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Old 29-11-2022, 08:25   #1
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Round Trip Distance - DC

I have a question about where a round trip distance for a device starts/stops. Is the source for a device the fuse panel (where the positive starts and negative return for that device) or the breaker that feeds the fuse panel?

In this scenario (see attached image), do I use 12 ga from the fuse panel to the dash socket or do I continue with the same gauge as the feed to the fuse panel?

In other words, is the round trip for the dash socket from fuse panel and back or from breaker and back?

A 20 amp breaker on main DC panel protects the 8 ga wire feeding the fuse panel

One way distance to main DC panel/breaker: 15 feet (8 ga)
One way distance from fuse panel to dash socket: 5 feet (12 ga)
Distance from fuse panel to light fixture: 7 feet (< 18 ga, but increase to 16 ga)


DC Consumers:
Dash Socket 12V DC with Watertight Cap, Blue Sea # 1011
6.75" LED Cabin Dome Light

Amp draw:
Dash Socket: 15 Amp max
Light: 0.23 Amp max

ATC Fuses in the fuse panel:
Dash socket: 20 Amp
Light: 1 Amp

Calculated wire gauge based on amps and distance:
Breaker to Fuse panel: 8 ga
Fuse panel to light: 16 ga
Fuse panel to dash socket: 12 ga
* If dash socket round trip is from breaker, the 8 ga goes up to 6 ga


Thanks y'all
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Old 29-11-2022, 08:49   #2
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Re: Round Trip Distance - DC

Each wire size would be an independent calculation, going all the way to the battery. Even then, if you don't account for simultaneous device actuation and even battery drop on common wires, there will be undervaluation.
Saying all that, I suspect most just use panel branch wire exit as the "start".
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Old 29-11-2022, 10:12   #3
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Re: Round Trip Distance - DC

The practical approach is that you assume that the wires to the fuse panels or busbars are oversized to handle multiple devices or longer runs and you only calculate the voltage drops for the segment that you are adding.

The purist approach dictates that you calculate the voltage drops for the entire path from the source (battery, inverter, etc), through any breakers, fuse panels, switches and the target device all the way back to the source again.

If I was driving a .23 amp light I would probably go the practical route. If I was driving a constant power device like an induction motor I would go the purist approach as voltage drop translates into increased amps, more heat and reduced lifetime.
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Old 29-11-2022, 10:17   #4
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Re: Round Trip Distance - DC

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Old 29-11-2022, 10:18   #5
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Re: Round Trip Distance - DC

my j5hbD&lt;;&lt;; at 67777&l
Quote:
Originally Posted by NPCampbell View Post
The practical approach is that you assume that the wires to the fuse panels or busbars are oversized to handle multiple devices or longer runs and you only calculate the voltage drops for the segment that you are adding.



The purist approach dictates that you calculate the voltage drops for the entire path from the source (battery, inverter, etc), through any breakers, fuse panels, switches and the target device all the way back to the source again.



If I was driving a .23 amp light I would probably go the practical route. If I was driving a constant power device like an induction motor I would go the purist approach as voltage drop translates into increased amps, more heat and reduced lifetime.
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Old 29-11-2022, 18:15   #6
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Re: Round Trip Distance - DC

from the breaker to fuse block I would cacl for all fuse block loads (and future) on at once and 3% drop.

then each load from fuse block could be done at 10%. with the idea that the feeder at 3% should have minimal overall effect.

but if you kept going from junction to junction to junction etc at some point you'd have to look at the whole system. not just lose 10% at each point.

with the picture you'd be 8awg for 3% from the breaker to the fuse block. and 14 would be fine for the cig plug (blue sea says 16 but I would not go that low)

if you want to add more to the fuse block consider 6awg

if the cig plug is charging phones it'll never see more then a few amps anways..
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Old 30-11-2022, 05:54   #7
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Re: Round Trip Distance - DC

Thank you for the input, I can proceed with more confidence and understanding.
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