I'm new to the RV world but definately not new to electricity. I have been having a lot of fun testing out inverters but I have a couple questions for you tech gurus.I purchased a 1990s model new old stock xantrex prowatt 600 inverter. The specs claim 600 continuous and 1200 surge with a MSW. I figured I would test it with a small 700w kureg coffee maker and see if it could take it. The darn thing drew over 62 amps and the inverter fan bogged down a hair but overload protection never kicked in and It spit out the coffee with no problem.On my trimetric meter at the battery, it showed voltage dropped from 12.6 to 11.9. When I measured the voltage at the inverter terminal however it was only 11.35 volts. I am using 10 ft total of 6 gauge wire even though the manual recommends 4ga however the wire at the inverter never really got warm. Just barely noticable and neither did the inverter for the 2 minutes it takes to brew the coffee. Just the voltage drop with barely any heat.1) Is this voltage drop acceptable or should i think about upgrading the wiring?2) I am calculating this draw between 700 and 750. On a 600w model why didn't it shut down? Is it safe to do this regularly?3) when calculating amp draw for an appliance do you use voltage near the battery or near the inverter?4) if the inverter overload protects itself, how long do you have to reset the inverter before it causes damage? (I fried a 100w china inverter trying to overload it. It went into fault mode then about 10 seconds later burnt a diode on the board)

upgrade the wiring... 12 volts needs lots of copper.why do you think in the 50's cars/trucks went from 6 volts to 12... half the current. and more reliable starting.and the rumor mill.. going from 12 volts to 36. a lot less copper.

Chuck_thehammer wrote:upgrade the wiring... 12 volts needs lots of copper.why do you think in the 50's cars/trucks went from 6 volts to 12... half the current. and more reliable starting.and the rumor mill.. going from 12 volts to 36. a lot less copper.Interesting!!! Thank you for the reply!!!! :)

Ten feet of #6 wire should have a voltage drop of 0.24v with a current of 62A. You have a voltage drop of 0.55v if I am reading your post correctly. Do you have 10' of hot and 10' of ground, total of 20'?I would go to #4 wire if not #2. You are making over 30 watts of heat.BTW, I would use my clamp-on ammeter to measure the current.

I have a newer xantrex 600 and it recommends O gauge and 4 as a minimum. I used 2 but dont run anything near that heavy.

Acceptable voltage drop for Xantrex 600w inverter? Overload?

pianotuna
Nomad III
Nov 01, 2015
Hi Salvo,

Thanks for taking us through the math.
Salvo
Explorer
Nov 01, 2015
No, the cables are not going to catch on fire. The main reason for big cables to enable a large dynamic voltage range. The battery is 100% at 12.8V. The inverter low voltage alarm is around 11.1V. That means you got a dynamic voltage range of 1.7V to work with.

If the cable drops 0.5V, then your usable voltage range is only 0.7V. Therefor you want as little as possible cable voltage drop. Let's go with a 5% drop. That means the cable voltage drop is:

Cable drop = 1.7V * 5% = 85 mV

85 mV is the magic number. You want to keep cable voltage drop to less than 85 mV.

We can now calculate the cable resistance and then look up resistance in a wire gauge chart to get the desired wire gauge.

Total wire resistance = 85 mV / 60 A = 1.4 mohm

If total wire length is 10 ft then wire resistance per foot is:

Wire resistance = 1.4 mohm / 10 ft = 0.14 mohm/ft

Go to this table to get the correct wire gauge.

2 gauge wire has 0.156 ohms per 1000 ft, or 0.15 mohm/ft. That's close enough.

Yes, 6 gauge will work. But if you want to keep the low voltage alarm at a minimum, go with 2 gauge.
2oldman
Explorer II
Nov 01, 2015
enblethen wrote:
More batteries will not alleviate voltage drop. It is caused by resistance in the wire to the load.
More batteries may slow down the discharge of the battery bank.
With that load on just 2 batteries I bet it would. Voltage drop is not always the wire's fault.
grizzzman
Explorer
Nov 01, 2015
tenbear wrote:
:B
kyle86 wrote:
I figured I would test it with a small 700w kureg coffee maker and see if it could take it. The darn thing drew over 62 amps---.

On my trimetric meter at the battery, it showed voltage dropped from 12.6 to 11.9. When I measured the voltage at the inverter terminal however it was only 11.35 volts. I am using 10 ft total of 6 gauge wire---.

Your original post said the the voltage dropped from 11.9v at the battery to 11.35v at the inverter. That's 0.55v volts being dissipated somewhere. You said the current was over 62A.

That means over 34 watts is being dissipated somewhere. 10' of #6 wire would cause a 0.24v drop, a loss of 14.88 watts so 20 watts is being dissipated somewhere else along the battery to inverter path.

How hot would a 20watt incandescent bulb get? That could get hot enough to cause a fire under worst case conditions.

Yes, I'm an old engineer.:B

So....... you are comparing say a 34 watt thin tungsten filament in a vacuum to 10 feet of insulated 6 guage wire? Realy?
I think the real issues are quality and duty cycle. As an example morningstars 300 watt suresine can be pushed to 600 watts for 10 minutes. and thats with a passive heatsink.

This is assuming the op is NOT running this load continuously, a few minuites of runtime isn't as likely to be an issue. That being said in electronics,"heat kills" I would suggest a pc type fan aimed at the back of the inverter that runs until you are done with coffee.
tenbear
Explorer
Nov 01, 2015
:B
kyle86 wrote:
I figured I would test it with a small 700w kureg coffee maker and see if it could take it. The darn thing drew over 62 amps---.

On my trimetric meter at the battery, it showed voltage dropped from 12.6 to 11.9. When I measured the voltage at the inverter terminal however it was only 11.35 volts. I am using 10 ft total of 6 gauge wire---.

Your original post said the the voltage dropped from 11.9v at the battery to 11.35v at the inverter. That's 0.55v volts being dissipated somewhere. You said the current was over 62A.

That means over 34 watts is being dissipated somewhere. 10' of #6 wire would cause a 0.24v drop, a loss of 14.88 watts so 20 watts is being dissipated somewhere else along the battery to inverter path.

How hot would a 20watt incandescent bulb get? That could get hot enough to cause a fire under worst case conditions.

Yes, I'm an old engineer.:B
kyle86
Explorer
Nov 01, 2015
Thank you all for the replies and edumication. I know on paper it has to be done a certain way kinda like the old engineer vs technician cliche. I guess my real question is... It's installed.. The coffee maker works... The dc wires are not getting hot at the inverter end.. Is there an issue running it this way? How can this cause a fire if it's not getting hot? Wouldn't buying a bigger inverter and bigger wire be a waste of money?
enblethen
Nomad
Nov 01, 2015
Only making reference back to the OPs issue.
pianotuna
Nomad III
Nov 01, 2015
Hi enblethen,

That depends on the load. If you do 200 amps of load on a 232 amp-hour battery bank there will be LOTS of voltage drop at the battery terminals.

enblethen wrote:
More batteries will not alleviate voltage drop. It is caused by resistance in the wire to the load.
More batteries may slow down the discharge of the battery bank.
enblethen
Nomad
Oct 31, 2015
More batteries will not alleviate voltage drop. It is caused by resistance in the wire to the load.
More batteries may slow down the discharge of the battery bank.
MEXICOWANDERER
Explorer
Oct 31, 2015
VOLT DROP "Here to there"

Dropped Voltage "Everywhere"

Example of excessive volt drop. 12.2 measured at the battery post versus 11.3 measured at the inverter terminal input.
Example of voltage droop. 11.0 measured at the positive battery post and at the inverter inlet terminal.
They are fixed by two entirely different methods.

Not trying to play hall monitor here - merely trying to maintain clarity.