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Old 09-21-2016, 08:25 PM   #1
Mini-Skoolie
 
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Electrical questions - wire size and more

It's time to wire the bus and I have several questions. 1st, I'm trying to get the right size of DC wire for the appliances. I've been using an online tool to figure it out and the size wire it says to use seems pretty big. Here are some examples. Any validation to these or suggestions would be appreciated.
Water pump: 7.5 amps, 50 ft (round trip) the result was a 3 gage wire.

Ceiling fan: 2.5 amps, 40 ft the result was a 8 gage wire.

I also have a question about grounding. I have an inverter, ac breaker before the inverter, and a dual ac/dc breaker panel. Do I have to ground all of the breaker panels? If so, do they all ground to the frame?

If I can upload it, I'll include a pic of my planned wiring scheme. I appreciate any help available.
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Old 09-21-2016, 09:14 PM   #2
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that sounds big, but dc suffers from drastic voltage drop across distance.

here is the chart i used in wiring my bus and i still think the wire was big.

http://assets.bluesea.com/files/reso...on_chartlg.jpg

anytime i comment on grounding i'm quickly corrected, there are lots of sites with good rv electrical ground explanations. but as i understand it, you do give everything a route back to ground through the shore pedestal.

you don't ground everything to the frame.... you give a potentially energized frame a way to ground back to a safe ground.

hope that helps
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Old 09-22-2016, 07:57 AM   #3
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Quote:
Water pump: 7.5 amps, 50 ft (round trip) the result was a 3 gage wire.
Not sure what you mean here, your bus can't be more than 40' long. In DC the return is ground, in this case the frame, you don't need a return wire. Wire size for 7.5A for 40' would be AWG 10. For the 2.5, AWG 14. That water pump at 30 ft drops to AWG 12. I usually wire most things with 12 unless it is a long run or heavy draw. For my LED lights I use 16 but 18 would be big enough.

Dick
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Old 09-22-2016, 08:58 PM   #4
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Thanks Turf and Somewherinusa. I appreciate the info and the wire chart. What I mean about the round trip is ... distance to the appliance and then the return through the negative. I'm not sure I understand about the return being the ground. So are you saying I run the positive source to the appliance and ground the negative to the frame? If I have 2 or 3 led lights connected together on one circuit (in parallel), do I connect them together and then on the last light connect the ground to the frame? Sorry if these are basic questions. This is my first crack at wiring DC. Thanks.
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Old 09-22-2016, 09:41 PM   #5
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Basically yes to both questions. You can have a grounding point and use it for many devices.
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Old 09-22-2016, 11:54 PM   #6
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nevittja, you're right: you do have to consider the wire length for both the positive (supply) and the negative (return) sides of the circuit. somewhereinusa and turf have in mind to use the frame as the return half of the circuit and to consider that resistance and voltage drop to be zero, so that only the supply half of the circuit is included in the wire sizing. For most loads this method is fine; there are a few where it's less desirable and often a separate return wire as you had originally contemplated is recommended (for example, a very high-current load like a large motor or a noise-sensitive load like an audio amp or radio equipment).

When using the frame/body as the dc ground/return path, take care to think about the size of the jumper wire from the frame to the battery bank and to the alternator (or engine block). It's a critical and often forgotten part of the circuit. When the supplies to a handful of circuits go out each on their own wire and fuse, we sometimes forget that ALL that current returns on that one frame-engine-battery jumper and that its size and connection cleanliness matter.
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Old 09-23-2016, 12:00 AM   #7
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Regarding the grounding of the ac system: there's a fun thread about ac systems there. I contributed a block diagram of sorts in this post; you might find it and the discussion that followed interesting. There are some grounding nuances to think about when the ac source changes between shore power at home or campground and local power from an inverter or generator.
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Old 09-29-2016, 08:23 PM   #8
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Thanks again for the info. I have another question. I am replacing an exsisting ceiling light with a new puck light. The existing light only has one wire connecting it. The new light has a positive and a negative wire. Using the exsisting wire, can I just cap the negative wire. And connect the positive?
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Old 09-30-2016, 05:46 AM   #9
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You have to ground the negative wire.
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Old 09-30-2016, 05:58 AM   #10
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Quote:
Originally Posted by turf View Post
that sounds big, but dc suffers from drastic voltage drop across distance.

here is the chart i used in wiring my bus and i still think the wire was big.

http://assets.bluesea.com/files/reso...on_chartlg.jpg

anytime i comment on grounding i'm quickly corrected, there are lots of sites with good rv electrical ground explanations. but as i understand it, you do give everything a route back to ground through the shore pedestal.

you don't ground everything to the frame.... you give a potentially energized frame a way to ground back to a safe ground.

hope that helps
never bond the AC neutral to the frame, thats the hot skin people talk about.
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