Amps - wire size - fuse

VanKitten · Sep 18, 2017

I think I may have been not thinking clearly about something.

The light may have just dawned on me

Amps = watts x volts

(of course, that equation can be flipped to derive any qualities needed)

But...30 amps of shore power is not the same as 30 amps battery power.

So...when I have 30 amps of 120v shore power....the wire gauge and fuse I need is
For 12v. 300 amps. Right? 30x120. 3600. Divided by 12v is 300 amps in my wiring.

Again for example....

I have 1800 watts of power required at 120v.
The power to drive that inverter will draw. Watts divided by volts. Then multiply by 12v 180 amps....

So... the wiring to the inverter must be able to handle 300 amps (plus fuse). Then from inverter to battery...same gauge wire. (Since it is only 2 feet away...I am thinking a 2 gauge wire for all of it)

I think I have that right.....yes?

Trebor English · Sep 18, 2017

RoamingKat said:
I think I have that right.....yes?

Close, but not quite.

Watts = volts x amps so amps = watts ÷ volts.

30 amp shore power is 30 amps so you need #10 wire for 30 amps. At 120 volts it is 30 amps. That is 3600 watts but still 30 amps.

If you wanted to get 3600 watts from an inverter fed by 12 volts the inverter input would be 300 amps (3600 ÷ 12 = 300) but this is not your shore power.

For the 1800 watt example, 1800 divided by 120 is 15 amps. To feed an inverter 1800 watts at 12 volts it is 1800 ÷ 12 = 150 amps. The wire from the battery to the inverter needs to be able to handle that to avoid overheating the wire.

To calculate inverter input amps a quick shortcut for mental arithmetic is to take watts and divide by 10 instead of 12. That gives a larger number of amps accounting for the loss in the inverter. For example, if the inverter takes 20 percent more than it gives, 1800 watts out would take 180 amps input rather than just 150.

VanKitten · Sep 18, 2017

I think you miss understood.

The wire from shore power TO the inverter must carry 30 amps X 120v.
But..the wire I am using is by gauge for 12v. So...
I need wire that will carry 300 amps...this is 2 gauge wire to be safe.
Then to carry the OTHER direction from the battery to the inverter....
1800w is not at twelve volts....it is
120v. This is the draw from the inverter to the outlet. So.. 180 amps of 12v from battery to inverter to support that

See..this is where I started out not paying attention to where the conversion is actually 120v not 12v.

Weight · Sep 18, 2017

30 amps is the rating of the 120 volt shore power outlet. Are you drawing the full 30 amps? Do you have a combination inverter/converter.? The converter acting as the battery charger and power supply while you are plugged into the 120 volt shore power outlet. What is the size of your inverter? Wire to carry 12 volts 30 amps is larger than to carry 120 volts 30 amps. So, where does the 300 amps come into this question? 300 amps to your inverter? If you have a 2000 watt inverter, you would need 2/0 awg cable, not 2 awg. 2/0 awg should not be longer than 10 feet. Mine is 18 inches. You would need a 300 amp fuse at the battery. The cable would have to be larger if you have a larger inverter. The inverter would draw 200 +/- amps from the battery when working to run something like a microwave. Less for other things. On the 120 volt side, 14 awg would work, 12 awg even better. I use 12 feet of 12 awg to take 120 volts to my kitchen, from my 2000 watt inverter. Your shore power cord should be heavy enough to carry the amps you will be using. 10 awg would be fine for a 50 foot cord.

John61CT · Sep 18, 2017

I think you've got it, but better to be precise in wording to avoid confusion.

RoamingKat said:
The wire from shore power TO the inverter must carry 30 amps X 120v.
...
the OTHER direction from the battery to the inverter

Inverter SOURCE is 12 V, OUTPUT is 120V.

A **charger** is reversed.

RoamingKat said:
1800w is not at twelve volts....it is 120v.

Watts is watts, power is power, no matter the voltage.

Only Amps are different according to voltage.

RoamingKat said:
180 amps of 12v from battery to inverter to support that

Correct.

Do you by chance have a combi unit where the charger and inverter are in the same box?

Spaceman Spiff · Sep 18, 2017

RoamingKat said:
. . . Amps = watts x volts

Watts = Amps X Volts

Amps is a count of electrons flowing past a point in the wire at an instant of time.
Volts is the electrical pressure pushing the electrons through the wire.
Watts is the measure of how much work the volt and amp combination will do.

. . . 30 amps of shore power is not the same as 30 amps battery power.

Amps in = amps out (excepting very special cases)

So...when I have 30 amps of 120v shore power....the wire gauge and fuse I need is

AWG 10 copper wire for 30 amps.
http://www.cerrowire.com/ampacity-charts

For 12v. 300 amps. Right? 30x120. 3600. Divided by 12v is 300 amps in my wiring.

For 12V, still 30 amps = AWG 10

Again for example....

I have 1800 watts of power required at 120v.
The power to drive that inverter will draw. Watts divided by volts. Then multiply by 12v 180 amps....
So... the wiring to the inverter must be able to handle 300 amps (plus fuse).

1800W ÷ 110V = 16.4A. Anything bigger than AWG 12 will carry 16A safely. Fuse to protect AWG 12 wire = 20A fuse.

Then from inverter to battery...same gauge wire. (Since it is only 2 feet away...I am thinking a 2 gauge wire for all of it)

Always wire for the amp draw a device uses (in the above case the inverter uses 16+ amps). Always fuse for the wire size as close to the source of power as practicable.

RoamingKat said:
I think you miss understood.

The wire from shore power TO the inverter must carry ~~30 amps X 120v.~~ 1800 ÷ 110 = 16.4 amps, plus a few more amps to run the inverter (the inverter spec should tell you how much current it draws).

But..the wire I am using is by gauge for 12v. So...

Wire size safety is determined by how many amps it can carry. This is a concern with 110V. For 12V wiring we are usually more concerned with voltage drop in the wire, which is why we use bigger wire than necessary for amp load.

I need wire that will carry 300 amps...~~this is 2 gauge wire to be safe~~. AWG 2 would be safe but way bigger than needed.

Then to carry the OTHER direction from the battery to the inverter....
1800w is not at twelve volts....it is 120v. This is the draw from the inverter to the outlet. So.. 180 amps of 12v from battery to inverter to support that

It is still 1800 watts. Your inverter will take 1800 watts from the grid (plus a little to run the inverter) and deliver a maximum of 1800W to the 12V side.
See..this is where I started out not paying attention to where the conversion is actually 120v not 12v.