Confused On Acceptable AWG For Short Distances.

[HRTKD]

I need to run wire from my converter to the bus bar. The one-way distance for one of the wires from the converter to the bus bar will be no more than 20", perhaps a bit less depending on how I route the wire. One wire gauge calculator says that 6 AWG is OK to use. That makes me happy since I have either 6 AWG or 2/0 AWG (2/0 won't fit in the converter ports so that's not going to happen). The wire calculator on the Bay Marine Supply website disagrees, saying that not even 1 AWG is acceptable.

Similarly, my 1000 watt 12v inverter needs wire also. The distance for that is a few inches shorter, but also includes a circuit breaker. I calculated that the maximum amperage throughput for the inverter would be 91 amps (1000 / 12 * 1.1). The manufacturer says the inverter is 85%-90% efficient. I'm optimistic and going with 90%. I don't push this inverter very hard. I previously had 6' of 1 AWG when the inverter was in a different location.

The distance from the bus bar to the battery is no more than 20". The bus bar to battery wire is 2/0 with all the necessary disconnects and fuses. This is a 12v system.

I thought I had this all figured out.

I need this system up and working in four days. I don't have time to get different cable. If the 6 AWG is acceptable that would be great. Otherwise, I'll try to make the 1 AWG work. It's very stiff wire so it's difficult to work with in small spaces.

 

[gnubie]

There's a few things to consider.
1. The outright ability of the wire to carry the current
2. The amount of voltage drop you application can handle
3. The environment the wire will be in, free air allows it to dissipate heat better, in conduit or a cable bundle means it will get hotter
4. The temperature the wire will rise above ambient at the current you are drawing, this determines your insulation temperature rating

6 AWG would carry the current, just rated for it in open air with a slight safety margin, and for the short runs copper 6 AWG wouldn't drop much voltage, around 0.2V. You always want a bit of headroom in your cabling so 5 AWG might be a better choice but since you are stuck with 6 AWG so long as you fuse sensibly it'll be OK IMO. If you wanted to play it safe, 4 AWG is heaps.

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

 

[snoobler]

You can also string 2X parallel conductors. 2X 6awg is like 3awg.

Are we talking the 91A or some other number?

 

[HRTKD]

Thanks for the feedback.

91 amps is what the inverter will pull at most. The converter is 55 amps, so I'm not as worried about that.

I considered using two 6 AWG wires, but I was thinking I would need to put them both in the same lug. Now that I'm thinking about that, maybe that isn't necessary. The limitation is the port on both devices. 1 AWG is very difficult to get into the inverter, but with some work it does make it.

 

[snoobler]

If 1awg would fit, then 2x 6awg at 3awg equivalent would fit.

I use the awg charts 90°C rating, so 6 is definitely under spec, but 4 is fine and 3 is better. If you already have the 6, it's an easy fix with doubled conductors.

 

[HRTKD]

I'll have to look at how much wire I have left. I may give the double conductor method a try.

 

[svetz]

Resistance = pL / A, where L is the length, A is the cross sectional area, and p is the resistivity of the metal (Copper is 1.7 × 10-8 Ω m).
Example math was recently worked out in #27 and #29 in this thread. @RCinFLA was also kind enough to provide resistivity for a number of metals in post #4.

 

[RCinFLA]

The distance from the bus bar to the battery is no more than 20". The bus bar to battery wire is 2/0 with all the necessary disconnects and fuses. This is a 12v system.

I thought I had this all figured out.

If the 6 AWG is acceptable that would be great. Otherwise, I'll try to make the 1 AWG work. It's very stiff wire so it's difficult to work with in small spaces.

At 91 amps 20" of #6 will have 0.061v drop but will have 5.6 watts of heating. It will get quite hot. Doubling it up would help make heating more acceptable.

#2 -> 0.024v drop with 2.2 watts heating (wire just a little warm)

 

[Zil]

If you are using marine grade cable; 6 awg will be fine for 100 amps at 48" round trip. Fuse for 100 amps. The voltage drop will be less than 2%. Not enough heat to worry.

 

[HRTKD]

At 91 amps 20" of #6 will have 0.061v drop but will have 5.6 watts of heating. It will get quite hot. Doubling it up would help make heating more acceptable.

#2 -> 0.024v drop with 2.2 watts heating (wire just a little warm)

Thanks! The wire size calculators I've seen don't show wire heating. I'll have to take that into consideration in the future.

What got me confused was picking the first calculator that came up on a Google search. It was from WireBarn.com and this is the result.



6 AWG wire looks totally fine! I could even use 10 AWG. It seemed too good to be true, so I checked Bay Marine Supply. What got me in trouble here was that I was exhausted from working on the trailer all day. I looked at the four lines on the right as a commentary on the numbers to their immediate left. I didn't realize they were color codes for the values on the left. My (incorrect) reading of the chart made me think that starting with 2 gauge the load exceeded the ampacity of the wire. In the case of the Bay Marine Supply calculator, 6 AWG is acceptable but it's on the margin.

 

[John Frum]

The most common cause of discrepancies between the calculators/tables is round-trip distance.
Some want the whole circuit length and others assume the positive and negative leg are the same length.
The other common issue is wire insulation temperature rating.
NEC has columns for 60, 75 and 90 celcius.
Most of the calculators we use including the bay marine one assume 105 Celsius.
Devil is in the details.
Finally voltage matters as in voltage drop.

 

[HRTKD]

I'm not trying to half-ass this. I really want it to be right, safe, efficient and effective. I am very concerned about voltage drop. The existing wiring in my trailer is a complete cluster. The wire from the battery on the tongue is maybe 8 AWG. It's hard to tell as it has zero printing on the sheath. The wire goes from the tongue, makes a pit stop in a distribution box just aft of the tongue (about 5') where the 7 pin cable is also wired in. Then it goes to about the middle of the trailer (10' more), up into the cabin to a cut-off switch, back down under the trailer where who-knows-what happens (because I lose track of it), then back up into the cabin and into the main distribution center.

I suspect that some of the problems that I've had with the motorized bunk bed (HappiJac) are due to voltage drop. It sometimes fails to work on battery. On shore or generator power (where the converter is running) it always works. The converter is located within the main distribution center and has only a 12" wire length to get to the main board.

If I think of it, I need to measure the volts at the battery and at the main distribution center.

The inverter is currently wired with 1 AWG that was from the previous setup. However, I forgot to put the circuit breaker between the inverter and the bus bar. I don't have 1 AWG lugs on hand to chop the existing positive cable in half and add ring terminals. That's what got me to thinking that maybe I could use the 6 AWG.

I got all the big stuff right on this install. It's the little things that are killing me. Like why oh why did I run 6 AWG to a PV port that uses 10 AWG?

I can't stand shoddy work. In the picture below, factory wiring is on the right (no support for the wires at all) and the start of my wiring on the left (multiple supports, grommet in the hole where the wires come out). The pieces of OSB and plywood near the top of the picture were added by me this weekend. The vertical 1"x2" board was wobbly and needed stabilization. The plywood is there for the hanger rod and shelf that were supported only by 1/4" Luan plywood. Everything is glued and screwed/stapled.

 

[gnubie]

In the case of the Bay Marine Supply calculator, 6 AWG is acceptable but it's on the margin.

Yup, what I mentioned way back there in my post

*more thoughts*
There's nothing wrong with being conservative when it comes to cabling. Things rarely burn down because too heavy a cable was used, but at the same time you need to keep your eye on the ball and work with what you have.