Ampacity Confusion

[Spire]

Hey guys,

So I'm trying to finalize my electrical system and determine what wire gauge is necessary for certain runs.

It's my understanding that I need to take into account voltage drop AND ampacity. I understand the voltage drop and have been using the calculators, but I'm a bit confused with ampacity.

I've seen the NEC ampacity chart posted in other threads, which shows the ampacity of a 2/0 conductor as 195A for a 90 degree C rating.




At the same time, there are ampacity tables for welding cable that give different values for max amperage. For example, this page shows 325A max for 2/0 cable: https://temcoindustrial.com/product-guides/wire-cable-and-accessories/welding-cable

The other place I've been looking is the Blue Sea Chart, which I understand is for determining the wire gauge needed to stay under a certain voltage drop, but why would it even show 2/0 cable as an option in the 200A column if the ampacity is, in fact, 195A?


Specific to my application, I have three 12V, 100ah lithium batteries wired in parallel, as well as a 2000W inverter. I was planning to use 2/0 cable to connect the batteries to each other, connect the batteries to the bus bars, and to connect the inverter to the bus bars. I had also planned a 250A fuse for the inverter to bus bar positive wire and a 300A fuse for the battery to bus bar wire. Now I'm second guessing if 2/0 is the correct choice.

I've seen other wiring diagrams posted that use 2/0 cable for similar sized battery banks and 2000W inverters: https://www.explorist.life/2000w-520w-60a-budget-friendly-camper-wiring-diagram/

What am I missing here?

 

[sunshine_eggo]

Start with NEC awg ratings.

Check voltage drop. If < 3%, done.

 

[RCinFLA]

NEC wire ampacity is based on watts per foot of heating and ability to dissipate heat based on confinement and other wires in the confinement space (like conduit) also generating heat from other wires also carrying current in close proximity.

It runs from about 1.5 watts per foot to about 6 watts per foot depending on wire gauge circular area which provides surface area to dissipate heat.

Cords like SOOW heavy insulated rubber cased cord like used for generator power cords allow less amperage for given wire gauge because the heavy insulation covering restricts their ability to dissipate heat.

The primary concern is degradation to wire insulation due to sustained long term heat exposure with cumulative heating eventually causing insulation to become degraded. There are different classes of insulation allowing more or less maximum heating. New polymers are getting much better for wire insulation. If you are old enough, you can probably remember taking a drill or vacuum cleaner apart to find the wiring insulation crumbling off the wires.

 

[John Frum]

Hey guys,

So I'm trying to finalize my electrical system and determine what wire gauge is necessary for certain runs.

It's my understanding that I need to take into account voltage drop AND ampacity. I understand the voltage drop and have been using the calculators, but I'm a bit confused with ampacity.

I've seen the NEC ampacity chart posted in other threads, which shows the ampacity of a 2/0 conductor as 195A for a 90 degree C rating.

View attachment 122447


At the same time, there are ampacity tables for welding cable that give different values for max amperage. For example, this page shows 325A max for 2/0 cable: https://temcoindustrial.com/product-guides/wire-cable-and-accessories/welding-cable

The other place I've been looking is the Blue Sea Chart, which I understand is for determining the wire gauge needed to stay under a certain voltage drop, but why would it even show 2/0 cable as an option in the 200A column if the ampacity is, in fact, 195A?


Specific to my application, I have three 12V, 100ah lithium batteries wired in parallel, as well as a 2000W inverter. I was planning to use 2/0 cable to connect the batteries to each other, connect the batteries to the bus bars, and to connect the inverter to the bus bars. I had also planned a 250A fuse for the inverter to bus bar positive wire and a 300A fuse for the battery to bus bar wire. Now I'm second guessing if 2/0 is the correct choice.

I've seen other wiring diagrams posted that use 2/0 cable for similar sized battery banks and 2000W inverters: https://www.explorist.life/2000w-520w-60a-budget-friendly-camper-wiring-diagram/

What am I missing here?

The nec table you reference is de-rated for "Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)"

This one is for "Based on ambient air temperature of 30 ̊ C (or 86 ̊ F), following are the allowable ampacities of single insulated conductors (rated 0–500 volts) in free air"

https://www.uglys.net/docs/default-source/uglys-documents/errata/UglysResidentialWiring2017_Errata_Page2.pdf

The other one you reference is probably the ABYC

https://boathowto.com/wiresize/wiresize_tables_abyc.pdf

The NEC free air table 90C copper column is fairly close to the ABYC 105C outside engine spaces column.

 

[chrisski]

The chart you used is the chart I build my wiring from. Actual amoacity is from a DECENT wiring spec sheet, not a title in an ad.

So to the formula I used for for max amps is the same I used for fusing:

(Inverter Wattage) / (Low voltage cutoff) / (Inverter Efficiency) X (1.15 for a safety factor)

2000 X/10.5 /.85 X 1.15 = 257 amps.

Going to that chart, I saw 90 C 4/0 wire is rated for 260 amps, so I decided on 4/0 wire. I opted on purchasing TECMO 4/0 welding wire, which has an ampacity a bit higher than 260, so I felt comfortable with my decision. I am a bit confused an ampacity for welding wire whether that is a long continuous run or on and off welding.

It is confusing. Many use 2/0 for the same wiring, but sometimes it is because the low voltage cutoff is not used In the calculations. A 2000 watt inverter will pull enough amps based off voltage to to get 2000 watts. Typically, I don’t pull that wattage unless my batteries can deliver 14 volts. The calculation at 14 volts is 197 amps which is a couple of watts over what 2/0 can deliver.

I fell its important to build the system for a low voltage cutoff of the inverter. I may never see my batteries get so depleted that the voltage i is that low, but I sometimes am not around to watch guests.

EDIT: I have since upgraded that system to 24 volts, kept the wiring in place, and have never noticed any heat rise on my wiring with wither 12 volts or 24 volts with an IR gun.

 

[Danke]