# Help in sizing cable and circuit breaker.

Discussion in 'Electrical Systems' started by Mark Cat, May 22, 2013.

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### Mark CatSenior Member

Large 24 VDC panel (1 Main and 16 branch circuit breakers Carlingtech C-Series UL489 lable) near helm powered by 1200 CCA bank. Boat follows ABYC.

New lighting branch circuit 25 feet round trip at 11 Amps continuous, requirement for 3% Voltage drop.

Using two conductor 600V 105 Degrees C boat cable. The new cable goes into a harness with two existing cables, for a total bundle size of three cables (6 wires). The bundle goes through the engine compartment.

Would anyone know how to size the circuit breaker and conductor.

Mark Cat

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### jehardimanSenior Member

You need to get the current (2012) copy of ABYC E-11. That will spell out the minimum conductor size and maximum amperage for a bundle of that size conductor x number (or if you want to bundle that many, you may need a larger conductor). Generally, the circuit breaker should not excede the capacity of the cabling. A 2008 version of E-11 can be found here.

http://plaisance-pratique.com/IMG/pdf/ABYC_E-11-2.pdf

Last edited: May 22, 2013
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### jehardimanSenior Member

Last edited: May 22, 2013
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### CDKretired engineer

My ITT reference data book says AWG14 for a less than 2% drop at 25 ft and I= 12 A.
Fusing current of AWG14 is 166 A, so the circuit breaker type should be more than 11 and less than 166 A! I would use the next available one above 11 A., which is probably 15 or 16 A.

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### Mark CatSenior Member

So I have just been able to get back to this post. First of all I would like to thank CDK and jehardiman for their help.

Here are my findings so far:

1200 CCA Battery rating Branch AIR = 2500 Amps. I have an option here for either the UL489 (which is overkill) Circuit Breaker or UL1077 Supplementary Protector (operable after fault) at >= 2500 Amps AIR.

3% Voltage Drop per ABYC, Conductor CM (Circular Mil) = 4106. 14 AWG = 4110 CM. So for the drop 14 AWG is good.

For ISO that's about 2.5 mm^2.

The Electrical Requirements for this recreational boat do not exclude DC from bundling derate like 33CFR183 does. Also, conductor continuous loads and non 100% rated breakers are derated to 80%.

So to avoid a large bundling derate I was able to route the cable with separation from other cables.

Breaker terminal temperature rating is 60 degrees C. Panel rating at 30 Degrees C ambient (ABYC and 33CFR183).

Continuous load at 125%. 11 Amps x 1.25 = 13.75 Amps.

Per ABYC Table VI - B at 14 AWG, 60 Degree C column, = 14.0 Amps. So for continuous current 14 AWG is good.

The CA3 (Handle type) series CBs can be ordered in a 12 Amp version. Also, in looking at the panel and its generous heat dissipation (ventilation + heat sinking to ambient), and that its located in a well ventilated area on the enclosed bridge. The decision was made to try a breaker first at a 100% rating. If this did not work then I could try something larger 13, 14 and 15.

So as a starting point the CB is 12Amps. DC medium trip time curve at 125% of rating.

Worse case over load; 12 Amps x 125% = 15 Amp.

15 Amp is greater than the 14 Amp table entry for two conductor DC boat cable. However the rating is within ABYC next size up rule. If the nature of the load could fail high overload then I would move to 12 AWG.

In going through the engine room (50 degrees C) using 105 degrees C cable the derate per ABYC = 20.8 Amps. 90 degrees C = 17.2 Amp. I could use a 90 Degree C cable, however we only stock 105 degree C at 600V.

So to sum up:

Two conductor Boat Cable: Marked "Boat cable UL 1426 ABYC" = 14 AWG 105 Degree C dry, 75 Degree C wet BC-5W2 600 Volt fine stranded tinned copper UL 1426.

Circuit Breaker is a Carlingtech Handle type 12 Amp (Starting Point) UL Rec STD 1077. Check to confirm UL1077 test CB operable after fault. If not, use UL489.

All for now,

Mark Cat

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