Dumb question

[Bluedog225]

With a very basic understanding of DC, I understand how to size a wire to account for voltage drop. And I get fuse stuff.

My question is what happens in a long DC circuit when the voltage drop becomes extreme?

Watts=volts * amps

So as volts drops to 0, amps has to rise for watts to stay constant. Right? And will this cause the wire to heat?

Or asked another way, at a fixed voltage, if I always size my wire for a maximum voltage drop of 10%, will that always serve to protect the wire from overheating?

Thanks

 

[sunshine_eggo]

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

 

[Bvillebob]

Look at it this way. Your wire is really just a low value resistor. You can look up the resistance of wire by size, then multiply by length.

If your wire has one ohm of resistance, to make it simple, and you're pulling 50 amps of power through it, Mr. Ohm's law says that you're dissipating 50 watts of heat in that wire. Consider a 60 watt light bulb, that's roughly how much heat you'll be generating. If that 50 watts of heat is spread out over 100 feet of wire, that's only 1/2 watt per foot, not enough to get worried about. If it's 6 inches of wire that's another story. If you have 5 ohms of reistance and you're pulling 100 amps, that's 500 watts of heat to dissipate, and something's going to get hot. Midnite Solar has some videos up showing what happens when you short a lithium battery through 2/0 welding cable, you can watch the insulation melt in seconds.

There are very good ampacity charts out there, carefull calculated by professionals, to tell you what size wire to run for the amount of current. you intend to run, there's no need to reinvent the wheel.

 

[RCinFLA]

10% voltage drop for a random cable length does not determine cable heating.

Wire ampacity is based on watts of heating per unit length (per foot for NEC) which determines heating per foot by I^2 * R per foot.

Wire gauge (circumference area to radiate heat), insulation, and enclosure (conduit) impacts ability of wire to dissipate heating,. For NEC, ampacity it is roughly based on about 2 watts per foot to 6 watts per foot of heating.

There is very dramatic impact for a DC input to AC output inverter. Inverters regulate their AC output voltage. If their input DC voltage drops they will draw more current to supply the same AC output load wattage due too much DC cabling loss.

Most inverters' efficiency drops as they draw more DC current so there is additional inverter loss due too much DC cabling loss.

Battery terminal voltage slumps more with the increased current demand resulting in another degrading impact of too much DC cable loss.

 

[Dzl]

I think your pondering are on the right track but I think you are missing a piece of wire sizing. Voltage drop is one big factor but safety is related to ampacity (safe current carrying capacity of the wire), which exists independent of length and voltage. Further, ampacity is effected by many things like the insulation temperate rating of the wire, the ambient temperature, and whether the wire is bundled and in conduit or in open air.

When sizing wire you need to consider both factors, or find a calculator that considers both (many calculators only calculate voltage drop/wire size).

The blue sea circuit wizard will factor both voltage drop and ampacity, as will the bay marine wire size calculator (but the data out is only as good as the data in so you have to understand the variables and assumptions of the calculators).

Or asked another way, at a fixed voltage, if I always size my wire for a maximum voltage drop of 10%, will that always serve to protect the wire from overheating?

No, as an extreme example: A 14AWG, 2ft round trip, 48v, circuit with a 900A load, would be below the 10% voltage drop threshold. Clearly 900A through 14AWG would be a bad idea. This is an extreme example obviously but shows with very small distances ampacity is going to be the limiting factor long before voltage drop is, since its independent of distance. Also note, the calculator used for this example (which is popular with many on this site) gives no indication that this would be unsafe, because its just a voltage drop calculator, doesnt consider safety/ampacity.

Generally speaking, the shorter the circuit the more likely that ampacity will be the limiting factor before voltage drop.

 

[Bluedog225]

Great info. Thanks. This is exactly what I was trying to get at.

I was looking at a blue sea chart when the question came occurred to me.

Knowing more, I’d ask my question differently.

If I follow the Blue Sea (or other) guidelines, it looks like ampacity (and therefore operating temp) already factored in to the wire size recommendations. Right?

And while we’re on the topic, does 110F-115F operating temp (Texas summer) need to be factored in?

Thanks again.

 

[rmaddy]

That Blue Sea Systems chart takes into account both ampacity and voltage drop.

For your 110-115ºF operating temps, it might make sense to use the "inside engine room" column in the fuse section.

Blue Sea Systems has a phone app version of that chart that I recommend. You can enter more specific details and get more specific results. The results actually show separate ampacity and voltage drop results along with the recommended choice.

 

[John Frum]

So as volts drops to 0, amps has to rise for watts to stay constant. Right? And will this cause the wire to heat?

Without current flow there is no heat.
Without voltage there is no current flow.

 

[Dzl]

Great info. Thanks. This is exactly what I was trying to get at.

I was looking at a blue sea chart when the question came occurred to me.

Knowing more, I’d ask my question differently.

If I follow the Blue Sea (or other) guidelines, it looks like ampacity (and therefore operating temp) already factored in to the wire size recommendations. Right?

Yeah, I'm pretty sure that the Blue Sea chart considers both, I know that the Blue Sea calculator considers both so I would think the chart does too. The charts will be specific to a particular voltage though.

And while we’re on the topic, does 110F-115F operating temp (Texas summer) need to be factored in?

Yes, I believe the baseline assumption for both the ABYC (marine) and the NEC, is 30 Celsius / 86 Fahrenheit. I believe I've got a screenshot of a temperature correction table for higher temperatures saved somewhere let me see if I can find it. IIRC its from the 2017 NEC code.

Here ya go:

Or, here is a table based on the assumption of 40*C / 104*F (a little low for you maybe)

 

[Dzl]

That Blue Sea Systems chart takes into account both ampacity and voltage drop.

For your 110-115ºF operating temps, it might make sense to use the "inside engine room" column in the fuse section.

The ABYC "inside engine room" would be perfect column to use, I think. IIRC that assumes 50*C which is 122*F

 

[Bluedog225]

Very cool. Thanks!