Circuit Breaker Amps re Fusebox Rated Amps

chadmichael

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I'm building something much like the 400W Classic from Will's book. But this question is more specific.

I'll have a dedicated circuit from the battery array's busbar to my fusebox. Fusebox is rated to 150Amps. I suppose that means something about the busbars internal to it. I'm using a less than 18" piece of 4 AWG to wire this; so, according to the chart in Will's book, this wire can carry up to 300 Amps. I'm using the fatter than necessary wire cuz I have it, and I understand bigger copper is never bad (correct me if I'm wrong; I already know it's unwieldy if your in cramped space).

My question is ... can I put a 150 Amp Circuit Breaker on this circuit? It definitley protects the much larger amperage of the wire, but am I allowing to many amps to hit the 150Amp fusebox?
 

timselectric

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I think that you are confusing 4 awg with 4/0 awg.
If you are using 4/0 , then you can use a 150a breaker. (Limited by the 150a panel rating)
If you are using 4 awg, then no larger than a 100a breaker.
These are approximations, because it varies depending on the type of cable.
But, the panel is fine with 150a.
 

chadmichael

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I think that you are confusing 4 awg with 4/0 awg.
If you are using 4/0 , then you can use a 150a breaker. (Limited by the 150a panel rating)
If you are using 4 awg, then no larger than a 100a breaker.
These are approximations, because it varies depending on the type of cable.
But, the panel is fine with 150a.
I'm reading from a chart. Perhaps incorrectly. It's a very short piece of 4 AWG, and the chart says it's good for 250-300A, and it does say that if it were 20 feet long, it would only be able to carry 105 Amps.
 

timselectric

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If you feel comfortable with it.
Length does affect its ability.
Also depends on what you are actually going to be putting on it continuously.
I personally wouldn't put 150a on a #4.
 

Zil

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The length will affect voltage drop, not ampacity. Marine Grade 4 awg wire can carry up to 160 amperes. No matter the length.
M.G. 4/0 awg cable can carry up to 445 amperes. No matter the length. The length affects voltage drop.
 

timselectric

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It actually affects both. Or is affected by both.
Depending on how you want to look at it.
The size of the conductor is calculated on what voltage and amperage is required at a given length.
Raising the voltage or lowering the amperage, will allow a smaller conductor to be used.
 

chadmichael

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I think that you are confusing 4 awg with 4/0 awg.
If you are using 4/0 , then you can use a 150a breaker. (Limited by the 150a panel rating)
If you are using 4 awg, then no larger than a 100a breaker.
These are approximations, because it varies depending on the type of cable.
But, the panel is fine with 150a.
Hey... I'm not trying to push back or anything but this is kind of important. Using the caclulator on this page ... a 4AWG pure copper wire, under 4 feet in length, with 2% loss, can actually carry more than 150 amps ... it actually says a 6awg can carry 150 amps at 4 feet.

If you think this is incorrect, I'd like to know more cuz I don't want to burn my place down :)
 
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Hey... I'm not trying to push back or anything but this is kind of important. Using the caclulator on this page ... a 4AWG pure copper wire, under 4 feet in length, with 2% loss, can actually carry more than 150 amps ... it actually says a 6awg can carry 150 amps at 4 feet.

If you think this is incorrect, I'd like to know more cuz I don't want to burn my place down :)
The ampacity limits table at this link agrees.

Pure copper wire with insulation rated for 105C is good for 160 fault amps un-bundled, in open air outside of engine spaces.
Since the closest fuse size under 160 amps is 150 amps then we are limited to 150amps.
150 amps * .8 fuse headroom = 120 service amps.

The next step is to factor for distanced induced voltage drop.
The calculator at the top of the page does that well.

That page is based on ABYC regs, NEC regs are usually a bit more conservative.
 

timselectric

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Hey... I'm not trying to push back or anything but this is kind of important. Using the caclulator on this page ... a 4AWG pure copper wire, under 4 feet in length, with 2% loss, can actually carry more than 150 amps ... it actually says a 6awg can carry 150 amps at 4 feet.

If you think this is incorrect, I'd like to know more cuz I don't want to burn my place down :)
I don't see it as pushing back. Just asking for more information, or clarification.
And yes, the right conductor, with the right insulation, installed in the right environment, can carry a higher amperage.
But, it's too close for my comfort.
There's a lot of times when Just good enough will do.
But, I do this for a living. And my reputation relies on minimal failure rates. So, I tend to always go an extra click on everything.
 

chadmichael

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One thing I'm having trouble understanding is what the ampacity rating actually is. It's not actually the limit of the conductors ability to carry current, it seems, but more like the amps at which the insulation will melt away and expose a very hot copper wire to your flammable ... is this correct?
 

timselectric

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One thing I'm having trouble understanding is what the ampacity rating actually is. It's not actually the limit of the conductors ability to carry current, it seems, but more like the amps at which the insulation will melt away and expose a very hot copper wire to your flammable ... is this correct?
It's the level of heat created on the conductor.
Which breaks down the insulation and loosens connections from expansion and contraction of the conductor.
 

chadmichael

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Next issue ... :)

I've decided that the 2AWG pure copper is enough to supply my 12v 150Amp fuse block. I intend to put a 150 Amp circuit breaker in the circuite as well.

The issue is that the fuse block itself, a Blue Sea product recommended by Will, seems to have terminals even smaller than a 1/4". Additionally, the circuit breaker has 1/4" terminals itself. Yet, my boxes of miscelleneous connectors seems to expect much larger terminals for 2 AWG. In fact, the smallest ring connector has a 5/16th diameter hole. While this will technically fit onto the terminal of the fuseblock, it doesn't seem quite right.

1) Am I doing something against the grain?
2) Do I just need to locate 2 AWG ring connectors in the 1/4 size?
 

HRTKD

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Next issue ... :)

I've decided that the 2AWG pure copper is enough to supply my 12v 150Amp fuse block. I intend to put a 150 Amp circuit breaker in the circuite as well.

The issue is that the fuse block itself, a Blue Sea product recommended by Will, seems to have terminals even smaller than a 1/4". Additionally, the circuit breaker has 1/4" terminals itself. Yet, my boxes of miscelleneous connectors seems to expect much larger terminals for 2 AWG. In fact, the smallest ring connector has a 5/16th diameter hole. While this will technically fit onto the terminal of the fuseblock, it doesn't seem quite right.

1) Am I doing something against the grain?
2) Do I just need to locate 2 AWG ring connectors in the 1/4 size?

I have made my own bus bars out of copper flat bar. I chose to use 1/4" bolts to hold the 2/0 cable lugs to the bus bars. For my situation, 1/4" studs are just fine. I'm managing the strain relief on the cable/lug to ensure there is no strain, so I don't need a large stud (5/16" or 3/8") that can provide a much higher level of clamping.

In my system, some off-the-shelf components use 3/8" studs and others use 5/16". I keep a supply of all three sizes. No big deal for me.

My Blue Sea Class T fuse holder uses 3/8" studs as I recall. A fuse block that has studs smaller than 1/4" probably isn't rated for the level of amps that a 3/8" stud fuse holder is.
 

Zil

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It actually affects both. Or is affected by both.
Depending on how you want to look at it.
The size of the conductor is calculated on what voltage and amperage is required at a given length.
Raising the voltage or lowering the amperage, will allow a smaller conductor to be used.
And that is why we use Voltage Drop Tables to select the wire gauge.
 

Zil

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If you use larger than 4 awg up the size of the fuse to the ampacity of the wire. The ampacity of the wire is the SAFE current the wire can carry. Not the melting point of the insulation. Larger fuses have less voltage drop.
 
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