Hardwiring an AC inverter (1 Viewer)

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Pete483

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I've got a 750 watt inverter that I want to hardwire. They recommend 6 gauge wire with a 150amp fuse. This is no problem, but I also wanted to relay this thing to the ignition so that it can't be left on on accident.

The internal fuses in this thing are three 35 amp's wired in parallel. A relay that would handle that kind of load would be pricey, but could I just wire three 35 amps relays in parallel? I'm not the greatest with electronics so I'm trying to learn this as I go.
 
You could get a solid state relay for around $100 that would handle 100 Amps at Mouser Electronics. If you look I'm sure you could find a 125 amp version, I only did about 3 minutes of research. The 40 Amp relays at Summit run $45.
Solid state relays work exactly like mechanical relays, the only difference is you need to provide a heat sink. They last for years under constant switching.

Just a thought.
 
I know I can get those, but I can get 30-40 amp relays for $3-4 ea at any autoparts store.
 
Pete483e483 said:
I know I can get those, but I can get 30-40 amp relays for $3-4 ea at any autoparts store.
Click to expand...


Summit also has some really nice 30/40A pigtail relays that are about $12 a piece...I just spliced two of them into my headlight harness when the onld ones corroded away....
 
The three fuses in parallel is a hack that I have never seen before. Three relays in parallel would be an even worse hack.
 
I'm electrically illiterate but wouldn't one of those 30 amp fuses blow at 30 amps same af if there were only one fuse?
 
I'm no electronics expert, but I can maybe help a little.

Three fuses in parallel, each one will still blow at 30 amps, the difference is you're splitting the load between the three of them, so theoretically you should be able to handle 90 amps. The only problem is if there is some variation in the wires, load, resistance, etc, and one of them blows, all of a sudden you've only got 2 fuses which are now trying to carry 45 amps each. So the weaker fuse will blow first, then the last fuse will be carrying all the load, and will blow. So essentially they will all go at once almost simultaneously.

Wiring 3 relays in parallel will also have the same effect. If one goes, the others will not be able to handle the additional load, so they will all go. I'd recommend just hardwiring straight to the battery and putting a switch on there. I'm not sure why you need 150 amp fuse for a 750 watt inverter, that only amounts to about 62 amps at full draw. Even figuring some inefficiency, a 100 amp should be plenty. And depending on what you're using it for, maybe you would want to use a 60 amp fuse to prevent overloading the inverter, sort of a like an easily replaceable safety valve.
 
ducktapeguy said:
I'm no electronics expert, but I can maybe help a little.

Three fuses in parallel, each one will still blow at 30 amps, the difference is you're splitting the load between the three of them, so theoretically you should be able to handle 90 amps. The only problem is if there is some variation in the wires, load, resistance, etc, and one of them blows, all of a sudden you've only got 2 fuses which are now trying to carry 45 amps each. So the weaker fuse will blow first, then the last fuse will be carrying all the load, and will blow. So essentially they will all go at once almost simultaneously.

Wiring 3 relays in parallel will also have the same effect. If one goes, the others will not be able to handle the additional load, so they will all go. I'd recommend just hardwiring straight to the battery and putting a switch on there. I'm not sure why you need 150 amp fuse for a 750 watt inverter, that only amounts to about 62 amps at full draw. Even figuring some inefficiency, a 100 amp should be plenty. And depending on what you're using it for, maybe you would want to use a 60 amp fuse to prevent overloading the inverter, sort of a like an easily replaceable safety valve.
Click to expand...
A good explanation.
Another consideration is that the relays will not all switch at exactly the same speed, so for a few millisec one relay will carry the entire load, for a few more millisec more two will share the load, finally the third relay will make and the load will be distributed. This may work for a while but fairly soon the faster relay will blow, resulting in the cascade effect Ducktapeguy described.

#6 wire is rated for around 150 Amps at 150 C, so I'm guessing that is where the 150 amp fuse came from.
 
Hold on guys.

Ar we talking about an automotive application?

Then we are dealing with a 12 volt DC system going to 115 volts AC. That means to get his 750 watts it takes 6.5 amps at 115 volts.

Where are is this 150 amp DC stuff coming from? Most homes are 200 amp services AC, which is actually less current!!

The three fuses in parallel would require a small ohm resistor downstream of each to distribute the current across each fuse.
 
^^^ I am trying to figure that out too, where did 150 amps come from?
 
The recommendation for 6 gauge wire with a 150amp fuse at the battery was the manufactures recommendation.

Let me explain my idea more thoroughly: I would fuse each relay with a 30 amp fuse between it and the battery. I also considered potential problems from slight differences in timing, but even at a few tenths of a second, it would take more that 100 amps to blow a 30 amp fuse.

I'm starting to lean towards not doing this parallel business, but the only reason I'm still thinking about it is that: 1. The inverter itself is fused in parallel and 2. I hate spending 3 times the price of this thing just to wire it up.
 
For my taurus fan I used two relays in parallel. Seemed to work fine. I made sure my wires were the same length so the time delay between the relays was minimal. I also used a fuseable link for a fuse.

For my inverter I bought a high amp automotive fuse and directly soldered it to my 4 gauge wire.
 
Kofoed said:
Hold on guys.

Ar we talking about an automotive application?

Then we are dealing with a 12 volt DC system going to 115 volts AC. That means to get his 750 watts it takes 6.5 amps at 115 volts.

Where are is this 150 amp DC stuff coming from? Most homes are 200 amp services AC, which is actually less current!!
Click to expand...
On the AC side, yes there is only about 6.5 amps, but you also have to consider the DC side. Since the DC side is at 12V at 750W, the amperage is 62.5A continuous (750W/12V=62.5A)

That's not even considering losses and load spikes. Figure about 10% increase to account for inefficiencies. I think most inverters say they can handle a spike of twice the continuous load so lets say the spike is 1500W. 1500W/12V=125A.

You can see why some people want manufacturers to switch to higher voltage automotive systems. A little power can increase current rapidly.
 
robinhood4x4 said:
On the AC side, yes there is only about 6.5 amps, but you also have to consider the DC side. Since the DC side is at 12V at 750W, the amperage is 62.5A continuous (750W/12V=62.5A)

That's not even considering losses and load spikes. Figure about 10% increase to account for inefficiencies. I think most inverters say they can handle a spike of twice the continuous load so lets say the spike is 1500W. 1500W/12V=125A.

You can see why some people want manufacturers to switch to higher voltage automotive systems. A little power can increase current rapidly.
Click to expand...

If you are using a cheap inverter that is of a "square-wave" waveform. Then it is not really AC and is switched DC--bad for certain electronic devices. Then your calc holds. And the effective wattage is way higher because of it (so they reduce the current draw as a result to get the advertised wattage).

For sine wave inverters my calc holds.

At this point, maybe Pete could speak up and tell us the working current draw that is indicated on the device.
 
The inverter is a 750 watt--1500 peak-- "modified" (just about square) sine wave and says up to 7.0 watts (AC). They don't specify it's current draw, but the internal fuses are three 35 amp blade fuses, so it couldn't be more than 105 amps. I figure the amount of time this thing would be at its peak (1500 watts) would be so short that it wouldn't blow 105 watts of total fuses.

The manufacturer recommends 150 amp fuse, but if it only has 105 amps fused inside, I don't see what I can't just use three 35 amp relays wired in parallel. Everything I've been able to find on matching wire gauge to fuses recommends 125 amp or less for 6 awg wire anyways, which leads me to not want to totally trust this manufacturer.
 
I just ordered a Cobra 850 inverter, 800 watts continuous/1600 watts peak. I won't get it for 1-2weeks, but now I'm interested in what it will spec for hard wiring...
 
Kofoed said:
For sine wave inverters my calc holds.
Click to expand...
Not sure I see the math there, that is, why sin/square, makes any difference. The DC side (the wires hooked directly to the battery) doesn't know that it's attached to an inverter. All the wires know is there is a continuous, smooth, linear load of 700w.

In other words, there is 700 watts going through both the AC wires and the DC wires, they're just at differrent voltages and currents.

.................115V AC wire @6A................12V DC wire@ 58.3A
Hair dryer ================Inverter==================Battery

Hair dryer = Wattage of AC side = 700w = Wattage of DC side

115V x 6A = 700w = 12V x 58.3A
 
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Regardless of the math, I'm not sure if relaying the inverter is such a good idea. For one thing, those kinds of current will probably kill a standard relay pretty quickly due to arcing, plus I don't think inverters were meant to be switched on/off on the DC side like that. if you left something plugged in when you turned on your ignition, you could possibly cause power spikes that would destroy your appliences. It's probably not good for the inverter either. I think most inverters recommend a certain power on sequnce for connecting them.

I'd just hardwire it directly with the factory wiring, if you defintely want it switched with the ignition, wire a relay to the factory switch, which is probably on the low amp/high voltage side of the inverter.
 
I was thinking about this last night and realized I lost a zero in my ciphering (not posted) back in post #9.

Low volts high amps = low amps high volts :frown:
 
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