Wiring to a distribution block

[FLJetBoater]

When you're sizing wiring to a distribution block, do you just need to plan for the distance to and from, and not the entire distance of the circuit? For example....

I will have a distribution block 2 ft from house switch.

Distribution block will power added accessories (GPS, VHF, 2nd bilge, amp in future). Say a total of 100 amps.

0-6 ft @ 3% drop @ 100 amps calls for 4AWG as per Blue Seas chart. So, the 2ft cable from switch to block needs to be 4AWG. The neg from neg block to battery would also be 4AWG.

Is that correct?

Then, each device is wired appropriately from the DIST BLOCK to the accessory? or from the SWITCH to the accessory? Say the GPS is 15 amps (I think high, but for example) and 10ft away from the BLOCK that would require 12AWG. But if the distance from the block to switch to battery needs to be considered then it's say, 13ft and would require 10AWG.

I guess in short, are you sizing for the total length of the circuit from battery to ground, or are you sizing the wire just for the part of the circuit that it is on? Like if I needed to carry 40A between 2 distribution blocks that are 5 ft apart, that would be 8AWG. But if there are 15ft of other wire on either side of the distribution block does that need to be considered?

Another example...for powering a 90A amp 15ft away (total 30ft circuit). If I run 12.5ft of 1AWG to a block, can I then run the last 2.5 ft off the block in 4AWG to the amp? Or does it all need to be 1AWG? I guess does the distribution block become the "power source" in the diagrams? Or do you have to consider the entire path from the battery and back in which case I would have to run 1AWG on the whole circuit to the amp and back?

 

[spatty99]

I believe your understanding is correct.

The American Boat and Yacht Council (ABYC), the organization that develops standards for boats, states that conductors for the panelboard/switchboard are sized for its total electrical load, whereas branch circuit conductors are sized by their loads.

So in your example, the wires TO the distribution block are sized for the total output from the distribution block, 4 AWG. The wires FROM the distribution block are sized for the branch device, 12 AWG.

A few notes... The 3% voltage drop is for critical loads, 10% is allowed for non-critical. Wire length considered for voltage drop is the round trip, not one way. Intermittent panelboard loads are counted as 10% of the their aggregate or the largest single draw, whichever is greater.

 

[FLJetBoater]

I believe your understanding is correct.

The American Boat and Yacht Council (ABYC), the organization that develops standards for boats, states that conductors for the panelboard/switchboard are sized for its total electrical load, whereas branch circuit conductors are sized by their loads.

So in your example, the wires TO the distribution block are sized for the total output from the distribution block, 4 AWG. The wires FROM the distribution block are sized for the branch device, 12 AWG.

A few notes... The 3% voltage drop is for critical loads, 10% is allowed for non-critical. Wire length considered for voltage drop is the round trip, not one way. Intermittent panelboard loads are counted as 10% of the their aggregate or the largest single draw, whichever is greater.

Ok thanks! Yeah I was aware of round trip but struggling to determine where the round trip starts/stops. For the device itself, is the round trip from the block to the device and back to the neg block? Or do you have to count the extra distance to the battery too? That doesn’t make sense to me I think it should be only the distance from the block.

 

[spatty99]

The round trip for the device is from/to the distribution block, not the battery. So no, you don't need to count the extra distance from the battery.