Author Topic: Wiring sizing  (Read 144 times)

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Todd j

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Wiring sizing
« on: April 22, 2022, 11:00:04 AM »
Anyone else question recommended wire sizing on branch circuits?   Seems way big to me.  I guess the charts are right.   I bought a pre made switch panel with resettable breakers.   It is forcing me to run larger wires than I would have otherwise.  Just something to think about that I let get by me.

Brian.Dixon

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Re: Wiring sizing
« Reply #1 on: April 22, 2022, 03:57:46 PM »
Branch circuit wires are sized for the combined max currents of the items on the circuit.  Branch circuit protection is sized to protect the wire (not devices).  Devices can be, and typically are, further protected by an inline fuse or a small circuit breaker (usually in the device) .. and that protection is device protection, not wire protection.  Feedline to the branch circuits terminals has its own breaker sized for its ampacity and is sized for the combined max ampacities of all the branch circuits that it powers.  These rules CAN result in larger than expected wires.  Another approach is to limit wire size and the number of amps a branch can handle, then add more branch circuits if you have more needs.
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Rbob

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Re: Wiring sizing
« Reply #2 on: April 22, 2022, 10:37:21 PM »
Blue seas has a great app for wire size, fuse selection and circuit protection. In "circuit protection you can choose branch or main circuit.  It is helpful. 

https://play.google.com/store/apps/details?id=com.bluesea.android.circuitwizard&gl=US

Blue Seas Circuit Protection part 1 and part 2 more in depth:
https://www.bluesea.com/support/articles/Circuit_Protection/1441/Part_2%3A_Select_a_Fuse_and_Fuse_Holder_For_Your_DC_Product_Installation

Dan Boccia

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Re: Wiring sizing
« Reply #3 on: April 23, 2022, 10:44:27 AM »
There are a lot of ways to end up with an undersized wire, which can lead to excess heat, which can melt the wire jacket, which can cause a short, which can cause a fire. Another leading cause of boat fires is lack of understanding of the explosivity of gasoline vapors and understanding of where to use ignition-protected devices. I am currently rewiring an ENTIRE boat, direct from the builder, because there were dozens of violations that all added up to that boat being a literal explosion waiting to happen.

I use the Blue Seas app on my phone regularly when I'm designing or wiring a boat. This app will tell you whether the ampacity of the wire or the voltage drop along the wire is the limiting factor. Most often, wire sizes have to go up due to voltage drop. Especially on circuits feeding electronics, you want to limit voltage drop to 3% or less, and that's even more critical when the house batteries are AGM or lead acid, because the voltage of these batteries may drop down substantially as they are discharged, whereas lithium batteries supply near constant voltage around 12.8 volts throughout their discharge cycle.

If you push the envelope and use a smaller wire than recommended, right there you risk a fire on your boat, period. Pay attention to how many wires are in a bundle, because it DOES matter. I often use the "7 to 24" wires in a bundle because often the wire runs through the main wire bundle on the boat. Also, the duration of the load - for instance a bilge pump should use a duration of a few to several hours if you really want it to be able to bail you out and get back to safe harbor.

Finally, a really little known problem, one of the few things that I don't recall reading about in Nigel Calder's book: The temperature rating of the device you're connecting to. Take for instance a very common Blue Sea A-series breaker. Look at the specs, and it will say "maximum operating temperature 85C". If you're using wire that is rated to 105C, and connecting that wire to this breaker that is rated to 85C, you must use the lower value to size that wire. In the Blue Sea app, you'd select 80C. Otherwise, you'll overheat the connection point at the breaker, which is typically in a congested area where you cannot afford to have an overheated device.

Another problem that catches people is the ampere interrupting capacity (AIC) rating of the breaker or fuse. If you use a device with an AIC rating that is too low for the power source that powers that wire, in a fault scenario, power could arc across the points of the breaker and literally weld the breaker shut, and now you have no circuit protection and you'll likely melt down the wire, causing a fire. Same with fuses. Read up about this in Nigel Calder's book, especially if you have lithium batteries!!

The physical act of wiring a boat is approachable to everyone with some care. Sizing wires and specifying circuit protection is a further stretch that requires focused study, an attitude of curiosity, and training/mentoring. Luckily, basically all the answers are in Nigel's book, and further explanations are available online, for instance some of the excellent material available at Pacific Yacht Systems.

Brian.Dixon

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Re: Wiring sizing
« Reply #4 on: April 23, 2022, 12:06:20 PM »
<snip>
Finally, a really little known problem, one of the few things that I don't recall reading about in Nigel Calder's book: The temperature rating of the device you're connecting to. Take for instance a very common Blue Sea A-series breaker. Look at the specs, and it will say "maximum operating temperature 85C". If you're using wire that is rated to 105C, and connecting that wire to this breaker that is rated to 85C, you must use the lower value to size that wire. In the Blue Sea app, you'd select 80C. Otherwise, you'll overheat the connection point at the breaker, which is typically in a congested area where you cannot afford to have an overheated device.

Another problem that catches people is the ampere interrupting capacity (AIC) rating of the breaker or fuse. If you use a device with an AIC rating that is too low for the power source that powers that wire, in a fault scenario, power could arc across the points of the breaker and literally weld the breaker shut, and now you have no circuit protection and you'll likely melt down the wire, causing a fire. Same with fuses. Read up about this in Nigel Calder's book, especially if you have lithium batteries!!
<snip>

Wow ... great stuff, Dan.  I didn't know about these two things.  And my brother's never returned my books on corrosion and Nigel's electrical book ... now claims he didn't borrow them grumble grumble grumble ... need to go buy the latest versions ...
The Great Alaskan - Professional performance - Easy to build! - https://www.glacierboats.com  ><((((> .`.><((((> .`.><((((> .`.><((((>

Todd j

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Re: Wiring sizing
« Reply #5 on: April 25, 2022, 03:28:52 PM »
All, thanks for the tips

tom e

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Re: Wiring sizing
« Reply #6 on: April 25, 2022, 08:48:59 PM »
My 2 cents, and you get what you pay for --

I wired up a wire size or two not because of the amps, but purely because of the voltage drop over a long distance and at the same time forgot to include the return leg.

The other place I occasionally over looked was actually running some scrap wire thru the same curves, holes and other not straight runs to account for the actual length of the path to be used.  I confess: I got lazy and tried to eye-ball it, that almost never worked out.  <shrug>

Best of luck to ya.
Building a Tolman Jumbo, but there's a lot of ideas and recommends here that apply to a Jumbo.

json

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Re: Wiring sizing
« Reply #7 on: April 26, 2022, 07:55:16 AM »
This thread is making me wonder how many runs I undersized. I definitely tried to use the sizing for amps/voltage drop as a guide, but probably only considered half the run length on some of them, and definitely didn't consider some of the other things brought up above. No time like the present to rehash the past!

Dan Boccia

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Re: Wiring sizing
« Reply #8 on: April 27, 2022, 12:05:50 AM »
Json, good point, the entire length of the circuit, positive & negative, must be included in the calc. Wise placement of negative busses (you get one for free with most breaker panels in the helm area, place one wherever the start batteries are, and one in the transom) helps limit total circuit lengths. This brings up another thing I am seeing regularly: bus bars that are too small...you have to size them for the max current that could run through them. All the negative busses get tied together with appropriately-sized cables. The transom bus is typically where negative, bonding, and ground wires are all tied together, so all these are at the same ground potential as the outboard engine. Aluminum fuel and water tanks, fuel fill fittings, and any through-hull seacocks are what typically get "bonded", whereas you typically get a ground from the VHF radio and sometimes other electronics, plus any inverters/AC systems. The only place these should tie together with the negative system is in the "final" bus, which again is typically in the transom for outboard boats. As usual, this is well-covered in Calder's bible.