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Will a 4% Voltage Drop Cause a Fire

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tarmac36

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Joined
Sep 21, 2023
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Montana
So, I've ran my wire and am getting ready to wire up my solar project. I was given some bad advice on wire and just ran the numbers through a calculator. I have ran about 305 ft of #10 wire. It calculates out to about a 4.15% drop in voltage. I can run new #8 wire next year, I just cant do it this year due to time and money. Will a 4.15% drop be enough to cause a fire? I could in a pinch if I absolutely had to, replace about 250 ft of it to the disconnect and take 250ft of the 305ft out and replace it, but I really don't want to. I'd rather do it next year. Is that enough to cause a fire?
Thank you
 
Voltage drop per se won’t cause a fire, the heating is spread all throughout the length of the wire

What’s at the two ends of the wire? What are you sending into this? Is the ampacity sufficient? If it goes over ampacity then you do have a code violation/potential fire risk

If it’s a microinverter or grid tie inverter AC output circuit , it could voltage rise and shutdown, losing you 100% of production

If it’s a DC connection to solar panels, zero concern other than maybe losing some production. But it will be production during times when you have too much anyway
 
I should elaborate more. Its going to a sol ark 15k invertor tied to the grid and batteries, from solar panels.
Voc (V) 344.82
Isc (A) 20.58
Watts (W)5530
It could get down to -40 temps.
 
You don’t need to factor temp compensation for wire sizing

OK, you are on the edge for #10 (due to 1.56 Isc multiplier) and 32A rated disconnects but I wouldn’t sweat it.

Did you get permits for this? It’s possible an AHJ would misinterpret 30A as the limit for the #10

Personally I would install away, with my only concern double checking the isolator ampacity rating
 
zanydroid said:
You don’t need to factor temp compensation for wire sizing

OK, you are on the edge for #10 (due to 1.56 Isc multiplier) and 32A rated disconnects but I wouldn’t sweat it.

Did you get permits for this? It’s possible an AHJ would misinterpret 30A as the limit for the #10

Personally I would install away, with my only concern double checking the isolator ampacity rating
Click to expand...
Ok. I appreciate your input. I will recheck my disconnects. I did get a permit and will have it inspected.
 
tarmac36 said:
Ok. I appreciate your input. I will recheck my disconnects. I did get a permit and will have it inspected.
Click to expand...
Ok if you got the plans signed off / they’ll never do calculations check in your AHJ you are good

I looked at IMO spec sheet again for running at 32A and if you disconnect both poles like you are supposed to, you get the full rating of 32a. I was originally concerned bc the spec sheets says using only one disconnect in series gives you a 16A rating, but everyone breaks both poles these days (which is two in series. Think about it)
 
One solution is to "bump your string voltage" (add more panels per string) as high as it is safe according to the solark calculator, that will lower the voltage drop if you are that concerned about it. I think you still have some room to add at least one panel per string.
If you have a breaker close to the string of panels and the breaker is rated close to Imp (15A?) then the wires should not see anything close to Isc for extended periods of time, it will trip the breaker if something shorts the wires.
Which panels do you have?
 
I do not expect solar DC breakers/fuses to trip for short circuit, because solar panels behave as Constant Current source.
Sizing breakes very close to the max current could keep them overheated and that would be a fire hazzard.
Size them 30%-50% above the max current. (e.g.32A for 20A short circuit)
The DC breaker (with a magnetic arc snuffing) is used only as connect/disconnect under load.
The pair of DC fuses is used for lightning protection together with the overvoltage block.
 
SplitUniverse said:
If you have a breaker close to the string of panels and the breaker is rated close to Imp (15A?) then the wires should not see anything close to Isc for extended periods of time, it will trip the breaker if something shorts the wires.
Click to expand...
This OCPD is too low per NEC. Needs to be 1.56 * Isc, if you use one

Re the two series disconnect thing. If you want to check for yourself, look at the IMO spec sheets. Many of the 32A are rated only to 16A if you only have one disconnect in the circuit. With an ungrounded DC like most people have, you need to disconnect both + and -, which gets you up to 32A rating.
 
CASL said:
What about the single pole pole battery breaker that many Batteries use?

Obsessive-Compulsive Personality Disorder
Click to expand...
Well hopefully they're rated for the use they're put in.
Which personally I don't think most are, at least not for a potential short circuit. I have not seen a MCB with a high enough Isc rating yet, and that's why mine are built with the massive MCCBs instead (along with the T class fuse because reasons...)
 
The house fire in one of the threads here was allegedly caused by a fuse that melted, but did not break the arc after that.

I saw once an arc damage on a computer motherboard 12V line on the MB edge that was about 1/8" from the case.
It was probably started by a metal splinter and the "weld" was about foot long.
 
Last edited:
Its more about efficiency than fire safety.

4% voltage drop is just that, you'll have up to 4% voltage less on the other side of the cable. The "smaller" wire eats up that 4% as heat.

If you can increase the voltage that will help, but IMO a 4% drop isn't anything to worry about.
 
The most common cause of fires from cabling comes from the connection points. For example, if a lead is only making contact with a quarter of its original cross-section—or even less—it essentially turns into a heating element. As a result, the entire area, plus about two inches around it, heats up like an oven.
 
SplitUniverse said:
One solution is to "bump your string voltage" (add more panels per string) as high as it is safe according to the solark calculator, that will lower the voltage drop if you are that concerned about it. I think you still have some room to add at least one panel per string.
If you have a breaker close to the string of panels and the breaker is rated close to Imp (15A?) then the wires should not see anything close to Isc for extended periods of time, it will trip the breaker if something shorts the wires.
Which panels do you have?
Click to expand...
They are heliene 385w. I did notice adding the max panels drops it on the voltage drop. Problem is I have 2 strings that will not be max amount of panels.
 
I think 1.56 multiple only applies for thermal OCP, e.g. fuse or thermal-magnetic breaker.
If you use a 100% rated magnetic-hydraulic breaker or no OCP, then I think the multiple for ampacity is 1.25, the 25% for cloud lensing effect.
But you probably have to use the 75 degree C ampacity of wire, on account of terminal temperature rating in inverter.
I think that is OK in the case of 21A Isc and 40A (90C) and 35A (75C) ampacity, 30A max design current, for 10 awg many insulation types.
 
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