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Old 10-28-2013, 10:12 PM   #1
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Mixing AC/DC

Hi to all,
I am currently in the research phase on helping by brother in law in converting his bus, but need some guidence on mixing AC and DC on the same vehicle. My background is in AC(house wiring), DC(worked at Circuit City in the past installing in the mobile electronics bay various items for 3.5 years) and low voltage(under 24 VDC/AC in alarm systems, coaxial cable, CAT5 in new home construction and HVAC control work). My questions are: 1. Does AC in a mobile environment need to be stranded? 2. When wiring a 120 AC panel to the bus, does the ground/neutral in the panel remain isolated from the bus chassis/does it get connected to the bus chassis(one or both ?)/ does the body of the bus get bonded to the chassis for 'hot skin' prevention(if so, how many places).....? 3. How big of an inverter does he need to run say a small fridge and a microwave while on the road?(1200W-2000W, i'm assuming). 4. How many batteries does he need to have for 'house power' and for the bus itself. 5. Can the AC ground(shore power recepticle) be connected to the DC ground (chassis of bus)or does it remain isolated in the AC panel? Bus is a 1992 Ward Senator(w/ 2 batteries on board at this time for all DC needs).
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Old 10-29-2013, 10:24 AM   #2
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Re: Mixing AC/DC

There are some big threads here disusing the stranded vs solid for AC wiring. In my opinion, using stranded is more problematic due to most residential plugs, light switches, ect are harder to connect properly. One loose strand can create a short, all ends need to be soldered. Solid Romex works just fine without all the fuss.

I won't be grounding any of my AC to the bus steel. It will have a completely isolated ground wire system, same as a house. The only ground will be in the shore power cable. Reason being if some one in the RV park has something wired wrong, your bus skin will become live. Also not good for galvanic corrosion. The less electrical charge running through the steel skin the better.

The body is already bonded to the chassis.

You will need a 2000 watt inverter to safely run a small fridge and a microwave. Microwaves like pure sine wave. For non full time use the Motor master brand inverters go on sale for half price from time to time. $200 should get you going. They also have a digital readout for consumption, battery voltage, charging voltage, ect.

The DC in the bus is already connected to the skin and frame. Don't mix the AC and DC grounds. AC needs to be grounded only to the shore power cable, or directly to your generator. If your generator is not neutral ground bonded, (Most inverter generators are not bonded) you will need to make up a simple jumper plug to use when on generator power. Same go's for running off a inverter. If it's not bonded, make a jumper plug.

Nat
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Old 10-30-2013, 10:11 PM   #3
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Re: Mixing AC/DC

I appreciate the feedback; I agree on the solid wire factor, also on the Gnd on the AC shore power. One question: how does and inverter work with it's Gnd and interconnecting the AC gnd from the shore connection. I'm assuming ground the chassis of the inverter to ground and then connect the AC to the panel? Then the ground of the shore power goes to the gnd terminal in the AC panel only?(Isolated from the chassis AND from the DC ground.)
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Old 10-31-2013, 09:04 AM   #4
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Re: Mixing AC/DC

http://www.jackdanmayer.com/rv_electrical_and_solar.htm This place has some good information on bus/rv electrical. If you plan to connect to power in a RV park you should use an inverter that switches the ground neutral bond. For your stand alone power you should treat it just like a house. If you search for rv conversion electrical lots of info comes up read with caution I have ran into some sites that have no clue as to ground/neutral bonds. and as you know that can be dangerous.

Chuck
Here is another link that discusses this issue. http://www.civicsolar.com/resource/e...-boats-and-rvs

Chuck
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Old 10-31-2013, 09:55 AM   #5
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Re: Mixing AC/DC

The "RV Electrical Safety" post here in Board index č Bus Conversions č Tutorials and How-to's is the go to for the AC ground neutral bond

Lots of good ingo there.
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Old 10-31-2013, 01:31 PM   #6
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Re: Mixing AC/DC

Quote:
Originally Posted by shutterbug3
I appreciate the feedback; I agree on the solid wire factor, also on the Gnd on the AC shore power. One question: how does and inverter work with it's Gnd and interconnecting the AC gnd from the shore connection. I'm assuming ground the chassis of the inverter to ground and then connect the AC to the panel? Then the ground of the shore power goes to the gnd terminal in the AC panel only?(Isolated from the chassis AND from the DC ground.)
Yes DC grounds only to chassis and negative battery post. AC out from inverter only to isolated ground in AC panel. Keep AC and DC grounds apart. If anything ever malfunctions, you will be happy you did. Finding the problem will be much simpler.

Also no steel plug or light boxes attached directly to the bus steel. Make it imposable for your bus chassis and skin to become a live circuit.

Nat
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Old 10-31-2013, 02:43 PM   #7
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Re: Mixing AC/DC

Quote:
Originally Posted by shutterbug3
I appreciate the feedback; I agree on the solid wire factor, also on the Gnd on the AC shore power. One question: how does and inverter work with it's Gnd and interconnecting the AC gnd from the shore connection. I'm assuming ground the chassis of the inverter to ground and then connect the AC to the panel? Then the ground of the shore power goes to the gnd terminal in the AC panel only?(Isolated from the chassis AND from the DC ground.)
A good inverter will have a built in relay that switches the gound/neutral bond between the inverter and shore power. One reason I dont like cheap inverters designed for home use is this feature is nonexsistant and requires the builder to make his own switching circuit. JMHO but I hate extra circuits and work more things to go wrong.
I personally belive in stranded wire for mobile applications. Not only for vibration but for ease of runs. As everyone says its a personal preferance. I work on Cummins/Onan generators and solid wire on those is the #1 cause of no worky. If you check there is even a group who talk about electron flow on the surface of solid core wire as compared to many paths for stranded. But I digress, what you are comfterable with is whats best in my book.
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Old 11-10-2013, 11:41 PM   #8
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Re: Mixing AC/DC

Quote:
Originally Posted by nat_ster
Yes DC grounds only to chassis and negative battery post. AC out from inverter only to isolated ground in AC panel. Keep AC and DC grounds apart. If anything ever malfunctions, you will be happy you did. Finding the problem will be much simpler.

Also no steel plug or light boxes attached directly to the bus steel. Make it imposable for your bus chassis and skin to become a live circuit.
I'll respectfully disagree on the ground advice based on personal experience. I have an older Xantrex modified sine inverter which specifically indicates in its instructions that its chassis is to be well-connected to the vehicle (and, thus, the dc negative/ground). One day while testing that thing on the shop floor I learned the "why" behind that instruction. At some point I found myself squatting beside the equipment with one hand resting on the shell of the inverter for balance while reaching to feel the temperature of an alligator clip on one of the battery terminals, and on touching that was much surprised to feel the familiar zing of 60 Hz ac biting back. It turns out that this inverter has its chassis internally connected to its ac ground and neutral terminals. This is a reasonable thing to do when fault currents and operation of GFCI protection are considered. Anyway, it turns out that if this inverter's chassis isn't connected to battery negative, then ~50V ac develops between those two and there's enough leakage current available to give a person a good zap when he unwittingly touches the wrong places at the same time. Think of a metal-bodied luminaire, a metal-chassis appliance which is either grounded or is double-insulated but has a fault, the grounded shell on the connector of the VGA/DVI/HDMI/USB/audio cable on that ac powered TV, and touching something on the bus body like a window or door or bench etc. Plenty of opportunity for surprises.

Consult the manual of your inverter to be sure. If the manufacturer doesn't specifically caution against it, I would definitely connect the inverter's ac ground to the bus chassis/body.

I can appreciate the idea behind avoiding metal junction boxes etc. It makes some sense in a shore-power-only scenario. But if there's an on-board inverter in the picture that's another story. Also I have some concern about whether such an approach defeats (ie prevents operation of) GFCI given that there's no ground to fault to.

One caution for any who might be thinking of both an inverter and shore power (I am): beware the inverter's internal ground-neutral bond. You want that bond when the inverter is sourcing the power, but when things are switched over to shore power, it should be done in such a way that the inverter's internal ground-neutral bond is disconnected. If your transfer switch arrangement switches only the hot leg between inverter and shore, then when the shore power cord is plugged in it'll be connecting shore ground and neutral improperly. You'd need a multi-pole transfer, a plug-and-socket transfer, or some similar thing to keep the bonding appropriate.
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