Correct wire sizing for DC panel feed?

[zephyr070]

Greetings all, and happy new year!
The re-wiring of my 1989 Hunter Vision 32 has been a long but rewarding road. Cut corners on the part of several previous owners, and on the manufacturer itself, has revealed some pretty "interesting" situations that I have resolved according to ABYC standards and countless hours of reading Nigel Calder. I decided to take the DC rewiring project in phases. Phase I, batteries-to-engine compartment switch, is complete. Phase II involves running new DC panel feeders from that switch forward to the panel at my nav table, and here's where I'd like some feedback.

The cables running from my house bank (FLA 2x6V in series, total 235 Ah capacity) to the main switch are 1/0, only about a 20' roundtrip. The main DC breaker on my panel is rated at 40A, and is a 30' roundtrip from the battery switch. Using Ancor's and/or Blue Sea's wire sizing guide, should I be sizing the wire for a 40A load? I know it should be selected for a 3% voltage drop, being mission-critical, and that I will need to fuse the circuit just beyond switch after installing the new wire, I'm just wondering what size the wire should be? The paralleled 10 gauge wires that currently serve to feed the DC panel don't seem adequate. Of course, I will have to bundle the pos and neg wires, so maximum O.D. is a restricting factor.

I'm confident in the work I've put into the DC system aft of the engine compartment, however, I really want to make sure this next phase is done correctly. Any and all input is welcome!

 

[dlochner]

In addition to reading Nigel's book, spend time on MarineHowTo.com for wiring and electrical system information.

Look at the Blue Seas calculator to properly size the wire. On longer runs and primary runs I like to go up at least one size, from 10 to 8, 8 to 6, because the line loss is cumulative; if you lose 3% from the switch to the panel, and another 3% from the panel to the device, the total loss is more like 5 or 6%.

The circuit protection should be sized to protect the wire. Look at the ampacity charts. If the wire can safely carry 20 amps, then a 20 or smaller fuse can be used.

In your situation with a 40 amp main breaker on the DC panel, I might use cable that can carry 50 amps and then fuse for 40 amps. This will reduce line loss and afford a higher degree of protection due to the larger cable size.

 

[Johnb]

I used #4 wire to run to the DC panel. This may appear over sized but
1 the drop in that section of the wiring affects the entire panel
2 it minimizes annoyances like the lights flickering when the domestic water pump starts etc

The distance is 15 feet one way
PM me if you want the calculations

 

[jssailem]

I am wondering why the batteries, the switch, and the panel are located nearly 25feet apart. granted you calculate the distance in 3 dimensions but still.
I would be looking at a way to reduce that distance.

The chart is suggesting with 50amps 1AWG to limit drop to 3%. Note a switch cabe add to your drop by 300-500mamps.

 

[zephyr070]

I am wondering why the batteries, the switch, and the panel are located nearly 25feet apart. granted you calculate the distance in 3 dimensions but still.
I would be looking at a way to reduce that distance.

The chart is suggesting with 50amps 1AWG to limit drop to 3%. Note a switch cabe add to your drop by 300-500mamps.

View attachment 188824

I'm not sure I'm "calculating the distance in 3 dimensions," as you say, but rather following the suggestions for 3% voltage drop over a particular roundtrip distance for wiring dimensions.

Yes, I also wonder why the hell Hunter decided to make the distance between battery and switch, and switch and DC panel so long, but that's not my mystery to unravel - instead, I need to engineer around it. The batteries are where they are going to be, and the switch and panel are where they are based on spatial restrictions, hence why I'm on this forum, asking this particular question.

Yes, technically, the total round trip distance from the house bank terminal posts to DC main breaker is something like 50', but does that mean that I need to size the wire from battery to panel as #1 the whole way, given that roughly 1/2 that distance is already routed as 1/0? Given this information, do I need to be worried about a .5A addition to my total load?

 

[dlochner]

I'm not sure I'm "calculating the distance in 3 dimensions," as you say, but rather following the suggestions for 3% voltage drop over a particular roundtrip distance for wiring dimensions.

I think @jssailem is referring to the run not being a straight line, rather it goes up and around different parts of the boat. This makes estimating distances a little more challenging.

Yes, I also wonder why the hell Hunter decided to make the distance between battery and switch, and switch and DC panel so long, but that's not my mystery to unravel - instead, I need to engineer around it. The batteries are where they are going to be, and the switch and panel are where they are based on spatial restrictions, hence why I'm on this forum, asking this particular question.

Ahh, the eternal DIY boat owners question, WTF were the designers and builders thinking?

Yes, technically, the total round trip distance from the house bank terminal posts to DC main breaker is something like 50', but does that mean that I need to size the wire from battery to panel as #1 the whole way, given that roughly 1/2 that distance is already routed as 1/0? Given this information, do I need to be worried about a .5A addition to my total load?

Two thoughts. The legs of the circuit do not need to be the same length. It may be possible to route the negative back to the battery or main negative bus bar by a shorter route. If so, then the circuit length will be shorter.

Can you mix and match cable sizes? Yes, sometimes it just make sense to do that. I've done for very short distances because it was impractical or difficult to run a heavier cable. But I try not to do that. Calculate your line loss from the battery to the switch. This will be the starting point for the next segment. If the voltage drop will be from 12.5 to 12.3, then this is the starting voltage for he next leg. Now calculate the line loss for the next leg and when figuring out the final voltage start with 12.3 v (making the math easier to do in my head) a 10% loss would result in a voltage of 12.3 - 1.23 = 11.07 v at the panel. Is this an acceptable level? (Again, I not suggesting you accept a 10% loss on the main cable, just avoiding looking for a calculator.)

 

[jssailem]

The wire gauge needed to address a 50’ round trip run that only has a 3% voltage drop is 1awg. The switch you place in the middle of the run adds a 300-500ma loss to that run.
You already have 1/0 to the switch. So 1awg can get you the rest of the way from switch to panel. Be sure to use 1awg for the return to ground.

 

[Ken Cross]

First off, there should be a difference between the panel circuit breaker and actual current draw. You should never get near the breaker's capacity. You need to size wire for the actual expected load plus a little for the odd what if. That is unless you like difficult to route stiff wire, higher cost, and excess weight. There are trade-offs in everything. Your boat your choice.

 

[dlochner]

First off, there should be a difference between the panel circuit breaker and actual current draw. You should never get near the breaker's capacity. You need to size wire for the actual expected load plus a little for the odd what if. That is unless you like difficult to route stiff wire, higher cost, and excess weight. There are trade-offs in everything. Your boat your choice.

Yes, ideally the cable and main panel breaker should not be at max load. However, the main breaker must be sized to protect the cable, or put another way the cable must be large enough to safely carry the current before tripping the breaker in an over current situation. To do otherwise is to place your boat and other boats in the marina at risk of fire. Some of our projects are about more than just our own boats.

 

[Ken Cross]

Yes, ideally the cable and main panel breaker should not be at max load. However, the main breaker must be sized to protect the cable or put another way the cable must be large enough to safely carry the current before tripping the breaker in an overcurrent situation. To do otherwise is to place your boat and other boats in the marina at risk of fire. Some of our projects are about more than just our own boats.

The current required to cause cable damage is way more than what's required to maintain voltage. As an example in your home, and most others you use 12 gauge cable for 20 amp loads all the time. And for 15 amp loads, some even use 14 gauge. Not a danger to the wire.

I'm suggesting with a 40 amp breaker you don't need to maintain voltage within 3% at 40 amps. You need the 3% at whatever amperage you will likely encounter. So if you do the math at 40 amps you need "0" cable to manage a 3-volt difference over 30 feet, but 12 gauge wire is rated for 41 amps for chassis installations. So anything bigger than 12 gauge will cause no wire damage. The problem with 12 is the voltage drop, not wire damage. You need to know your expected load.

 

[dlochner]

The current required to cause cable damage is way more than what's required to maintain voltage. As an example in your home, and most others you use 12 gauge cable for 20 amp loads all the time. And for 15 amp loads, some even use 14 gauge. Not a danger to the wire.

I'm suggesting with a 40 amp breaker you don't need to maintain voltage within 3% at 40 amps. You need the 3% at whatever amperage you will likely encounter. So if you do the math at 40 amps you need "0" cable to manage a 3-volt difference over 30 feet, but 12 gauge wire is rated for 41 amps for chassis installations. So anything bigger than 12 gauge will cause no wire damage. The problem with 12 is the voltage drop, not wire damage. You need to know your expected load.

I think the cable sizing question is really the last question to be asked because at this point, we don't have enough information to make a good recommendation. There is a difference between what will work, what will work safely, and what will work more efficiently. I have an admitted bias towards efficiency, I don't want to waste electrons on heating wire, when I want them to be doing some other work. From my perspective, we need to start at the very beginning.

First off is a question of efficiency, how much line loss is acceptable between the battery and the panel. While 3% is often cited as acceptable, it should be remembered that line loss is cumulative. Lose 3% between the battery and panel and another 3% from the panel to the device and now there is about a 6% line loss.

Next up is typical loads. How much current is being regularly used, lights, stereos, instruments, etc.

Close behind that question is, what is the maximum that may be used? For example, on my boat when sailing I typically draw between 5 and 10 amps, however, if the refrigeration starts up the draw will increase by 30 amps for a maximum of about 40 amps.

Now we can begin to determine cable size and breaker size. For the 40 amp draw, I would up the max draw a little and call it 50 amps. This yields a safety margin and room for error or unusually high current draws. Since I abhor line loss, I calculate cable sizing to yield less than a 1% line loss.

Now it is just a matter of knowing the wire distance/length and calculating the necessary cable size to yield less than a 1% loss at 50 amps over that distance. The circuit breaker will be a 50 amp breaker. The system will be safe and efficient.

 

[Ken Cross]

I think the cable sizing question is really the last question to be asked because at this point, we don't have enough information to make a good recommendation. There is a difference between what will work, what will work safely, and what will work more efficiently. I have an admitted bias towards efficiency, I don't want to waste electrons on heating wire, when I want them to be doing some other work. From my perspective, we need to start at the very beginning.

First off is a question of efficiency, how much line loss is acceptable between the battery and the panel. While 3% is often cited as acceptable, it should be remembered that line loss is cumulative. Lose 3% between the battery and panel and another 3% from the panel to the device and now there is about a 6% line loss.

Next up is typical loads. How much current is being regularly used, lights, stereos, instruments, etc.

Close behind that question is, what is the maximum that may be used? For example, on my boat when sailing I typically draw between 5 and 10 amps, however, if the refrigeration starts up the draw will increase by 30 amps for a maximum of about 40 amps.

Now we can begin to determine cable size and breaker size. For the 40 amp draw, I would up the max draw a little and call it 50 amps. This yields a safety margin and room for error or unusually high current draws. Since I abhor line loss, I calculate cable sizing to yield less than a 1% line loss.

Now it is just a matter of knowing the wire distance/length and calculating the necessary cable size to yield less than a 1% loss at 50 amps over that distance. The circuit breaker will be a 50 amp breaker. The system will be safe and efficient.

The OP stated he has 1ot (20 feet round trip) cable to the battery switch. 30 feet round trip to the 40 amp breaker.

Now about your boat. You need a new fridge. Ours uses 3.7 amps and per the installation manual, it doesn't run through the panel.

Ken

Ken

 

[dlochner]

The OP stated he has 1ot (20 feet round trip) cable to the battery switch. 30 feet round trip to the 40 amp breaker.

Now about your boat. You need a new fridge. Ours uses 3.7 amps and per the installation manual, it doesn't run through the panel.

Ken

Ken

In terms of daily use, my refrigerator uses about as much energy as yours. Mine will run for less than an hour a day except in the hottest weather or when I've just loaded it up with warm stuff. It has 2 very large cold plates that hold the temperature for 10 -16 hours. When the temp rises about 6 degrees above the set point the refrigerator comes one and runs about 30 minutes or less. Then its good for another 10-16 hours.

It is a water cooled Grunert refrigerator.