Sanity Check - calculating DC circuit lengths

[TheLake]

Hi all,

I just need some reassurance that I haven't made an error in my thought process.

When calculating DC cable length to put into a sizing calculator, it is the total length. However, should it be broken down by segments.

Example:

From the breaker panel to a fuse block:
- 100 amp capacity at the fuse block
- 4 foot run from the breaker to the fuse block (8 feet total)

At the fuse block:
- multiple small circuits (branch circuits) of varying length.


My question (or need for validation):
- I believe that I should calculate the total run between the breaker and the fuse block and size that wire for the max amp at the fuse block (AWG 6 cable)
- then I should calculate the runs from the fuse block to the devices
- for example - if a run is from the fuse block for 40' (return) for a <1amp LED light I would use AWG 18 on this run (sheathed)


I wanted to assure that I wasn't mistaking this by calculating the run length from the circuit breaker through the fuse block and to the end device when calculating the size (as this would give a very large cable size).

Just a sanity check please.

Thanks,
Chris

 

[READY]

That makes sense to me. One possible nuance is that technically you need to account for the voltage drop from the source to the terminal block as the starting voltage for the branch circuit. But unless you are simultaneosly running multiple large loads I suspect this is insignificant.

 

[Gdavid]

You are doing it right, keep in mind that there will be voltage drop at each point so your device's amperage requirements increase as voltage drops. Not a concern for a LED nav light but could be for a higher load item.

 

[Gdavid]

Oops, READY already beat me to it.

 

[TheLake]

Thank you. I feel better with just a sanity check!

The points about voltage drop is well taken and I will recheck my calculations.

 

[psneeld]

Might have someone with current ABYC double check minimum wire size. Thought it was 14 ga for everything but signal wire.

 

[TheLake]

11.14.1.2 - Conductors shall be at least 16 AWG

Exemptions:
1. 18 AWG conductors may be used if included with other conductors in a sheath and do not extend more than 30 inches (760mm) outside the sheath.

and

4. Conductors on circuits of less than 50 volts having a current flow of less than one amp in communications systems, electronic navigation equipment and electronic circuits.

 

[ksanders]

My opinion is that you are making a big deal out of it with no real reason to do so.

Choose wire based on the circuit protection...

15 amps use 14 AWG (this is really the smallest size wire to use)
20 amps use 12 awg
30 amps use 10 awg
50 amps use 6 awg

Install your equipment and go about your happy life.