how to size ground cable from house battery to frame

Hi. My sprinter based 23 foot motorhome as an existing 2/0 negative cable from the house battery terminating into a lug bolted to the frame 40" away. The original OEM batteries would have been 200Ah, whereas mine are now 260Ah on the same 2/0 ground cable. The existing power feed from the chassis battery is a 1 gauge red cable providing charging from the alternator to the house batteries, while the vehicle is operating. Recently, I've added a 45 amp of solar controller, about 55 potential amps of misc devices (eg, fans, outlets, an espar diesel heater, etc.). When I say potential amps, that's max the devices can draw. Also, I added a sterling battery to battery charger that's a max 60 amps into the house bank. While I assume the B2B charger is a wash to some extent with the existing alternator charging, the alternator probably only contributes 30 amps. 

I want to be safe and am not concerned with cost. Should I upsize the house battery ground to include these additions? 

Thanks, 

John  

Comments

  • TecnodaveTecnodave Registered Users Posts: 412 ✭✭✭✭
    Outwestbound,
    your existing cable is large enough, not to worry about it. If you were running a 2000 watt inverter a full output it is a bit small but what you have described it is conservately large.
    david
    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
  • outwestboundoutwestbound Registered Users Posts: 34 ✭✭
    Tecnodave said:
    Outwestbound,
    your existing cable is large enough, not to worry about it. If you were running a 2000 watt inverter a full output it is a bit small but what you have described it is conservately large.
    david
    Thanks. I forgot, I DID add a Xantrex XC2000 watt inverter ALSO. My bad. I checked the list, I also forgot to add the DC 6 foot refrigerator. Think I should bump it up to the next size? Also, since I reoriented the bank, I could shorten the ground from 40" to only about 28" pretty easily. 
  • BB.BB. Super Moderators, Administrators Posts: 29,496 admin
    Do you have a battery isolator (switch) to disconnect the house batteries from the van battery(ies)? I.e., will the starter/other vehicle loads draw current from the house batteries?

    Here is a simplified chart from the NEC regarding wire AWG and current:

    https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

    For off grid solar power systems, I suggest that you should use the NEC derating of wire and fuses for maximum continuous current flow. of 1.25x upsizing of cable. For example, if you have a 150 Amp alternator/charging source, then:
    • 150 Amps * 1.25 NEC derating = 187.5 Amp minimum rated branch circuit rated wiring and fuses/circuit breaker.
    Note: In reality, most automotive type Alternators, even if they are rated at 150 Amps don't really supply that amount of current when charging your batteries... As the alternators heat up, their output current falls quite a bit--Plus the extra wiring length (increased voltage drop) also reduces average maximum current flow.

    Regarding voltage drop, for wiring between your charge controller and battery bank, a 12 volt bank should be around 0.05 to 0.10 maximum drop ideally. And for your inverter and DC circuit wiring, around 0.5 volt maximum. You can use a simple voltage drop calculator to estimate the voltage drop (and play with the AWG numbers to find the right size/length of wiring):

    https://www.calculator.net/voltage-drop-calculator.html

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • outwestboundoutwestbound Registered Users Posts: 34 ✭✭
    BB. said:
    Do you have a battery isolator (switch) to disconnect the house batteries from the van battery(ies)? I.e., will the starter/other vehicle loads draw current from the house batteries?

    -Bill
    Thanks Bill. Yes, my house battery bank is isolated/disconnected from the van's chassis battery. I have a solenoid that closes (puts both batts in parallel), only when the engine is running. When the engine IS running, the alternator provides about 40 amps max to the house batteries at max, whereas 20-25amps is typical. My concern was that I've added so much to the motorhome, that the existing negative off the house/habitation area battery bank to the frame may be undersized. I don't know anything about sizing the ground wires, so I posted. 
  • BB.BB. Super Moderators, Administrators Posts: 29,496 admin
    You are in an "awkward zone". Vehicles typically use the frame/chassis metal as the "return path" (negative) for battery bus power. And solar power systems and our "standards" use wiring for +/- battery bus and only use the greenwire/chassis ground for "safety" (someplace for short circuit current to flow and trip a fuse/breaker if a "hot wire" gets shorted to metal).

    Your starter and alternator use the chassis for current flow--And the starter may hit 100-200 amps (depending on your engine type/size/etc.).

    For "safety ground" chassis grounding... A 6 AWG cable between negative house battery bus and chassis ground is usually "good enough" for up to a 200 Amp battery bus (and fuse/breaker).

    So, now you have to look at the 12 volt loads... The AC inverter, solar and house battery charger are "easy"--They have Hot and Return wires directly from the devices back to the battery bus... No normal current flow in chassis.

    The diesel heater and various DC vehicle devices may (for example) use the chassis ground for their return current path. Using the NEC link and derating, you would be looking at a 55 amp design:
    • 55 amps * 1.25 NEC derating = 68.75 Amp minimium rated fuse wiring (round up to next "standard" size fuse/breaker) Say 75 Amps
    • NEC Table: 75 Amp rated branch circuit = 6 AWG to 2 AWG (depending on wiring insulation rating)
    And then looking at voltage drop... Say 55 Amps with 10 foot one way wiring run and 0.5 volt maximum drop (note, most of the time, chassis ground resistance is less than the return copper cable designed for the branch circuit):

    https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=1.296&voltage=12&phase=dc&noofconductor=1&distance=10&distanceunit=feet&amperes=55&x=69&y=23

    6 AWG:
    Voltage drop: 0.43
    Voltage drop percentage: 3.62%
    Voltage at the end: 11.57

    And a similar set of calculations for other loads and your charging sources connections (Voltage Drop 0.5 volts for loads, voltage drop of 0.05 to 0.10 volts for charger to battery bus drop). Note that the Alternator voltage drop to house battery may never be that low--Vehicle charging systems are just not designed for optimal deep cycle power system charging.

    Other issues with using chassis ground for the return leg... The engine, frame, and chassis tend to be separated by rubber mounts. And these mounts should have a heavy cable (or braided cable) that "jumps" the mount to electrically connect the motor/frame/chassis to battery ground.

    For a vehicle, they have relatively small loads (lights, radio, etc.). And if you add a bunch of larger DC loads, then it is possible that you will exceed the current capacity of the chassis ground bonding cables. Probably not an issue, but it is something to understand.

    Also, if you do something "strange", you may have to add bonding wires. For example, you put your diesel heater on the back door (yea, you probably would never do it, just an example), you would need to add a ground bonding strap from the door back to the door frame/chassis. Otherwise, the return current flowing through the door hinges/bearing will spot weld/ruin the bearings.

    Again, you probably will not have "rebuild" your chassis ground system. But there are issues and, in some cases, extensive ground bonding is required (for example, HAM radio folks find that they may need to add ground bond wires "everywhere" like doors, gates, etc. to reduce RF electrical interference from poorly ground sheet metal/electronics.

    Is this even helping, or just confusing you more? Grounding is a complex subject and easy to make mistakes.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • outwestboundoutwestbound Registered Users Posts: 34 ✭✭
    BB. said:
    Is this even helping, or just confusing you more? Grounding is a complex subject and easy to make mistakes.

    -Bill
    Yes, this is helpful. Thanks for taking the time Bill. 

    Safety is essential and I'm rebuilding this little motorhome to go to South America for a year (from the US). I went through the vehicle and checked the Winnebago (HOUSE side) installed grounds to frame e.g., load center, slide and stair motors, generator, etc. Some had to be sanded down and cleaned up. On the CHASSIS side, I inspected the engine's braided strap and the starter battery grounds - they are good. So the "basis" is good, then I turned my attention to the modifications to the vehicle I've made. I've been at this for a year part-time, so in some cases, I'm going back and making decisions that are a re-work of prior work, but that's ok, I'm learning better/safer ways of doing things. Because I'm essentially "boondocking" for a year, because shore power is almost non existent (and of very poor quality when it is available), the power systems are of great importance. 

    I want to wire it as best I can and avoid being lit up standing on the ground in the rain touching the vehicle (hot skin I think it's called.)

    Some "RV" installers ignore it, but because I'm ignorant, I have installed the green ground wires to the frame on equipment I've installed, like the inverter, per the manufacturers recs. 

    I follow what your saying on the chassis grounds, then you turn to 12 volt loads. I checked on my installs. The controllers and inverter, but also the DC refrigerator, diesel heater, and some fans all were returned to the house battery bank, rather than grounded into the frame. I'll attach all those grounds to a common negative post, then from that post into the load side of a shunt for my Trimetric 2030. The other post on the shunt goes to the negative battery terminal (12V, 260Ah AGM bank). The shunts load side goes to the chassis ground. I believe this will capture all the grounds so that the Trimetric meter is accurate. 

    In your example, you added 55 amps, which you scaled up to 75, for a 6-2AWG ground, depending on insulation. 

    In your email example, you estimated 6 AWG for general chassis purposes, the added 6 AWG more for your 75 amps of DC additions. 2, 6AWG cables is about is 27mm2, so that's 2AWG 32mm2 cable minimum. Am I following you correctly? 

    If my existing chassis ground is a 1/0, which it is, this should be fine. (?) Is this logical? Your 55 amp example is actually much lower because I returned these to the house battery. However, the vehicle's DC load center is grounded to the frame, so maybe 55 amps is close anyway. 

    There is another issue, which is that I'm contemplated adding a Sterling B2B charger model BB1260 - a 60 amp charger. Sterling is adamant about not using the chassis as a ground. Rather, they require a cable sized the same as the red, to go between the house and chassis batteries. Since I plan to follow their instructions, it's looking like a 1AWG minimum, probably 1/0 AWG cable. 

    Wouldn't a ground between the batteries be "safer" and improve the overall electrical system, in addition to serving just the Sterling BB1260? 

    Thanks for your time. 





     
  • BB.BB. Super Moderators, Administrators Posts: 29,496 admin
    edited May 22 #8
    I (and everyone here) are very happy to help.

    Regarding:
    I follow what your saying on the chassis grounds, then you turn to 12 volt loads. I checked on my installs. The controllers and inverter, but also the DC refrigerator, diesel heater, and some fans all were returned to the house battery bank, rather than grounded into the frame. I'll attach all those grounds to a common negative post, then from that post into the load side of a shunt for my Trimetric 2030. The other post on the shunt goes to the negative battery terminal (12V, 260Ah AGM bank). The shunts load side goes to the chassis ground. I believe this will capture all the grounds so that the Trimetric meter is accurate. 
    I just want to be clear here... The battery side of the shunt goes to all of the batteries (if parallel) negative posts/bus.

    The load side of the bus, is where all of your load returns, and the chassis ground cable, are connected.

    If you put any return wires on the battery side of the shunt, those currents will not be "seen" by the shunt and the Trimetric will not log those currents (and time).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • outwestboundoutwestbound Registered Users Posts: 34 ✭✭
    BB. said:
    I (and everyone here) are very happy to help.

    Regarding:
    I follow what your saying on the chassis grounds, then you turn to 12 volt loads. I checked on my installs. The controllers and inverter, but also the DC refrigerator, diesel heater, and some fans all were returned to the house battery bank, rather than grounded into the frame. I'll attach all those grounds to a common negative post, then from that post into the load side of a shunt for my Trimetric 2030. The other post on the shunt goes to the negative battery terminal (12V, 260Ah AGM bank). The shunts load side goes to the chassis ground. I believe this will capture all the grounds so that the Trimetric meter is accurate. 
    I just want to be clear here... The battery side of the shunt goes to all of the batteries (if parallel) negative posts/bus.

    The load side of the bus, is where all of your load returns, and the chassis ground cable, are connected.

    If you put any return wires on the battery side of the shunt, those currents will not be "seen" by the shunt and the Trimetric will not log those currents (and time).

    -Bill
    understood. I have 2, 6V batts in series, so I'm going to the single negative "bank" post. 
  • outwestboundoutwestbound Registered Users Posts: 34 ✭✭
    Bill, do you think I'm in the ballpark in understanding your first post? 
  • BB.BB. Super Moderators, Administrators Posts: 29,496 admin
    Yes, I think you have it all down correctly. 

    There were a few different "posts" in the text, and reading on my phone, I just wanted to make sure everything was clear.

    Miss wiring the shunt is a sort of common debugging issue here.

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 29,496 admin
    Throw in grounding questions, and it can be confusing.

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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