly. I know I need to put a fuse on the wire by the battery that is just lower than the max the wire can carry. I imagine I need to keep the fuses close to the equipment to protect the equipment, but the statement “the fuse is there to protect the wire and nothing more” makes me question that.
 
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GordDavey

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Pacofortacos,

Just looked up the seat heaters and they are 27watts (+/-3 watts) for each pad. 2 pads to a seat, so 60 watts per seat, that'd be 5 amps min, 6.25 amps per seat with the 125% safety. I'd guess a 15 amp circuit could likely run both seats then and with 14ga as a minimum (16ga max 13 amps likely a bit too tight) ? Then I think if I used that circuit to also power one of the USB's, then that would be just over 15 amps. So still a 14 ga wire and a 20 amp fuse on it should be good to power both sets of seat heaters and 1 USB? If I'm understanding this correctly that is. Doing it this way the USB would have constant power, and to get it to work only when the key is on, I'd have to add a relay.
 
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pacofortacos

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I am not sure what the USB are referencing, sounds more like input power not output - output is always 5V on a usb as that is the standard.
I personally think the power outlet wire would handle both USB adapters and would just use that fuse.

IF you want to test it before finishing everything up, just wire both adapters in (in parallel) and put a max load that you would put on every usb out port and then feel the wire after 10-15 minutes.
IF the truck wire to your usb ports is warm then you are near max current, if ambient temp you are good, if hot disconnect and separate your power source or rewire with larger guage wire from the fuse.
Quick and dirty way to test.

On the heaters, the cost for you to go up to 12 ga. is minimal and I would go with 12 ga. if you are going to fuse at 20A - I personally even would if fusing at 15A. Reason is cost is minimal and benefit is great. Say over time, for some strange reason (due to flex, pinch, whatever) a few strands break in you power or ground wire - if using 14 ga. you now are getting close to not having the current carrying capacity, with 12 ga. you are most likely still fine with room to spare.
As a general rule, when wiring, I want my fuse and the load to be the weak links - not a buried wire that you cannot access. But maybe that's just me.

A corp. can't do that due to costs, but it is very little cost to upgrade doing it personally.

I even very seldom use 14 ga when doing wiring in the house unless I know the load is and always will be a light load.
 

pacofortacos

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Pacofortacos,

Just looked up the seat heaters and they are 27watts (+/-3 watts) for each pad. 2 pads to a seat, so 60 watts per seat, that'd be 5 amps min, 6.25 amps per seat with the 125% safety. I'd guess a 15 amp circuit could likely run both seats then and with 14ga as a minimum (16ga max 13 amps likely a bit too tight) ? Then I think if I used that circuit to also power one of the USB's, then that would be just over 15 amps. So still a 14 ga wire and a 20 amp fuse on it should be good to power both sets of seat heaters and 1 USB? If I'm understanding this correctly that is. Doing it this way the USB would have constant power, and to get it to work only when the key is on, I'd have to add a relay.


Ideally IMO, you don't want to run your circuits near max capacity at all times. Your wire may be "free air" but in reality it isn't in free air as it will be buried under carpet, in harnesses, etc.
The "free air" is because that allows for cooling, which is why when protected or covered the rating is lower.
A fuse is used for short protection, not a slight over current protection.
You can do what you want as far as making it one circuit, but it would be wiser to use 12 ga. wires and soldier and shrink tube all connections that carry any decent current IMO.
Spades at the final connection should be fine if they are tight since each connection by itself doesn't have a huge load.
 
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GordDavey

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Pacofortacos

Thanks. One more question on the fusing. With each item coming with its own inline fuse. Do I just need to use one fuse for the main wire or do I need that and the ones for the devices too?

Each seat heater comes with a fuse on the main wire and a fuse on the the relay wire. Should I keep those in place? I don’t like the idea of buried fuses and would rather just have a single fuse under the hood. But I don’t know if I can eliminate those or not.

I ran some 12ga for the seat heaters this weekend. Just to be safe.
 
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pacofortacos

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The factory just uses fuses in the fuse block or under the hood ;) That being said....

On the heaters esp., if there is a convenient place to locate the secondary fuses keeping them certainly wouldn't hurt anything. It is nice to have the lower rated fuse at each load so that if the part goes bad you pop that fuse before pulling enough current to pop the main fuse.

It's also nice that if one piece goes bad and it pops the lower rated fuse you don't lose the whole circuit.

So say if one seat heater shorted -
with just the main fuse, you would lose everything until you can disconnect or repair the shorted item.
But
with separate fuses, you would just lose the one seat heater and everything else would work fine until you get the time to take care of the shorted item.

PS. I have seen a lot of scorched foam from seat heaters (Jeep Grand Cherokees) when they fail, how many never caught fire I don't know - maybe the foam isn't overly flammable or the wires used on them is so small it just burns and breaks before it can do any serious damage.
 

chrisbh17

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I went with a main fuse (20a) right at the battery, then 10a inline from the relay to each seat heater. I also used 12ga wire even if somehow I could finagle the numbers to use something smaller. Considering I needed about 10 feet, did it really save me any $$$ or time to use a larger wire? Not really.

From the relay to each seat heater I did knock it down to 14 ga just because that what the hard-wired seat heater controls used.

The company that makes Posi-Tap connectors makes an inline fuse holder. Crazy easy to use and even looks professional when its done.
 
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GordDavey

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The factory just uses fuses in the fuse block or under the hood ;) That being said....

On the heaters esp., if there is a convenient place to locate the secondary fuses keeping them certainly wouldn't hurt anything. It is nice to have the lower rated fuse at each load so that if the part goes bad you pop that fuse before pulling enough current to pop the main fuse.

It's also nice that if one piece goes bad and it pops the lower rated fuse you don't lose the whole circuit.

So say if one seat heater shorted -
with just the main fuse, you would lose everything until you can disconnect or repair the shorted item.
But
with separate fuses, you would just lose the one seat heater and everything else would work fine until you get the time to take care of the shorted item.

PS. I have seen a lot of scorched foam from seat heaters (Jeep Grand Cherokees) when they fail, how many never caught fire I don't know - maybe the foam isn't overly flammable or the wires used on them is so small it just burns and breaks before it can do any serious damage.
Pacofortacos,

Perfect thank you so much for helping me understand all of this. I’m confident now I’m what I’m doing and sure I can work out what I’m planing for in the future.

Thanks to KC Cryptkeep and chrisbh17 as well for your part in this too.
 
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