Double-check my math please?

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Chazz Layne
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Double-check my math please?

#1

Post by Chazz Layne » Mon Oct 05, 2009 12:10 pm

I'm working out the parts list for the setup I'm building in the back of my truck, and have been trying to figure out what guage wire I need to run the 15-20 feet to get back there from the batteries. I'm going to be running the following off this feed:

~5 amps - Kenwood D710A
~10amps - Yaesu 857D
~.5 amp - CB radio
~5 amps - Engel MT60 Combi (this has a 15A fuse, so should I plan this as a 15A load?)
40amps - onboard air
~45amps - 500 watt inverter that will be rarely used, and probably never over 25% load
~1 amp - assorted low-wattage LED interior lighting
~5 amps - occasional use on a couple of 12v power ports I'm installing back there too


The Engel will be on 24/7.

The radios will be on most of the time.

I do not want to have to power down anything to use the air compressor OR the inverter, but I highly doubt I will ever want to use both at the same time so I figure they can share that available amperage. The most use the inverter is likely to see is charging the batteries for handheld HAMs and cameras, and perhaps 90 watts to charge my notebook at camp. The compressor would only be used to air up after a trail.


Based on the above I'm thinking I'm safe if I design around ~80 amps of max usage (which means I only need one 100-amp breaker, yay!). I am thinking 2-gauge wire will be sufficient to run that power 15-20 feet and allow for the extra drop from things like the breaker, fuses, terminals, etc.?

Also, would I need to install a circuit breaker on both the positive and ground (both run to battery) or just the positive?
Chazz Laynedotcom

sdnative

Re: Double-check my math please?

#2

Post by sdnative » Mon Oct 05, 2009 12:28 pm

Just a couple of comments:

Make sure you factor in losses. For example a 50W mobile transceiver if it were 100% efficient would only draw ~5A, but I would be on the safe side and say 8A or 10A (you should be using a 10A fuse for this one).

I would factor in a 50% loss on the inverter. Maybe an 80A fuse?

Cable sizing takes into account the round trip distance (from the battery and back), so your 15-20 feet is more like 30-40 feet.

Maybe consider two runs and keep the radios on seperate distributions than the other stuff.

Instead of a circuit breaker, consider one of those big mega fuses.

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Chazz Layne
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Re: Double-check my math please?

#3

Post by Chazz Layne » Mon Oct 05, 2009 12:49 pm

Good point on the radios - I do want to keep them on a separate circuit, both to minimize potential interference and so I can keep them on by themselves if something goes wrong with the higher amperage gadgetry.

I forgot about having to factor round trip, and efficiency of the device itself, thanks. I'm thinking one run on 4-gauge for the radios, with a 50-amp breaker. A second run on 0-gauge with a 100-amp breaker would then handle the rest, which should be ok since I would only run one of the high-amperage devices at a time (and then only occasionally) anyhow and the rest is small (lights, fridge). I don't mind powering down the inverter to use the compressor, or the compressor to use the inverter. That puts me at:

50-amp breaker w/4-gauge wire:
~10amps - Kenwood D710A
~20amps - Yaesu 857D
~1 amp - CB radio

100-amp breaker w/0-gauge wire:
15 amps - Engel MT60 Combi (since it has a 15A fuse)
~80amps - onboard air (40A at full load, though it will probably never see that)
~80amps - 500 watt inverter that will be rarely used, and probably never over 25% load
~1 amp - assorted low-wattage LED interior lighting
~5 amps - occasional use on a couple of 12v power ports I'm installing back there too

That does put the 100-amp side kind of close if the Engel and the compressor ever both wanted to draw full load at the same time (~95 amps). That would be a pretty short window in which that could happen though (filling tires).



I like circuit breakers since I don't have to carry spares for them, but I think I'll add a few of those mega-fuses to my parts bag just-in-case. :D
Chazz Laynedotcom

sdnative

Re: Double-check my math please?

#4

Post by sdnative » Mon Oct 05, 2009 7:19 pm

You can also ground the negative side of the 100 run to the chassis. It's not really necessary to run "dirty" power back to the battery. Save yourself a couple of bucks.

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Chazz Layne
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Re: Double-check my math please?

#5

Post by Chazz Layne » Thu Oct 08, 2009 12:48 pm

Thanks, I'm going to go that route with it. I'm also going to run a Vlair system that tops out at ~35 amps instead of the big ExtremeAire unit I was looking at. Still $70 for a wire though... :shock:

After looking at the price of wire I also decided to put the radios under the passenger seat where the factory CD changer used to be, cutting the runs down to less than 8 feet. That'll also save me from having to buy the extension cable kits for the remote face cables since they'll only be 3-4 feet away.
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Re: Double-check my math please?

#6

Post by Chazz Layne » Wed Feb 03, 2010 5:17 pm

Another related question: what kind of current draw should I expect on the control side of a generic 30A automotive relay (I'm guessing in mA)? I've been searching for a while and can't seem to come up with a good average.

Reason: roof lights and other powered goodies on the rack. I currently have 7 devices (or groups) planned for the roof that would need to be switched on/off independently. I'm not too sure I like the idea of running a complex series of power cables up there for each group of devices, adding a bunch of wiring under the hood, and another complex series of control cables through the firewall to the switches. Rather than doing that it would be nice to run a single lite gauge cable to the entire switch panel on the dash, a single control cable from there to the roof (such as a Cat 5e network cable - effectively eight 22AGW wires), then a single heavy gauge supply cable to a relay box hidden on the roof rack. If I did this, it would also make it easier to have a single hole/pathway for power & control and a single hole/pathway for antenna cables (smaller holes and less chance of leaks).
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cruiserlarry
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Re: Double-check my math please?

#7

Post by cruiserlarry » Wed Feb 03, 2010 6:51 pm

sdnative wrote:You can also ground the negative side of the 100 run to the chassis. It's not really necessary to run "dirty" power back to the battery. Save yourself a couple of bucks.
Actually, it is, if you want to avoid possible resonant hum on the radios. When you ground a larger current load to the frame, it still has to make it's way back to the battery, and the uninsulated metal frame can act like a large transmitting antenna :o

Also, the inverter (and most inverters I've worked with) recommend a power wire run of 3 feet optimally, and no more than 6 feet, to prevent a large drop output efficiency. As you extend that cable run longer (even with larger gauge cable) you can dramatically reduce the available 120V output of the inverter. Keep that in mind when deciding what you will use the inverter for...

And the most overlooked part of this equation - reserve power. Use high quality, vibration resistant, temperature resistant batteries (at least 2 if possible) - I suggest Odyssey batteries - and use the largest you can fit and afford. This greatly reduces the strain on your charging system, allows more reserve when not driving, and will maximize the inverters output (most call for a 2-4 battery bank dedicated to the inverter for extended / constant use in the field).
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Chazz Layne
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Re: Double-check my math please?

#8

Post by Chazz Layne » Wed Feb 03, 2010 7:22 pm

I'm planning to replace the main battery with a largish Odyssey (Discos have a large battery bay), then add dual Odysseys in the back where most of the non-driving load will be. This is also where I'll be running the solar to for charging, since it is primarily for keeping the fridge running. I'm planning to keep the main battery completely isolated except when driving (for charging) or emergency starting, and will be running all of the "RV" type systems/accessories off the rear bank. The inverter will also be back there, so I'm guessing between 6-18 inches of cable to it from the rear battery bank. The only non-stock load on the main battery will be lights, radios, and the computer (basically everything used from the "cockpit").

I think I will just go ahead and run the ground wires back to battery. A Discovery is short after all, so the cost of cable is negligible.
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Re: Double-check my math please?

#9

Post by toms » Wed Feb 03, 2010 8:06 pm

You might want to consider using a rig runner to terminate the leads from the battery for the radios. Then use Anderson power pole connecters on all your radios to plug into the rig runner. This will give you a very flexable arrangement for additional radios or guests.
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Re: Double-check my math please?

#10

Post by unwiredadventures » Thu Feb 04, 2010 9:54 am

toms wrote:You might want to consider using a rig runner to terminate the leads from the battery for the radios. Then use Anderson power pole connecters on all your radios to plug into the rig runner. This will give you a very flexable arrangement for additional radios or guests.
x2 on this. This gives you some real flexibility for adding on and changing.

I try to avoid inverters whenever possible. You can get good 12vdc power supplies for most everything, laptop (http://www.igo.com), camera battery charger, etc. I have a small inverter only to charge things that don't have 12vdv adapters (or where the manufacture wants to charge a high price).
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