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Old 30-10-2013, 19:08   #1
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Voltage Drop Across a Breaker

Im getting ready to install the electrical for my refrigerator and I have some questions.
My boat, Orion, is wired for 12V. I have two 8D gel batteries wired in parallel. The wire run from the battery to the compressor is 15 feet. I plan to use 6 AWG wire and connect the compressor directly to a battery (suggested in the installation instructions) rather than to the bus at my breaker panel. This is suggested as a way to reduce voltage drop.

The installation instructions suggest using a 30 amp inline fuse (as close to the battery as possible) and a switch but Id like to use a 30 amp breaker instead. This seems like it would reduce the number of connections and would be more convenient because the switch would fit into my breaker panel. The 30 amp breaker my panel takes has a maximum resistance of 0.003 Ohms +- 30% (breaker data sheet attached). The breaker has a #8 screw, which seems small compared to the #6 wire...

My questions:

Given the same temperature range, will the voltage drop from this breaker be significantly more than an inline fuse and a switch?

Is there a breaker that would be better suited for this installation?

How much voltage drop is detrimental and/or makes a system less efficient?

If I were to use a fuse, what type of fuse would you recommend?

With the batteries being connected in parallel does it matter which of the two batteries I connect the compressor to? The one that connects to the breaker bus is a further run than the other battery connected in parallel.
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File Type: pdf Breaker Datasheet.pdf (90.1 KB, 26 views)
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Old 30-10-2013, 19:30   #2
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Re: Voltage Drop Across a Breaker

I ran the power to my compressor from the DC panel off a 15 amp breaker and so far no problems. I think the drop across the breaker would be insignificant, especially since you're using 6 gauge wire.

I am also about 15' from the panel to the compressor but I do run a large cable from the battery to the panel so very little voltage drop at the panel.
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Old 30-10-2013, 19:51   #3
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Voltage drop across breaker should be insignificant. As long as the cable and components are sized properly for the distance then voltage drop should not be a big issue. Going thru properly sized bus bar, breaker, etc should create negligible voltage drop...cable run is potentially a much bigger factor.

But, 30 Amps for a fridge...wow...what are you installing?
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Old 30-10-2013, 20:19   #4
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Re: Voltage Drop Across a Breaker

i installed an adler barbour ten years ago. following the instructions, i wired directly to the battery bank (four golf carts) using the 10 guage wire recommended for the 12 foot run. rather than fuse it, i put in an ordinary home depot circuit breaker of the recommended size - 15 amp i think. i use the breaker as an on-off switch.

ten years later it's still working...
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Old 30-10-2013, 20:30   #5
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Re: Voltage Drop Across a Breaker

Look at it from another view. Any voltage drop represents a resistance. This resistance will dissipate power. The formula is R x I^2 = P if you know the resistance or if you measured the voltage drop then V x I = P. If you have enough power dissipating the breaker will trip or burn up. The breaker should have a very low resistance or else it is faulty.

A good breaker should be fine.
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Old 30-10-2013, 20:46   #6
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Slightly off point, but many (though not all) references recommend sizing OPD based on wire size. For 6 AWG, single strand, outside the engine room, that's 125 amp. Since manufacturer recommends 30 amp OPD, that's probably based on much smaller than 6 gauge wire, which implies that current is fairly low, which implies that voltage drop will be insignificant in 6 AWG.

http://assets.bluesea.com/files/reso...ence/20010.pdf
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Old 30-10-2013, 20:50   #7
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Re: Voltage Drop Across a Breaker

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Originally Posted by belizesailor View Post
But, 30 Amps for a fridge...wow...what are you installing?
Kinda wondered that myself. I think the max I've seen from my BD50 compressor was 6-7 amps on startup with a totally warm box.
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Old 30-10-2013, 21:01   #8
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Re: Voltage Drop Across a Breaker

How far from your battery to the breaker panel?

I think what the specs. call for is a fuse or breaker as close to the battery as possible. Usually this is off the positive bus bar near the battery enclosure.
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Old 30-10-2013, 21:59   #9
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Re: Voltage Drop Across a Breaker

if you are running direct to battery. and want to be able to turn it off and on. with larger wire. use one of these

187-Series Circuit Breaker - Surface Mount 30A - Blue Sea Systems


if you have heavy wire feeding your main dc panel. I would probably just put a reg breaker in there.
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Old 31-10-2013, 00:13   #10
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Re: Voltage Drop Across a Breaker

Thirty amps does seem like a lot, I had to look a couple times to be sure I was reading the manual correctly. But that's what it calls for. The system uses a Danfoss BD-80. The manual says to use a standard 30 amp automotive fuse and to put it close to the battery. The battery is about 6 feet from breaker panel. I'll have to look at the wire size that feeds the DC panel, if it's large enough I can go from there.
The 187-series Blue Sea breaker looks like it will work well for #6 wire, thanks for the tip on that. The Blue Sea chart suggests a Maxi fuse which I'll consider as well.

If the batteries are connected in parallel does it matter which battery I connect it to?
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Old 31-10-2013, 00:53   #11
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Re: Voltage Drop Across a Breaker

I had some problems with the fridge dropping out when the inverter came on, assumed it was the inverter dropping the voltage to far. Turned out it was that when the fridge turned on the high start up current caused a voltage drop a cross the plug and socket at the fridge (it's a portable to can be unplugged from the boat) Normally it only draws about 2.5a but at start up I think was momentarily drawing 15-20 so became very sensitive to line resistance. Changed plug for a heavier one and the problem when away. Think that's why they say heavy wire and minimal connectors
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Old 31-10-2013, 01:22   #12
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Re: Voltage Drop Across a Breaker

with the maxi fuse you'd still need a swtich (you said you wanted one) with that breaker it'll do both.
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Old 31-10-2013, 08:17   #13
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Re: Voltage Drop Across a Breaker

Quote:
Originally Posted by jonahmulski View Post
If the batteries are connected in parallel does it matter which battery I connect it to?

i've got a few things directly connected to the battery bank - fridge, autopilot, solar panel. i connect the pos and neg wires at opposite ends of the bank; in your case, i would put the neg on the neg of one battery and the pos on the pos of the other battery.

i don't know dirt about electricity but i always get the feeling that if i just connect to one battery i will somehow not be using the other battery, even though they are directly connected.

maybe someone who really understands this stuff will have better advice...
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Old 01-11-2013, 05:26   #14
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Re: Voltage Drop Across a Breaker

If you're going to use AWG6 wire and connect directly to the house bank, be very careful of the type of fuse or breaker you choose. To be ABYC compliant, you must use one of three types of fuses:

ANL, MRBF, or Class-T

The MAXI fuses and "standard automotive fuses" are NOT appropriate, as they do not have a high enough ampere interrupt capacity (AIC) to be used in this way.

The above-referenced breaker is the only one readily available now which has a high enough AIC to connect directly to the house bank, i.e., at least 5,000 amperes.

Devices without sufficiently high AIC can fail in a shorted condition, causing a direct short and, possibly, a fire. That's the reason for the standard.

FWIW,

Bill
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Old 01-11-2013, 07:49   #15
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Re: Voltage Drop Across a Breaker

That compressor running high is listed for 14 amps. Start up could be close at 30 amps.
It really does not matter which battery in a two unit parallel bank you attach.
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