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Old 03-20-2017, 08:27 AM   #1
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Default M400/4 wire size

I purchased an M400/4 to run off of a Fusion UD650. Pushing 2 MX650s and 2 MX770s. JL says to run to run a 40A fuse and 4 gauge wire for the amp. With my current electronics, I have a 50A breaker and it have never tripped. My boat has a 6 gauge wire from the charging relay to the electronics. This is would be VERY hard if not impossible to replace or upgrade. When I run wire size calculators, it shows I will be fine with 90 amps @ 12V. My question is: Is this just a CYA thing from JL or is there is a really good reason to run 4 gauge wire?
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Old 03-20-2017, 10:17 AM   #2
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Yes, there is good reason. A reduction in the pos/neg supply cables means a voltage drop when driven hard. That means less amplifier power, less operating efficiency, and greater thermal stress.
The correct size cable calculation is based on both the current demands and the length. Your circuit will be the total of the pos/neg cable lengths. 6 gauge might be fine for 4 feet + 4 feet, but not for 15 ft. + 15 ft. at the current the amplifier might draw. The existing breaker and cable is suitable for the existing draw, but not the additional draw. It matters if you want to get all the power that you paid for.
You could cheat if you want because you are driving an easier 4-ohm load all around instead of a more demanding 2-ohm load which would draw more current. But I wouldn't personally.
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Old 03-20-2017, 10:21 AM   #3
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Btw, you add, not replace. The intention is to keep separate circuits.
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Old 03-20-2017, 10:37 AM   #4
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You didn't say how long the wire run is, which makes a difference in voltage drop.

If less than 15' from the source (30 ft total wire), then the voltage drop difference between the two wire sizes (4ga and 6ga) is only about 0.12v. And that is if the UD650 is putting out every ounce of power it can deliver.

EDIT... oops... I somehow misread the OP and thought you/he was replacing a receiver with the UD650 instead of adding the M400/4 to the existing UD650.

That said, you'd still be OK with the M400/4 because it's max output at 12.5v and 4ohms is not much more than I was looking at for the UD650 (which was peak not RMS). Refiguring the voltage drop difference.. it came out to 0.15v difference.

If you want to push the M400/4 to full power, you'd be better off putting in a DC-DC converter and running the output at 14.4vdc into the M400/4. This way the voltage drop on the 6GA wire doesn't come into play at all and you'd get 10-25% more power out of the M400/4.

Last edited by km1125; 03-20-2017 at 10:52 AM.
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Old 03-20-2017, 10:45 AM   #5
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Originally Posted by David Analog View Post
Yes, there is good reason. A reduction in the pos/neg supply cables means a voltage drop when driven hard. That means less amplifier power, less operating efficiency, and greater thermal stress.
The correct size cable calculation is based on both the current demands and the length. Your circuit will be the total of the pos/neg cable lengths. 6 gauge might be fine for 4 feet + 4 feet, but not for 15 ft. + 15 ft. at the current the amplifier might draw. The existing breaker and cable is suitable for the existing draw, but not the additional draw. It matters if you want to get all the power that you paid for.
You could cheat if you want because you are driving an easier 4-ohm load all around instead of a more demanding 2-ohm load which would draw more current. But I wouldn't personally.
I understand, I will be changing out the breaker to a 100A. Thanks for the help. I am just trying to figure out if it makes sense to try and replace it, I think it will be a serious PIA. With that said, if I have 50A now PLUS the amp needs 4 gauge, do I need to go bigger? I guess it depends on what voltage drop I assume. Total run is 10' so I am using 20' for the positive plus negative runs. If I take 10%, 4 AWG is fine, if I choose 3%, I need 2 AWG!?!? This sounds like overkill to me.
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Old 03-20-2017, 10:47 AM   #6
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Originally Posted by km1125 View Post
You didn't say how long the wire run is, which makes a difference in voltage drop.

If less than 15' from the source (30 ft total wire), then the voltage drop difference between the two wire sizes (4ga and 6ga) is only about 0.12v. And that is if the UD650 is putting out every ounce of power it can deliver.
Its about 20' round trip (positive and negative). Keep in mind the UD650 is not putting out much power - I am not using the internal amp at all. Only the pre-outs to the JL M/400/4.
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Old 03-20-2017, 12:29 PM   #7
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Its about 20' round trip (positive and negative). Keep in mind the UD650 is not putting out much power - I am not using the internal amp at all. Only the pre-outs to the JL M/400/4.

Thanks.. especially after your post about changing to a 100A breaker, I was wondering.

Why are you changing the breaker? The amp has got a total draw of 40A, right? What else is on the circuit?

I can see (perhaps) going to a larger breaker, but 100A seems like WAY too much!
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Old 03-20-2017, 12:49 PM   #8
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Originally Posted by km1125 View Post
Thanks.. especially after your post about changing to a 100A breaker, I was wondering.

Why are you changing the breaker? The amp has got a total draw of 40A, right? What else is on the circuit?

I can see (perhaps) going to a larger breaker, but 100A seems like WAY too much!
Because I only have one run of wire to my electronics (that includes the amp). Where I previously had a 50A breaker, I will be adding the amp which requires 40A (I will have a 40A fuse right before the amp). That is 90A and they dont make that so I am going 100A.
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Old 03-20-2017, 12:51 PM   #9
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Keep the amp breaker an amp breaker. 40A breaker for M400/4
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Old 03-20-2017, 01:02 PM   #10
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Keep the amp breaker an amp breaker. 40A breaker for M400/4
I have 1 run that I do not think I can replace. I MAY be able to but it will be hard. It currently has a breaker on it. I dont want to put a 40A amp and fuse behind that without upping the main breaker.
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Old 03-20-2017, 02:44 PM   #11
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But (I'm guessing) most of that 50 Amp was used/dedicated to the UD650 output, which you are replacing with the M400.

What else is on the "electronics" circuit that is now fed with the 50A breaker? You want to add that max current draw up (minus the output stage of the UD650) and add the 40A to that. I'll bet you're closer to 60A total.

That breaker protects the 6GA line. Anything else connected to the 6GA should have it's own protection (inline fuses or fuseblock) to protect the wire feeding the device and the device.
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Old 03-20-2017, 11:50 PM   #12
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I can feel your pain about running wiring but sometimes you have to. A 20ft run of cable pretty much calls for 4ga. 6ga would work if the room is tight. Combining this with what you already have is going to give you some issues. First off the possibility of adding noise to your system is pretty high. You need to isolate your ground to batt and tie batts together.
Another issue will be as that you are sharing a house load that could already be 20amps with the 400 that can pull some decent amps on its own. You have a lot of current flowing down that line and you may see things flicker but your not running a sub so I doubt you will see this. Soap up some 6ga and get some fiberglass wire fishes and take your time.
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Old 03-22-2017, 05:46 AM   #13
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I can feel your pain about running wiring but sometimes you have to. A 20ft run of cable pretty much calls for 4ga. 6ga would work if the room is tight. Combining this with what you already have is going to give you some issues. First off the possibility of adding noise to your system is pretty high. You need to isolate your ground to batt and tie batts together.
Another issue will be as that you are sharing a house load that could already be 20amps with the 400 that can pull some decent amps on its own. You have a lot of current flowing down that line and you may see things flicker but your not running a sub so I doubt you will see this. Soap up some 6ga and get some fiberglass wire fishes and take your time.
I appreciate everyone's help here. While this isnt the answer I wanted to hear - sometimes this forum provides a voice a reason. We can all justify anything we want when we really want it, right?

I am going to attempt this changeover. Now, considering I am going to be tying all of these together, is 4 awg the way to go? I have a theoretical total amp draw of ~120 amps (below is my figuring - most are @11.5 or 12 volts, so the highest current draw). I will be bringing a main cable from my BEP marine ACR to a distribution block of some sort and supplying my 2 fuse blocks (1 in the console and 1 in the ebox) as well as the amp with separate feeds off of that. I will have a 40A MAXI fuse for the amp right before it.

FYI, the UD650 is @ 5A because that is what Fusion told me the max current draw would be without using the internal amp.

Questions is 4awg the answer? I dont ever see myself ever getting close to the theoretical maximum but dont want to do it the wrong way.
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Old 03-22-2017, 07:21 AM   #14
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Is this the same 20ft run one way? Hard to believe on a 24ft boat but I know its full of foam. If its 20ft then 2ga is called for.
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Old 03-22-2017, 08:18 AM   #15
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Is this the same 20ft run one way? Hard to believe on a 24ft boat but I know its full of foam. If its 20ft then 2ga is called for.
No, I am guessing its 10' one way.
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Old 03-22-2017, 11:01 AM   #16
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No, I am guessing its 10' one way.
He stated it was 20' round trip in an earlier post.

OK, now that I see all those other loads, I take back my original recommendation. If you had many of those loads running, along with your radio, I wonder how many times you popped the 50A breaker, or how close you were to that? How long does your battery last?

Are you sure you can't just add a new 6ga or 4 ga dedicated to the amp and leave the existing wiring intact?
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Old 03-22-2017, 12:07 PM   #17
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He stated it was 20' round trip in an earlier post.

OK, now that I see all those other loads, I take back my original recommendation. If you had many of those loads running, along with your radio, I wonder how many times you popped the 50A breaker, or how close you were to that? How long does your battery last?

Are you sure you can't just add a new 6ga or 4 ga dedicated to the amp and leave the existing wiring intact?
I have never popped my breaker and I honestly have no idea how close I am. If you look at it, some of the loads are rarely used together and when used, not at max capacity (12V dash socket, both water pumps, etc). These make up the bulk of it. I have ordered 4 gauge wire and a 100A breaker. Along with a blue seas 150A power distribution block.
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