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Thread: Basic Electrical Diagram Reply to Thread
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Topic Review (Newest First)
02-15-2008 07:50 PM
tsweeney No worries about the Hijack. In fact as a electrical engineer I can appreciate that indeed that details matter. Indeed I agree that items upstream of the fault are those that are damaged by a fault at there is no potential across the load once a short is established. Indeed motors are a bit different than other generally purely resistive loads. Continue on if you must. I'll post an updated diagram this weekend.

Regards,
Tim Sweeney
02-15-2008 12:17 PM
rhaley
Quote:
Originally Posted by xtatico1404 View Post
You are correct that it will find an alternate path to ground. However, in DC systems components of the system contribute to the short circuit. There are inductors, capacitors, etc, that contribute to the short circuit, and is the reason why equipment fails. All electronic equipment have pretty much all those components.

On AC systems, motors actually become generators during a short citcuit and contribute a good deal of current. I am not sure if the same happens on DC systems, but here is a good link about it http://www.brainfiller.com/documents...ntribution.pdf
How exactly are the components of an electronic device the reason why it fails? In an functioning piece of electronic equipment, all the components you have listed are powered up and operating. If a short occurs between the source and the device, the only repercussion to the electronic components is they will no longer have enough voltage to operate. The capacitors will bleed off their charge and the inductors will drain off their stored current, and then the device will cease to operate until sufficient voltage is provided. For an internal fault inherent to the device itself, then these components may well suffer damage depending on where in the circuit the fault occurs, but we are talking about over current protection for the source. A short circuit damages components UPSTREAM of the fault due the high current demands a short places on the source and connecting components.

The AC motor article is accurate, and agrees with what I am trying to convey. When you have a fault between the source and the motor, the source will dump its energy into the short circuit, and for a few cycles, so will the motors. The article emphasis the importance of sizing the over current protection to be able to handle the fault current from the source PLUS the motor contribution. The locked rotor current from the motor will not harm the motor (for the short amount of time it exists), it draws the same current every time it is started. The over current device(s) are there to protect the feeders to the motors and need to be sized as such.

tsweeney, sorry for the thread hijack, I promise to take this offline from here on out

xtatico1404, I'll be happy to continue offline, feel free to PM if needed.

Ryan
02-15-2008 11:25 AM
xtatico1404
Quote:
Originally Posted by rhaley View Post

If you have a short circuit, it is either in the wiring run to the device, in which case the potential will find an alternate path to ground prior to reaching the load and the only thing that needs protecting is the wire between the source and the fault, or it is in the device itself, which should have it's own protection (ie the small fuses on the back of a VHF, stereo, chart plotter, etc).
You are correct that it will find an alternate path to ground. However, in DC systems components of the system contribute to the short circuit. There are inductors, capacitors, etc, that contribute to the short circuit, and is the reason why equipment fails. All electronic equipment have pretty much all those components.

On AC systems, motors actually become generators during a short citcuit and contribute a good deal of current. I am not sure if the same happens on DC systems, but here is a good link about it http://www.brainfiller.com/documents...ntribution.pdf
02-15-2008 09:52 AM
sailingdog One of the nice things about the BlueSea dual circuit plus battery switches, is that they isolate the starting side from the house side, and when you start the engine, your electronics don't take a hit from the drop in voltage.
02-15-2008 09:31 AM
tsweeney Just a quick update as I've yet to fully digest everything but appreciate all the advice.
Indeed the two group 31s are in parallel as a singe bank.
The acr would be the 120Amp model (the charger on the inv/charger is ~50A)
The battery switch (A dual circuit plus?) allows combining the house and start batteries when neccessary.
Fuse and wire size issues are still pending.
It's good to know that I could switch the XBM to get basic stats on the house bank.

Thanks again folks.
02-14-2008 05:00 PM
sailingdog One other thing... make sure that the ACR unit you're using is rated for the amperage of the inverter/charger or alternator, which ever is higher output. They have three models, rated at 60, 120 and 300 amps... and if you've got the long rectangular one, that is only rated at 60 amps. The square one is rated at 120 amps, and is the one that comes in the Dual Circuit Plus kit with the battery switch.
02-14-2008 04:24 PM
rhaley
Quote:
Originally Posted by xtatico1404 View Post
Wire is sized based on the load and taking into consideration voltage drop, temperature & bundling, which might require an increase in wire size.
Agreed.

Quote:
Originally Posted by xtatico1404 View Post
Depending on the location of the protection, the fuse size will be sized to the wire, but most of the time will be sized to the load as you'll see.
As you said above, the wire size is based on the load. Short circuit protection is based on the wire size. Obviously they are related, but there is a difference.

Quote:
Originally Posted by xtatico1404 View Post

The fuse for the battery is chosen based on the wire size..
Agreed, as should all other branch protection.

Quote:
Originally Posted by xtatico1404 View Post
My example called for 5 Amps of load, on a #18 wire, which allows 20 Amps. If I were to size my fuse to the wire (20 Amps), then I will not be protecting the load at all.
80% of 20A I agree you are not protecting the load with this fuse, nor is that fuse intended to protect the load.

Quote:
Originally Posted by xtatico1404 View Post
If my short circuit happens to draw 18 Amps, the 20 Amp fuse will never blow and the equipment connected to the circuit will be damaged
If you have a short circuit, it is either in the wiring run to the device, in which case the potential will find an alternate path to ground prior to reaching the load and the only thing that needs protecting is the wire between the source and the fault, or it is in the device itself, which should have it's own protection (ie the small fuses on the back of a VHF, stereo, chart plotter, etc).

Quote:
Originally Posted by xtatico1404 View Post
The fuse size cannot be larger than the current carrying capabilities of the wire, you are correct on that. The fuse for any particular circuit on the panel will have to be at least equal to the sum of all loads on that particular circuit.
Agreed, or else you'll wind up with nuisance trips/fuse failures.

Quote:
Originally Posted by xtatico1404 View Post
The 80% rule of thumb is a design standpoint type of thing and it always applies to the wire, and in the end to the fuse/breaker if it is sized to the wire.
It takes into account that you do not want to run things constantly at 100% capacity...sort of like you do not run your engine at 100% power. The thing is that as current passes through the fuse, heat builds up and degrades the fuse over time, which is why you have to replace them every 5-10-15 years. Most of the time you have to replace it not because of a short circuit, it's that mechanically, it was no longer good. It also takes into account that you might add future loads to a circuit and that there will probably be multiple loads on a circuit. It's just the accepted design philosophy.
I agree that mechanical failure is much more common in fuses than most realize, particularly on boats. My personal and professional design philosophy, from this standpoint, is that the wire size is chosen to account for future additions to the circuit, not the fuse size.

Quote:
Originally Posted by xtatico1404 View Post
Once again, My recommendation is to read the manufacturer's recommendations to wire & fuse size. Their recommendation already have in place all the 80% bull^%$, etc, and you just have to forget about all of it. This is especially true for load that have electric motors on them, where things change big time.
I agree that the manufacturer is your best source of information on wire (assuming it is a dedicated circuit) and fuse sizes, with the caveat that the fuse size they will recommend will be to protect their device, and as such will be located at said device, not at the source.
02-14-2008 03:48 PM
xtatico1404 chuckles I took it as him having two batteries in parallel to increase the capacity of the house bank. Certainly that's the way to go, one instead of two house battery banks....BTW, your electrician has a very informative website.
02-14-2008 03:38 PM
xtatico1404 rhaley, after re-reading my answer, I can see that some confusion might come up and I understand your point. Hopefully this will clear things up.

Fuses are meant to protect the wire and indirectly the load.

Wire is sized based on the load and taking into consideration voltage drop, temperature & bundling, which might require an increase in wire size.

Depending on the location of the protection, the fuse size will be sized to the wire, but most of the time will be sized to the load as you'll see.

The fuse for the battery is chosen based on the wire size.

The DC Panel Main Breaker is chosen based on the total load that the panel will handle, 50A, 100A, etc.

From this point on is where it is very important to size your fuse to the loads. All fuses/breakers except the battery & DC Main breaker fall under this catergoy, which is why I said that typically you size the fuse to the load, not to the wire. My example called for 5 Amps of load, on a #18 wire, which allows 20 Amps. If I were to size my fuse to the wire (20 Amps), then I will not be protecting the load at all. If my short circuit happens to draw 18 Amps, the 20 Amp fuse will never blow and the equipment connected to the circuit will be damaged

The fuse size cannot be larger than the current carrying capabilities of the wire, you are correct on that. The fuse for any particular circuit on the panel will have to be at least equal to the sum of all loads on that particual circuit.

The 80% rule of thumb is a design standpoint type of thing and it always applies to the wire, and in the end to the fuse/breaker if it is sized to the wire. It takes into account that you do not want to run things constantly at 100% capacity...sort of like you do not run your engine at 100% power. The thing is that as current passes through the fuse, heat builds up and degrades the fuse over time, which is why you have to replace them every 5-10-15 years. Most of the time you have to replace it not because of a short circuit, it's that mechanically, it was no longer good. It also takes into account that you might add future loads to a circuit and that there will probably be multiple loads on a circuit. It's just the accepted design philosophy.

When talking about boats, you want your system set up as individually as possible. You want one circuit for bilge pumps, another for lighting, another for heater/cooler, VHF radio, etc, which is why most of the time we see a fuse/breaker sized to the particular load.

Once again, My recommendation is to read the manufacturer's recommendations to wire & fuse size. Their recommendation already have in place all the 80% bull^%$, etc, and you just have to forget about all of it. This is especially true for load that have electric motors on them, where things change big time.
02-14-2008 03:23 PM
chucklesR I don't think your ACR is wired correctly. Either way two suggestions,
1. Combine your two Group31's into one bank - it's far better to have one large bank as compared to two smaller ones. There are many threads here discussing that.
2. Having a ACR does not obliviate the need for a battery isolater switch. I also prefer and emergency combiner switch, nothing worse than calling Seatow after a night of too much music and lights.

BTW the XBM works fine, and can be wired to monitor both starter and house banks (the rocker works both ways, you can cheat it).

My simplified version (from www.pkys.com - my electrician):
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