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Old 02-28-2016, 01:57 PM   #1
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Very basic electrical...

Newbie. About to lift shell, install subfloor, rewire, re-plumb, etc.

In contemplating electrical, I've come to realize I have little idea what I'm doing in some areas I thought I had a handle on. Paralysis through analysis, perhaps, but here's a question I need answered before I start drawing up schematics.

What does it mean to say a particular terminal device - anything from a single LED bulb to an air conditioner - "draws x amps"? In other words, if I say a replacement LED bulb "draws" .18 amps, does that mean that, in one hour, that bulb draws .18 amps from the battery (or whatever the source of power may be)? And, if I say an AC unit has a "start load of 35 amps and then draws 12 amps", is that to say that as soon as I switch it on it will immediately deplete my power supply of 35 amps, just to start up, and that, thereafter, it draws 12 amps from my power supply every hour?

Conversely, if a given battery is said to have a capacity of "400 amp hours", does that mean that it will power, say 40 terminal devices drawing 10 amps each, for one hour and then be fully depleted?r 10 terminal devices drawing 40 amps, also for an hour, before it's exhausted?

If my 12V distribution box originates say, 6 circuits, each protected by a 15 amp fuse, does that mean I can populate each of them with terminal devices adding up to no more than 15 amps per hour? Beyond that, if I put a greater load than that on any one of those circuits (say, terminal devices drawing, in the aggregate, 16 amps), and turn on every one of them, that I'll blow that fuse?

If so, and if I'm reading LED material correctly, it would seem one could power virtually an entire trailer full of LED lights, interior and exterior, on a single circuit of relatively modest amperage, right? Many of these LED bulb/fixtures purport to "draw" fractions of a single amp, say .15 amps. If I use, say, 10 such bulbs inside my trailer, on a single 15 amp circuit, together drawing a paltry 1.5 amps per hour, am I drawing 36 amps every 24 hours from that circuit? Can I do that on a single, 15 amp circuit? In fact, couldn't I (in theory) simultaneously run 100 such bulbs on that single 15 amp circuit?

Basic stuff for a basic mind.
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Old 02-28-2016, 02:37 PM   #2
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I would say that in general you are on the right track. A few notes:

1. If you have a 400 ah battery set, you should not draw more than 200 ah (50%) out of it. If you use more than 50% of a standard lead acid battery you decrease it's life significantly.

2. The 35 a start load and 12 a running load does not come out of your 12v power supply, it comes out of your 120v AC distribution sytem. Doesn't affect your battery capacity.

3. There is a slight decrease in battery capacity depending on how quickly you draw power out of your batteries. Look up "Peukert Factor"/
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Old 02-28-2016, 04:33 PM   #3
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Don't confuse "amps" which is purely a measurement of flow rate, with "amp-hours" which measures battery capacity. A 2 amp flow rate that continues for one hour equals two amp-hours.

Some people teach electricity by comparing it to water, and it works for the part we're talking about here. Amps measure flow--like gallons per minute. Volts measure pressure--like 40 psi.

You'll probably run into this next, a watt is a measurement of power (work done) and equals one volt times one amp. So, your 30 amp AC supply for your trailer will run approximately 120 x 30 = 3600 watts of electrical stuff. All AC appliances should be labelled with the number of watts they draw. You'll have to manage your loads to get by on 30 amps--in other words, turn off the air conditioner before you try to run the microwave, etc.
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Old 02-28-2016, 04:43 PM   #4
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If one installs lithium iron phosphate batteries, one has access to at least 80% of their rated capacity. Thus a 400 amp hour lithium battery can provide 320 amp-hours of power. A lead acid battery or GSM battery can only provide 50%m of its rating, so in the case of a 400 amp hour array of lead acid batteries, one could only use 200 amp hours of power.

If one has a refrigerator running on 12Vdc power and draws 3 amps when running, in ten hours it has used 30 amp-hours of power from the battery.
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Old 02-29-2016, 09:18 AM   #5
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Thanks so much. Each of those responses are helpful. Please keep 'em coming.

If I replace my Univolt with a PD 9245C 45 Amp RV Converter/Charger, how should I best divide the circuits? The current Univolt setup has the shore power going first to a breaker box that has three 20-amp breakers. Of those, one foes to AC, one to outlets and the last to the DC panel. I'm inclined to go with two 20 amp circuits to go to the DC panel, one 30 amp to go with the DC outlets and one 30 amp dedicated solely to AC. Is that do-able?
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Old 02-29-2016, 10:49 AM   #6
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The original trailer wiring size was predicated on the existing fuses/circuit breakers.

#16 wire is rated 10 amps
#14 wire is rated 15 amps
#12 wire is rated 20 amps
#10 wire is rated 30 amps

Oversizing the circuit breaker or fuse puts the wiring at risk of failure if there is a short.

Also per the National Electric code, one must derate the circuit 20% for continuous loads. So a 30 amp breaker is okay for a 24 amp continuous load (like a dryer or water heater in a house).

The 20 amp breaker is rated for 16 amps of continuous power. Note that during the air conditioner start up, the instantaneous power surge will briefly exceed the breaker capacity but the 13,500 BTU unit will draw close to 13 amps when running, so a 15 amp breaker would be undersized.

In a 22' trailer, the wire runs are so short that voltage drop is not a major consideration for the AC power. It is a concern for the 12Vdc power lines. Thus the much larger wire sizes for the battery and its connection to the charger versus the 12Vdc lighting.
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Old 02-29-2016, 04:41 PM   #7
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Quote:
Originally Posted by splyb View Post
Thanks so much. Each of those responses are helpful. Please keep 'em coming.

If I replace my Univolt with a PD 9245C 45 Amp RV Converter/Charger, how should I best divide the circuits? The current Univolt setup has the shore power going first to a breaker box that has three 20-amp breakers. Of those, one foes to AC, one to outlets and the last to the DC panel. I'm inclined to go with two 20 amp circuits to go to the DC panel, one 30 amp to go with the DC outlets and one 30 amp dedicated solely to AC. Is that do-able?
No. It sounds like we are mixing up the AC circuits and the DC circuits. The converter at most will draw 9 amps at 120v AC. It puts out 45 amps at 12v DC.

The AC circuits and the DC circuits only have one point where they connect, and that is through the converter. It takes in 120v AC (9 amps) and puts out 12v DC (45 amps).

Stay with a 20a breaker for the AC, use a 15a for the converter/charger, a 15a breaker for lights (unless all your lights will be LEDS cming from the DC panel), and you still have another 20a available for a water heater or space heater or microwave or whatever.
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Old 02-29-2016, 09:29 PM   #8
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120 volts X 9 amps = 1080 watts. A 45 amp converter provides 12 volts X 45 amps = 540 watts or @ 5 amps when you consider losses.
Circuit breakers and fuses are there to protect the wire. Do not oversize these protection devices.
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