Fusing and busbars

Zil

Just another ass on the web
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dixonge, nice build. My inverter cautions against horizontal mounting because of ventilation fans.
My sorry. ....warns against vertical mounting so stuff can't fall into ventilation fans or vents.
 

Hedges

I See Electromagnetic Fields!
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You'll probably get away with 125A, which exactly meets the usual recommendation, given some assumptions:

900W load,
10V low-voltage disconnect
90% inverter efficiency at full load

900W / 10V / 90% x 1.25 = 125A fuse

My contribution to the recommendation is that battery current isn't DC; at high load the capacitors in inverter can't keep up with current draw, instead looks like a rectified sine wave. RMS average current heating wires and fuses is about 1.12x "mean" average current that delivers power.

900W / 10V / 90% x 1.25 x 1.12 = 140A fuse

The 90% efficiency figure might be generous.
900W is anticipated load; better to use inverter maximum continuous output rating.

Blowing fuses is inconvenient, should only happen if there is a short. Overloads should result in shutdown due to thermal or other sensors.
 

FilterGuy

What, me worry?
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I disagree. A fuse sized to protect the wire is the largest fuse with ampere rating under the wires ampacity. In our mobile, limited power, dc systems voltage drop is a very real issue that must be considered. Even across fuses as well as connection points and wire resistance. If I have a 2/0awg that can dead short over 300 amperes with out overheating there is no reason to fuse that wire at lower amperes.
OK...... to each their own opinion. We can agree to disagree.
 

Zwy

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Jan 3, 2021
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re: wire gauges - I have finally learned the big end of wire gauges and am so relieved to be rid of this confusion! 2AWG, 0AWG, 00AWG, etc. I kept seeing 2/0 and thought it was the same as 2AWG!

re: fusing - I thought that the range of possibilities was from 'possible load' on the low end to 'cable will melt and/or catch fire' on the high end. Common sense says the fuse size should be somewhere in this range.

The wire has to be large enough to handle the load with minimum voltage drop for the length. Even though a wire might be able to carry the amps, the voltage drop might be over 2% and this in turn will cause more amps to flow due to the rules of wattage.

Take for instance a 10ga wire, it is rated for 30 amps. If you double the length from 20 feet to 40 feet at 25 amps, the VD goes from 7.2% to 14.4%. If instead 6ga wire was used, we would see 2.9% VD for 20 feet and 5.7% for 40 feet.

Blue Sea has a chart for critical and non critical circuits. I prefer to have VD as low as possible as it keeps the load such as a blender running efficiently and cooler. More amps means more heat, not just on the wire but the also the load as it has to work harder due to less voltage. For this reason, I oversize the wire for the given load and prefer to match fuse size according to load size and ampacity of the wire. Too large a fuse and you might find under heavy load the fuse might not blow leading to load damage.

My first thought would be to size it about halfway between nuisance tripping and melting.

Yes, that is how to correctly do it.

In my case my average load is 5-10 amps DC (per battery monitior), high end would be 100 amps when running blender (900w AC, 82.8amps DC according to online calculator). So having a 125amp ANC fuse on the 2AWG cable from my battery seems fine, but if it blew a few times I'd go for 150 or 200. Sound about right?

What's the length? Plenty of voltage drop calculators online, here's one. https://www.freeonlinecalc.com/voltage-drop-calculator-dc-ac.html

2AWG at 10 feet, 100 amps would be 2.8% VD at 12v. At 20 feet, it is 5.6%.
Related note - am now officially doubtful if all my big cables are 2AWG vs. 2/0 - need to check this today!
1/0 might be sufficient, it depends on length. All circuits need to be run thru a calculator if they are critical and high load such as an inverter. A light bulb that is LED wouldn't be high load, it wouldn't be necessary.
 

Zil

Just another ass on the web
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Cables and wires have a recommended maximum amperes they can safely carry. It depends on the insulation rating which temperature chart is used. I only use marine grade cable so use 105 degree chart. Such as Blue Sea circuit wizard. Or this; https://marinehowto.com/wp-content/uploads/2018/02/04-Fusing-Voltage-Drop.jpg
The ampacity of a cable has nothing to do with voltage drop. Voltage drop is calculated to find the awg you need for the device.
My 2 awg cable is fused at 200 ampere. My 2/0 awg cable is fused at 300 ampere.
 

dixonge

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ok, FINALLY got this diagrammed out to-scale (approximately) - I *think* everything will fit this way. I hope...

[UPDATED IMAGE]

RV Solar-Page-2.jpg
 

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HRTKD

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What magic allows the two batteries to connect? Have you solved Tesla's wireless transfer of electricity? :)

A disconnect on the portable PV should be included. When I have to pack up my ground deployed PV during the day, I first flip my circuit breaker on the input side of the solar charge controller. Then I disconnect the MC4 connectors. Necessary? I dunno, but at upwards of 80v, it's safer.
 

dixonge

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What magic allows the two batteries to connect? Have you solved Tesla's wireless transfer of electricity? :)
I didn't have room in the drawing for the portable wireless electricity generator tower :ROFLMAO: They were mainly in there to represent space where I am unable to place anything on the wall (too close) - there are 2 sets in serial which are then paralleled together.
A disconnect on the portable PV should be included. When I have to pack up my ground deployed PV during the day, I first flip my circuit breaker on the input side of the solar charge controller. Then I disconnect the MC4 connectors. Necessary? I dunno, but at upwards of 80v, it's safer.
I'm going to have SB50 connectors, max of 35v or so. I was thinking I wouldn't need anything else. Guess I'd have to lay the panels face down to be safe. Or get another switch :(
 

rmaddy

Full-time Solar-powered Trailer Life
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I'd like to see a fuse between the inverter and the positive bus bar.
 

HRTKD

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Not that I'm trying to bust your budget, but now is the time to put in a hard wired EMS. If you spend a lot of time connected to different shore power pedestals, an EMS is highly recommended.


I have the portable version of the above EMS.
 

dixonge

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Not that I'm trying to bust your budget, but now is the time to put in a hard wired EMS. If you spend a lot of time connected to different shore power pedestals, an EMS is highly recommended.


I have the portable version of the above EMS.
We spend as little time as possible connected to shore power
 

dixonge

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Never mind then. I'm in the same boat, so the portable unit stays home most of the time.
I am aware of the issues involved, and we've connected to a few sketchy hookups here and there. But I haven't yet been able to overcome the cost/benefit issue. I should, but probably won't 🤷‍♂️
 

Hedges

I See Electromagnetic Fields!
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You can check voltage and grounding of a pedestal manually before connecting.
A lightning arrestor, transient voltage device like MOV, could be good. You can buy bare MOV cheaply enough from DigiKey.
 

Hedges

I See Electromagnetic Fields!
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Related note - am now officially doubtful if all my big cables are 2AWG vs. 2/0 - need to check this today!

Single wires in free air (not a cable with multiple conductors), 90 degree insulation, 30 degree ambient, 2 AWG is good for 190A. 2/0 good for 300A
Seems like either works from ampacity perspective, and 125A or 150A fuse at battery would protect wire to inverter.
Yours are short enough that IR drop isn't likely an issue if within ampacity limits.


125A is the OCP I use for 2 AWG in cable or conduit (derated lower if other circuits in same conduit). 200A for 2/0 (rounding up from 195A ampacity)
This chart assumes 3 current-carrying wires; with split-phase at most two carry full current, so wire runs a bit cooler.

 

PerryB67

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Sep 27, 2019
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Fuses protect the wires. You haven't protected the wire to the inverter.
I don't see anything with a larger draw than his inverter. The ANL fuse next to the battery should be sized to what the inverter manufacturer recommends, protecting his inverter wires. The smaller devices need their own fuses. I have a 250a T fuse next to the battery, a 30 amp fuse for the solar controller, and Escapes circuit breaker for the WFCO. However we ass u me the ANL sizing next to the battery matches his inverter draw.

Enjoy,

Perry
 
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