Electric Load Estimates, DC/AC Wire Separation Rqmts, DC Bre

[jmedclay]

I'm about wire my TD and have some questions. Here are the relevant DC load estimates.

Individual DC Circuit Loads
Fantastic Fan: 3A
Cabin O'Head Lights (2): 3A
Cabin Reading Lights (2): 3A
Cabin Sconces (2): 3A
Cabin 12V Outlets (3): (Fan, 3A max each??)
Porch Lights (LED) (2): 1.5A
Galley O'Head Lights (2): 3A
Galley 12V Outlet (1): 3A?

Max Load Estimate
Fantastic Fan: 3A
Cabin O'Head Lights (2): 3A
Cabin 12V Outlets (Fan, Cell Charger): 4A
Galley O'Head Lights (2): 3A
Galley 12V Outlet (Fan): 3A
Porch Lights: 1.5A
Total: 18A

That's a very high estimate and unusual/unlikely conditions involving somebody with a heavy lounging load in the cabin while dinner is prepared on a hot day. A more realistic estimate in very warm weather, I think:
Fantastic Fan: 3A
Cabin Reading Lights (LED) (2): 1.5A
Porch Lights (LED) (2): 1.5A
Cabin 12V Outlet (Cell Charger): 2A
Total: 8A

Given all of that:

1) What size DC circuit main interrupt device would be appropriate for feeding the fuse block?
2) What are the guidelines for sizing this device?
3) What is generally preferred, fuse or breaker?
4) A number of wiring diagrams I've seen show a single AC breaker feeding both the battery charger and AC duplex outlets. Since our AC duplex circuit will be designed for 20A (our std kitchen equip is toaster oven and hotplate) it seems to me that the battery charger ought to have its own breaker, and 20A seems adequate. Please let me know if that seems reasonable, or if I'm missing something here.
5) Does 3A seem a reasonable max load to assign to the 12 outlets? We're not lighting cigarettes and I'd guess a fan is about the max load we'd impose; reasonable?
6) Physical separation of DC wiring from AC wiring; are there any requirements for, or details to observe when routing AC and DC wiring?

Thanks for any assistance you can provide.

[wannabefree]

This is what I would do. Others may disagree.

1) What size DC circuit main interrupt device would be appropriate for feeding the fuse block?
-- Your worst-case load is 18A. Add 25-50%, so 25-30A would be ballpark. Sizq your battery for at least 4x the max load. DO NOT use a thermal resettable breaker (one that resets itself). This is your battery's last line of defense. If you use a thermal breaker and there's a short, the breaker will cycle back on when it cools. Eventually the short will burn out or your battery will boil over. Use a breaker you have to push a button to reset, or a fuse.

2) What are the guidelines for sizing this device?
-- see 1

3) What is generally preferred, fuse or breaker?
--see 1

4) A number of wiring diagrams I've seen show a single AC breaker feeding both the battery charger and AC duplex outlets. Since our AC duplex circuit will be designed for 20A (our std kitchen equip is toaster oven and hotplate) it seems to me that the battery charger ought to have its own breaker, and 20A seems adequate. Please let me know if that seems reasonable, or if I'm missing something here.
-- No need. A 10A charger will draw at most 1.5A at full charge. But there's no harm. Parks provide 30A and 50A service. You can put 2 20A circuits on a 30A service. Sounds flaky, I know, but add up all the breakers in your 200A house panel and you'll come up with way over 200A. They bank on the fact that you never turn on everything at once.

5) Does 3A seem a reasonable max load to assign to the 12 outlets? We're not lighting cigarettes and I'd guess a fan is about the max load we'd impose; reasonable?
-- Mine are fused at 3A, but I don't run a laptop off them. Figure your max load, add 25-50% and fuse at that level. A laptop will need 10A minimum. Size wire accordingly. At 120V, 12ga is good for 20A. At 12V I use 8ga for 20A because I don't want to waste power in the wire. Skycraft Surplus in Fla is a good source for wire. A good site for wire size calculation: http://www.electrician2.com/calculators ... ver_1.html

6) Physical separation of DC wiring from AC wiring; are there any requirements for, or details to observe when routing AC and DC wiring?
-- Just keep them electrically isolated from each other. Don't tie 12V ground to 120V ground or neutral. There are differences of opinion as to whether AC neutral and ground should be "bonded" or tied together at a single point. For house wiring this is required; for trailer wiring it is a bad idea (IMO). Here's why: New parks are installing GFCI circuits on their feeds. If you tie ground to neutral in your trailer the park's GFCI will see leakage current in the ground line and disconnect your trailer. You'll be safe, but powerless.

Good luck with your project and keep us posted.

[jmedclay]

Thanks much for the information! A single 20A/120VAC breaker makes sense now, but the type.... I'd like to avoid a panel if reasonable, safe, code compliant. Options? DIN Rail mounting without enclosure OK? I'll search for surface mount breakers too.

Thanks,
John

Edit:

Not finding any surface mount AC breakers. My gut says it won't be code compliant. Still looking.

At this link:
http://forums.sailinganarchy.com/index. ... pic=117664
I read a couple of things that caught my attention:
"solid wire has no place on a boat due to risk of breakage and subsequent fire"
"main breaker must be double pole to guard against polarity reversal"

That seems prudent and reasonablly applicable to me. They also have a link to part of the ABYC E-11 document on AC-DC wiring, here: http://www.paneltronics.com/atimo_s/new ... cerpts.pdf

Edit 2:

It looks like this breaker can be mounted to the inside of the front panel of, and in a small, simple junction box. That will work I think.
http://bluesea.com/category/3/11/produc ... verview/19

[wannabefree]

That would work. So would DIN rail ($$$). When you find out the price, check this out: http://www.skycraftsurplus.com/circuitb ... style.aspx
Buy two and screw the bat handles together to gang them.
Double pole is a good precaution, but I didn't do it. I have a cheapo 12A thermal breaker for AC and a $4 outlet checker for testing park power. If it's wired wrong I don't plug in.

[Engineer Guy]

Hard to add anything to 'wannabefree's' superb advice. Also, great Product Line discoveries above. Some random thoughts...

1. Square D makes AC/DC Breakers:

Square D AC/DC Circuit Breakers

2. 4 Miniature Breakers [or more] can fit into 'the right' small Box, like one used for a Hot Tub, or in a Sub-Panel application. Blue Sea, or other nice Products, could then be wired in after 1 or 2 'large' DC Breakers to protect multiple DC Circuits individually. There are now Circuit Breakers that fit the form factor of Euro 'Blade' Fuses.

High Wattage Car Stereo Products can protect key Battery Circuits. Off-Grid Solar Home Products include medium Amperage Fuses and Hardware.

Personally, I would use a Tester to confirm proper Neutral and other Park Pedestal Wiring. Using 1 AC Breaker, as at a House, would leave valuable space for a Breaker to protect something else. A Tester also confirms intact Earth Ground; mighty important in my mind...

The State Code Inspector who liked and signed off my new, self-installed House Wiring mentioned not liking 2 GFIs in series, due to false trips. This scenario would include having the first of two GFIs in the Park Pedestal, if one thinks about it. Also, Neutral has to be intact for GFIs to protect properly. All this is worth confirming with a Tester.

[Optimistic Paranoid]

This is what I would do. Others may disagree.

6) Physical separation of DC wiring from AC wiring; are there any requirements for, or details to observe when routing AC and DC wiring?
-- Just keep them electrically isolated from each other. Don't tie 12V ground to 120V ground or neutral.

OK, this confuses me.

Say I'm doing a conversion on a Cargo Trailer with both a steel frame and sheet metal walls. Don't I need to ground it via the green AC grounding wire?
If I don't, and the insulation on the hot wire somehow gets cut or rubbed off, and the now bare wire makes contact with the frame or metal skin, the frame and skin will be energized and give me a shock, possibly a lethal shock, if I touch it while standing on the ground. Or am I missing something?
(I am taking it for granted that the frame will also be connected to the 12V ground.)

Regards
John

[wannabefree]

OK,opening a can of worms here. As I said before, others may (will) disagree. This is how I wired my trailer; it works for me, but circumstances differ. One important point - I only rarely charge from the TV. When I do I use an inverter to provide 110V to the on-board charger.

There are 3 electrical systems: 12V TV, 12v trailer, 120VAC. Each is isolated from the other. 12V TV ties to the frame. 120VAC does not because there is no way a short to the frame can occur short of 3/4" ply rotting away and 2 layers of insulation wearing away, and a hot wire shorting to the frame. IOW, extremely low probability. In cheap power tool vernacular, I am "double insulated."

If you want to charge from the TV and not use an inverter, you will necessarily tie 12V TV ground to 12V trailer ground. No way around it.

If you have a metal trailer, you should tie 120VAC ground to it for safety (I think I just ate my words here)

Why did I do it the way I did? I have a woodie, so I feel pretty safe. And this is how we wired the seismic trucks I worked on ~30 years ago. TV power is noisy and subject to some pretty wild transients. It messes up sensitive electronics, though it is doubtful you would have anything as sensitive as a seismograph.

Sorry if I generated confusion - what I did is not necessarily right for the metal trailer crowd. So much for rules of thumb. There's no substitute for thinking.

[Corwin C]

There's no substitute for thinking.

+1 and I would add understanding...