12V vs 24v

seymoursteve

I have been using a 24v system with a 24v-to-240 power inverter, mainly to power my well pump. We have a lot of power outages where I live, and want to make sure that I always have water available.  My question is: am I better off using the 24v system or would it be better to switch to a 12v system using a step-up transformer (110v to 220) to run the well pump?  Any thoughts would be appreciated.

The pump is a 1.5 HP, 350 feet deep.

Estragon

As a rule, 24v is better than 12v for an application like a fair sized pump. Going to 12v would more than double the current going from the bank to the inverter. Higher current increases resistance, voltage sags requiring more current, and so on.

seymoursteve

Thanks for the response. That is what I thought, but it's nice to get confirmation!

mcgivor

Estragon said:

As a rule, 24v is better than 12v for an application like a fair sized pump. Going to 12v would more than double the current going from the bank to the inverter. Higher current increases resistance, voltage sags requiring more current, and so on.

Just a clarification for educational purposes only, higher currents do not increase resistance, resistance is a constant, using 24V vs 12V with the same conductors would result in half the losses for given conductors, but in essence the statement holds true.

Estragon

Yes, increases wire losses is what I meant, although, and just for educational purposes, my understanding is that resistance is not constant.

As wire losses increase, the loss shows up as heating in the wire, and resistance is affected by temperature. Low temperature superconductors illustrate this notion.

mvas

Just for clarification ...
Amperage can and does increase the resistance.
Resistance is not constant.

Example:
Incandescent light bulb ...
Initially cold, low resistance and high amps in-rush
Quickly hot, high resistance and lower amperage

Same with toaster element & toaster oven infrared heater.

If the size of the battery wire is too small for the amps then the wire will heat up and the resistance will increase.
If there is poor / bad crimp then that connection can heat up and the resistance will increase.
If there is too much heat for the wire or at the connection then we get thermal runaway (more heat = more resistance  = more heat)
We could eventually see smoke, or even fire.

Typically, higher voltages (12v > 24v > 48v) and lower amps = less losses

Horsefly

The loss due to wire resistance is compounding as well.  Say you need 240W at 24V, which translates to a current of 10A. To get the same watts out of a 12V bank you need twice as much current, so 20A. Say the resistance in the wire is R, then the voltage loss in the 24V battery is 10 x R, but for the 12V battery it is 20 x R. Here's the compounding part:  This voltage drop represents a percentage drop from the original battery voltage. The fractional drop for the 12V battery is (20 x R) / 12, whereas for the 24V battery the loss is (10 x R) / 24.

That is to say: For a given watts pulled across the same wire, the loss for a 12V battery is four times as much as it is for a 24V battery. 

Lumisol

mvas said:

Just for clarification ...
Amperage can and does increase the resistance.
Resistance is not constant.

Example:
Incandescent light bulb ...
Initially cold, low resistance and high amps in-rush
Quickly hot, high resistance and lower amperage

Same with toaster element & toaster oven infrared heater.

If the size of the battery wire is too small for the amps then the wire will heat up and the resistance will increase.
If there is poor / bad crimp then that connection can heat up and the resistance will increase.
If there is too much heat for the wire or at the connection then we get thermal runaway (more heat = more resistance  = more heat)
We could eventually see smoke, or even fire.

Typically, higher voltages (12v > 24v > 48v) and lower amps = less losses

Do they even still make incandescent bulbs? lol Who still buys them?

12<24<48 not the other way around.

BB.

I think the ">" was intended to say if 12 volts, go to 24 volts. If 24 volts, go to 48 volts.

-Bill

littleharbor2

Could also mean 12 volt = higher amperage, etc.

mcgivor

The following paragraph  is from a physics book

Materials that obey Ohm's Law are called 'linear' or 'ohmic'; those that don't are called 'non-ohmic' or 'non-linear'. Ohm's Law isn't by any means a universal law; it doesn't apply to all conductors! Ohm's Law simply states that 'the current flowing through a wire, at constant temperature, is directly proportional to the potential difference across the ends of that wire'. This doesn't apply to, for example, a tungsten filament lamp, whose ratio between voltage and current changes as the voltage increases (due to its resistance changing as its temperature increases). The so-called 'Ohm's Law equation' (R = V/I) is, in fact, derived from the definition of the ohm, and not from Ohm's Law. For this reason, the equation applies even when Ohm's Law does not. If the ratio of voltage to current remains constant over a wide range of voltages, then Ohm's Law applies for that range of voltages. If the ratio of voltage to current changes over a range of volages, then Ohm's Law does not apply. 

For the purpose of calculation one would use the resistance value as a constant in an Ohm's Law equation at a given ambient temperature, dynamically there may be changes in the conductors resistance if grossly undersized which is why an over current device is of such importance, to protect the conductors.

So I stand corrected, the resistance can change due to temperature rise, but a properly designed system would not encounter temperature rises in the conductors  that would have a significant impact on the resistance, unless a fault occurs.

mvas

Like most metals, copper wire has a Positive Temperature Coefficient (PTC)
For every 10° C increase in temperature, copper's resistance increases by 4%.

That is significant enough for NEC to note ...
wires inside of conduit may need to be de-rated to a lower Ampacity Rating vs the exact same wire in free-air.

50,000 homes will have an electrical fire this year.
The more we understand, the less likely it occur in our home.

Lumisol

Were the fires from added wiring the owners did or from sub par wiring the builders did?
It makes a huge difference.
Few people go through all their home wiring and replace it with higher rated components.

westbranch

No mention of 'year of build'...  that would be a good first point of analysis.

Lumisol

There were a lot of aluminum wired houses built that were fire hazards up to the middle 70's.