|Topic Review (Newest First)|
|02-15-2008 06:50 PM|
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.
|02-15-2008 11:17 AM|
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.
|02-15-2008 10:25 AM|
Originally Posted by rhaley View Post
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 08:52 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 08:31 AM|
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 04:00 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 03:24 PM|
|02-14-2008 02:48 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 02:38 PM|
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 02:23 PM|
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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