House system: 12 or 24 volts points to consider

[BG6]

Having just finished helping someone set up their house system (actually a Mobile Command Post), here are a few things to help you pick which voltage to use, if you have a 24V coach.

NOTE:  This is a SEPARATE system just for the house, NOT the coach power system.

24V Pro:
Coach generator can directly charge house system
House batteries can be used to jump-start coach in emergency (note -- this can KILL deep cycle batteries)
Battery equalizer can run 12VDC accessories from 24V battery bank
Twice the current capacity on any wire
Computer-type Uninterruptible Power Supplies often run on 24V, and can be found cheap with dead gel cells

12V Pro:
Most accessories run on 12V
Most gensets start on 12V and will recharge start batteries (moot point if you have separate genset btty)
24V chargers, lights, etc may be harder to find and cost more than 12V versions

Other considerations:
Solar panels can be wired in series (say, 4A 24V) or parallel (8A 12V)
Battery equalizer or 24V / 12V adapter can charge 12V bank from 24V coach system
Most common equalizers are 50 or 60 amps max 12V load

In the case I've just been dealing with, we opted for a 12V house system, with a 24V-to-12V charger from the coach system.  There are 4 deep-cycle batteries, giving 12VDC at 600AH.  We grounded the house system to the same lug that the coach system uses.

Using marine terminals, we left the option to easily convert to 24V at 300AH to back up the coach batteries, or if the day comes that changing to 24V makes more sense.

[Sean]

... Twice the current capacity on any wire ...

Actually, this is not correct.  The "current capacity" of a wire, called "ampacity" in the trade, is independent of the voltage.

I suspect what you actually meant to say was that any given load will require only half the current at 24 volts than it would at 12.  For example, a 20-watt, 24-volt lamp will require only about 0.83 amps to operate, whereas a 20-watt, 12-volt lamp would require twice that or 1.67 amps.  Alternatively, you could say that a wire capable of running a single lamp on 12 volts could run two such lamps at 24 volts.

Incidentally, I wrote a five-page article on this very subject (12 vs. 24 volt house systems) which appeared in the April, 2010 issue of Bus Conversions Magazine.  At the end of that article is a list of resources for obtaining 24-volt parts and equipment.

-Sean
http://OurOdyssey.BlogSpot.com

[Jerry32]

I would say that the 12 V stuff doesn't use much power but that things like the inverter does and you will be able to have twice as much output from a 24 V system .

[bevans6]

I don't know why this is so difficult.  It's a zero sum game.  If you have 6 12 volt batteries, 100 AH each, you can have 600 AH at 12 volts, 300 AH at 24 volts, I suppose 200 AH at 36 volts, or 100 AH at 72 volts.  Each is exactly the same amount of power - 7200 "watt-hours".  Since power - ie. "watts" in this case, is what does work, each battery configuration has exactly the same potential to do work.

The only thing that changes in this case is the current, which varies inversely with voltage as power remains constant.  You need 600 amps at 12 volts and only 100 amps at 72 volts to do the same amount of work.  And hence the sole benefit of running a higher voltage -  since wire is limited in capacity by amperage, you can use a smaller wire to carry a given amount of power when the voltage is higher.  Which is, in my opinion, a totally valid and compelling reason to run as high a voltage as practical and safe for any application that consumes a lot of power.  An awful lot of commercial plant runs at 48 volts DC for that reason.

Brian

[BG6]

... Twice the current capacity on any wire ...

Actually, this is not correct.  The "current capacity" of a wire, called "ampacity" in the trade, is independent of the voltage.

I suspect what you actually meant to say was that any given load will require only half the current at 24 volts than it would at 12. 

You are, of course, correct.

I also didn't mention anything about stranded v. solid (stranded has less voltage loss).

[Bill B /bus]

I don't the question to be a difficult one to answer. Actually very simple.
You can either
1) rewire the coach for 12V
or
2) run the house on 12V through the equalizer from 24V(typically) to 12V.
Answer 1 means rewire the entire coach to 12V. For instance MCI uses 16ga wire for the clearance lights and 14ga for the headlights. Fine for 24V but marginal for 12V. Now change out the starter, alternator, skinner valves and relays to 12V.
Answer 2 means find a second equalizer for the house side, they are sized to at least 75Amps, use a 24V input inverter/charger, ie Trace SW4024 and wire as per standard for the house 12V.

And yes, I am predisposed to this dual system as both the GM and the MCI I converted were 24V buses.
I am running solar at a nominal 48V to reduce wire size needed.
Bill

[BG6]

And hence the sole benefit of running a higher voltage -  since wire is limited in capacity by amperage, you can use a smaller wire to carry a given amount of power when the voltage is higher. 

At the design board, you are correct.

However, the most important thing is that whatever the voltage, you need to do something WITH it.  Most DC stuff is on 12 volts, so running a higher voltage is only useful during the distribution stage -- and you will have to convert it to a voltage that you can use, negating the advantage on short-run stuff like a coach conversion.

Early DC appliances were on 36 volts, because the DC wind generators of the 1920s were the primary source of rural power until REA came along.  6VDC was standard for vehicles.  12VDC didn't become popular until the 1950s, when most cars were shipped with electric wipers, heaters with fans, more lights and radios.  

Today, just about everything running on DC is 12 volt negative ground -- our coaches, military vehicles and some aircraft are about all you'll find on 24 volts.  

[BG6]

I don't the question to be a difficult one to answer. Actually very simple.
You can either
1) rewire the coach for 12V
or
2) run the house on 12V through the equalizer from 24V(typically) to 12V.
Answer 1 means rewire the entire coach to 12V. For instance MCI uses 16ga wire for the clearance lights and 14ga for the headlights. Fine for 24V but marginal for 12V. Now change out the starter, alternator, skinner valves and relays to 12V.
Answer 2 means find a second equalizer for the house side, they are sized to at least 75Amps, use a 24V input inverter/charger, ie Trace SW4024 and wire as per standard for the house 12V.

And yes, I am predisposed to this dual system as both the GM and the MCI I converted were 24V buses.
I am running solar at a nominal 48V to reduce wire size needed.
Bill

Think about this.  To run 110VAC household stuff takes a LOT of inverter.  The ones that run on 24 volts are significantly more expensive and more difficult to find than the ones that run on 12 volts, and you can't replace a dead one with a quick trip to the local store.  Of course, you could buy a big enough equalizer to get that much 12VDC, but why bother, when you can simply run a 12 volt system in the first place?

Most stuff that isn't 110 volts is 12 volts.  Again, you need the equalizer to get this if you are running a higher voltage battery bank.

In a few years, 36 VDC or 48VDC accessories may become common (hybrid cars run higher voltages), and changing around the battery wiring will let us put whatever voltage we want on the DC power bus.  For the time being, the only reason that 24VDC might be of value is using military commo gear, and what little of that we expect to use can be run from converters which take 12VDC or 110VAC.

Thus, by going to 12 for the house stuff, we have a simple system with replacement components which are readily available.

[bevans6]

I am really used to 48 volt dc for power plants (Telephone type stuff), but I never really  stopped to consider the engineering level pro's and con's.  Given that this discussion has centered on a con for 12 volt DC - what are the con's to higher voltage DC?  Is there a safety angle I am overlooking?  cost of stuff running on the system goes up - not just 12 VDC stuff is cheap and common, does the inherent engineering level cost go up?  What are the down-sides to higher voltage DC in a vehicle?

My personal long term plan is to evolve to a 24 volt system tied into the bus system in some cunning way, for the primary use of running an inverter to power OTR roof airs.  I don't use much 120 volt AC stuff other than that.  I already have two  equalizer/power converters on the bus to create 12 vdc (one the stock MCI installed on that I use for radio, electric fans, seats, stuff that I consider "bus" use while driving, and one dedicated to supplying 12 VDC for the trailer lights and brakes).  Now that I understand that an equalizer draws power from both 12 vdc battery banks pretty equally, I think there is a lot to be said for using them for specific use.  But I see a lot of waste in this plan, it's pretty luxurious to set up a 24 volt 3,000 watt inverter just to run the roof air for a few hours a month, but it seems to solve a few issues that I have, so maybe it's the right way to go.

Brian

[Sean]

... what are the con's to higher voltage DC? ...

Above 30 volts the law changes.  Most low-voltage exemptions to NEC requirements are for systems 30 volts nominal and below.

Telephone facilities, BTW, are exempt from the NEC by definition.  A whole different set of guidelines, developed by the industry, applies in the common carrier environment.

This is the reason most vehicle house systems are 12 or 24 volt.  Land-based AE systems often go to 48 volts, but the additional conduit and other requirements then imposed are less of a burden there.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

[Hard Headed Ken]

 I think there is one other thing to consider. The higher the initial DC voltage the more efficient the inverter is because it has less work to do. Compare the efficiency rating of 12 volt, 24volt, and 36 volt inverters. It's been a while since I've done this research. If you have a 24 volt charging system then a 24 volt inverter maybe a little more efficient than a 12 volt. If my previous research is still true and you use an inverter it's a few percentage points better than a zero sum gain comparing 12 to 24. That small gain maybe offset by other cost factors.

Ken

[Jerry32]

120 volt use, TV, Microwave ,Satellite, fridge, computer ,datastorm ,hot water tank, induction cooking, heat pump, air conditioner, air compressor, tools,
Seems there are a lot of uses for 120 volts and very few for 12 v Jerry

[BG6]

I am really used to 48 volt dc for power plants (Telephone type stuff), but I never really  stopped to consider the engineering level pro's and con's.  Given that this discussion has centered on a con for 12 volt DC - what are the con's to higher voltage DC? 

The only real con is finding stuff that runs on the higher voltage, and that stuff tends to be more costly.

My personal long term plan is to evolve to a 24 volt system tied into the bus system in some cunning way, for the primary use of running an inverter to power OTR roof airs.  I don't use much 120 volt AC stuff other than that.  I already have two  equalizer/power converters on the bus to create 12 vdc (one the stock MCI installed on that I use for radio, electric fans, seats, stuff that I consider "bus" use while driving, and one dedicated to supplying 12 VDC for the trailer lights and brakes).  Now that I understand that an equalizer draws power from both 12 vdc battery banks pretty equally, I think there is a lot to be said for using them for specific use.  But I see a lot of waste in this plan, it's pretty luxurious to set up a 24 volt 3,000 watt inverter just to run the roof air for a few hours a month, but it seems to solve a few issues that I have, so maybe it's the right way to go.

Invert as close to the batteries as possible -- AC is less lossy than DC.  And, if you hook up close to the batteries, you can run a 12 volt inverter more cost-efficiently than a 24 volt unit.  The major cost difference is in the cable to the inverter, which is negligible unless you have long runs, and you can save quite a bit of money buying a 12 volt inverter instead of 24.

The problem is simple availability.  There are 12 volt inverters everywhere -- every truck stop, every Costco, every Walmart -- and when you build and ship a lot of something, prices go down.

You are already running a split system (12 / 24) -- and you are already getting your 12 volts from the battery bay (where the equalizer is), so you aren't getting any of the advantages of 24 volts.  It would be simpler (and less likely to fail) if you used your equalizer to charge a bank of batteries at 12 volts, and ran your 12 volt stuff from batteries in parallel.

[BG6]

120 volt use, TV, Microwave ,Satellite, fridge, computer ,datastorm ,hot water tank, induction cooking, heat pump, air conditioner, air compressor, tools,
Seems there are a lot of uses for 120 volts and very few for 12 v Jerry

In my coach:
120VAC: Microwave, fridge*, antenna rotators, convection oven, washer/dryer*, TV, DVD player, fast charger for power tools, electric heater, some lights*, garbage disposal, hot plate*, electric griddle*, printer, chest freezer*, downdraft stove fan and the battery chargers for the 24 and 12 volt banks.  Most of this stuff runs fine from the inverter, some needs either genset or shore power.  Stuff with * I only use on short power.

24VDC:  Coach chassis systems, charger for 12VDC bank, jumpstart socket to / from my M151A2 military jeep.

12VDC:  Inverter, radio, CD, amplified speakers, chargers for phone and most other stuff, laptop, main computer (converted the power supply), monitor, lights, ham radio gear, toad lights / power, satellite, fridge (solenoid to run propane, run on 120 when hooked to shore power).