Help me understand 12v wire gauge

[Scott & Heather]

So I'm trying to wire up my new 12 V pump. The pump is located at my water Bay which is the rear most bay. and it's probably a total of about a 30 foot run up to the front where my batteries are and my 12 V system is. I'm not sure what gauge of wire I need for this run. The pump is a max of 17 apps at 12 V. It's the 5.5 GPM pump. Can anyone help me figure out what wire gauge I need for that run? The charts online are confusing me and showing me that I need 6awg wire. That's huge! What am I missing?

[Geoff]

10 ga is good for a 30-40' run, plus you can use for more things.

[richard5933]

It really depends on what's an acceptable voltage drop for the application. Here's a chart which shows the distance you can run different gauge wires for both 10% (non-critical) and 3% (critical) drops:

https://www.bluesea.com/resources/1437

Since your pump draws 17 amps, I'm using the 20-amp column. It shows that 6ga would be necessary if you had a circuit where minimal voltage drop was required, and up to 12ga otherwise.

Sounds like you're trying to connect this thing directly to your batteries or that your 12v load center is near the batteries. Another option would be to run a 6ga wire to the other end of the bus closer to your wet bay and install a remote 12v load center. That would give you a convenient place to connect the pump and any future expansion you might have need for.

[Bill B /bus]

Max pump draw is 17A. That is maximum. Unless you have large diameter piping you will never pump at the 5,5 GPM. Most likely less than 1-1.5 GPM. I would expect a flow rate of somewhere around 0.5 to 0.75 GPM. Therefore #10 AWG (American Wire Gauge)  is sufficient. You might want to put a local switch near the pump for when you have to perform some maintenance on pump or piping.

Bill

[Jim Blackwood]

12ga will generally handle that amperage ok but the long run drops the voltage and will make the pump run slower than it otherwise would. If that's a concern, use a relay and bring the power in from a closer source.

Jim

[bevans6]

To take it back to basics, electricity 101, wire is rated in ampacity.  That is the amount of current in amps it can carry, in normal circumstances.  That ampacity rating is good for typical household runs, 50 to 100 feet, and normal applicances.  Ampacity is measured in amps, and the voltage can be AC or DC, and most common wire is rated up to 300 volts (which is an insulation rating, not a wire rating).  10 gauge wire is rated to 30 amps, 12 gauge is rated to 20 amps, 14 gauge is rated to 15 amps.  In a normal world, you would use a similarly rated breaker or fuse to protect the wire.  Breakers and fuses are actually designed to support 85% of the rated ampacity on a continuous basis, in a certain temperature range, and running higher amperage continuously (over three hours) or in a hotter environment is supposed to blow the device.  There is nothing bad about going larger in wire gauge than required (unless the wire is too big to fit in the terminals, which can happen) and there is nothing good about under-sizing the wire for the requirement.  It's kind of rare for a wire run on a bus to be over 20- 30 feet, mostly the device is located somewhat near the source, but it's always good to keep voltage drop in mind.

[Scott & Heather]

All great info. Thanks for the schooling on pump GPM capacity and for the schooling on electrical wiring. I do have a general concept of wire and resistance, load, and length of run, but all the charts online directed me to use 6awg wire for a 20-30 ft run at 17amps 12v. Turns out that isn't necessary and I think I'll use 10 instead. Right now I'm just merely trying to get the pump up and running before a trip so we have water. But I'm going to be purchasing a 12v panel this summer to do this right.  Coaches always been 110 V only. But now that I've had a camper trailer for a while, I'm seeing the advantages of a 12v system. So I am moving as much as I can over to 12 V. Lighting, water pump, heat (furnace), roof vents, and water heater (tankless).

[Bill B /bus]

Tankless water heater on 12V!  Now you're talking 0000 AWG wire in copper.  Tankless heaters are flow sensitive. Low flow will not satisfy the safety for burner/coil operation.
I agree on 12V for the systems. Use LED lighting everywhere, including the coach lighting.
Wintering here in the RGV our monthly electric usage is running at about 400 KWH with a household refrigerator, and electric hot water heater. i attribute that usage to LED lighting low current flow.


Bill

[Scott & Heather]

Got the pump installed. Used 10g wire. Took an empty gallon jug and a stopwatch and used a 1/2" pex line as my source feed and timed how long it took to fill the jug. 15 seconds and some change. That amounts to around 4gpm. Flow is like house flow even with another sink running while taking a shower. Very very impressed.

[Jim Blackwood]

Two things you should NOT use 12v for are the water heater and the furnace. Oh, also the A/C. You cannot possibly carry enough capacity to last you more than a couple days of heavy use without a hookup or generator. For the heat and water propane is a very practical choice, diesel for heat if you have a Webasto. Those instant water heaters just absolutely suck the amperage. Yes they are compact, but for an RV they're a terrible idea. They can easily exceed your shore line's capacity, even with two cables, and they aren't good at all with saving water. Just my opinion though.

Jim

[richard5933]

I think we're confusing 120v appliances and 12v appliances here...

We have both a number of 120v electric heaters - they are run on shore power or generator only. We also have a furnace which uses LP for heating. It runs off the 12v house battery system and can go for days without a problem - the only draw on the batteries is the 12v blower fan and the thermostat circuit.

Our water heater is 120v electric only, but eventually we'll be replacing it with an LP water heater. That will require 12v for the control circuit and will also be able to run on batteries for a substantial amount of time. Continuous (tankless) or conventional water heaters which run on LP only require minimal 12v draw.

I agree that 120v furnaces and water heaters are not a good idea when on batteries, but it was my assumption that when someone mentions running these on 12v they are talking about LP appliances. Or, that they have a massive battery bank and some really decent solar on the roof. The guy in Arizona that just showed his 4106 solar install on YouTube has enough capacity to run the heat on his mini-split overnight off the batteries, and then enough solar to recharge the next day.

[Scott & Heather]

Yes Richard, my thoughts too.

Let me clear up the confusion at least on my end:

1. A propane tankless water heater will work fine for us. I have a small one in our 23foot camper trailer and it's absolutely lovely. Doesn't use an insane amount of propane by any stretch and you get endless hot water. It self ignites using two D cell batteries. those same batteries are still inside it and we fulltimed for three solid months in that thing (my wife and I and our two girls). The batteries haven't died yet. They make even nicer propane tankless units and I have my eye on a few. The key is to filter the water before the unit to keep sediment buildup down. Anyway, my current 40 gallon electric water heater has two 2000 watt elements and as nice as it is, it's not conducive to life without the pole.

As for 12v furnace, the gas furnaces or even webasto style units all work perfectly fine on 12v. Our current heat is three electric space heaters. Also not useful when not connected to the pole.

When it comes to 12v stuff, having a good battery bank and using the appliances wisely can give you some serious boondock ability and that's what we need more of.

Our Samsung fridge will run off of inverter when the bus is driving down the road. Remember, our 50DN alternators on our buses can put out almost 6,000 watts of juice and since my OTR climate control is removed, I have access to most of that through an inverter. So running an inverter high efficiency fridge and some other minor stuff isn't a big deal. I'd like to not have to run my generator when driving down the road.

[TomC]

You all are getting way to scientific. A water pump doesn't run continuously. Listen to Geoff-17amps will be covered by 10ga wire. Good Luck, TomC

[Jim Blackwood]

10 ga will be fine. 12 ga would be fine too, the only concern being voltage drop. 14 ga might even work but I'd not do that myself without some testing. However it might turn out to be OK.

Jim

[eagle19952]

Got the pump installed.  Very very impressed.

So. Name the pump please.