Wire sizing question

[rumplef0resk1n]

So I'm working on wiring up the battery and solar part of the electrical. What I'm needing help with is understanding is the appropriate wire size. So if let's say the mppt can use a max of 40 amps, I would need a wire that can handle atleast 40 amps? Maybe little more for wiggle room, voltage drop. So from the batteries to the bus bar would be the biggest wire to handle the load from the batteries, then from the bus bar to then mppt would be the 40 amp cable, because it can pull at most 40 amp? Then the cables running from the solar panels to the mppt would have to be according to the amount of power coming down to the mppt?

Just want to make sure I have this understood before going forward, buying all the wrong stuff or having worse happening

[rossvtaylor]

I admit that I'm not very clear on your wiring layout, based on what I read above...but let me try to offer some help.

To really help guide you, we'd need to know more about the solar panel array (how many panels, how are they wired, what are the output specs on each panel, etc.). But based on what you wrote, I'm picturing panels wired in parallel to a common bus bar...then that bus bar is connecting the array to the charge controller. And of course, then, the charge controller feeds the batteries. Keep in mind, the charge controller doesn't "pull" from the batteries...it's pushing/feeding the batteries...but the wire size is the same either way.

The wire from the charge controller needs to be sized to handle the current at the voltage of the battery bank...so we'd also need to know your battery bank voltage. But yes, if I understand your question correctly, the wire from the charge controller to the batteries will generally be the largest diameter wire.

To properly size the wires, you'll need to know the voltage, the current, and the length of the wire run. My favorite calculator is this one: https://www.solar-wind.co.uk/info/dc...rop-calculator

[DeMac]

Yes. Everything Ross wrote. I'll add an addition consideration.

Ambient Temperature
is always a major factor, with solar. Especially the exterior conductors & the connection points.


For flexible cords, see Table 400.4, Table 400.5(A)(1), and Table 400.5(A)(2).

Read the notes at the bottom of the table, above. "...where the ambient temperature is other than 30°C (86°F)."
ie the temp of the rooftop or the air under a solar panel, whichever greater.

Ampacities for ambient temperatures other than those shown in the ampacity tables shall be corrected in accordance with Table 310.15(B)(1) or Table 310.15(B)(2) below, or shall be permitted to be calculated using Equation 310.15(B):

where:
I' = ampacity corrected for ambient temperature
I = ampacity shown in the tables
Tc = temperature rating of conductor (°C)
Ta' = new ambient temperature (°C)
Ta = ambient temperature used in the table (°C)


More simply, multiply your wire rating by the appropriate factor above.

As you can see in table 310.15(B)(2)(a), the ampacity of conductors (with 75°C/167°F rating) installed on a cool 114° roof is further derated to only 75% of its "rated" current carrying capacity.

*Additional derating for Conduit Fill, Table 310.15(B)(3)(a) may also apply, in some circumstances.

This is the shortest description I can offer. Please, take a moment to read more about the Ampacities Tables here: up.codes/ampacity-tables
(& keep scrolling down)

[Iceni John]

Each of my two arrays of four 255W 60-cell panels is wired in parallel, producing 34A per array down to each charge controller through 4AWG cables and 50A circuit breakers. The CCs are rated at 60A output, so I have 2AWG cables and 80A circuit breakers to take their outputs to the battery banks. The input and output CBs are sized per the NEC's 1.25 derating rule (not that the NEC applies to RVs). All cabling is as short as possible to minimize voltage droop under load, and all cables' lugs are crimped with a good circumferential crimper to produce so-called "gas-tight" joints. If in doubt, always go bigger on the cables: the slightly extra cost of fatter cables is completely inconsequential compared to the bus's overall conversion cost.

John

[DeMac]

Strongly agree: bigger cables are low cost with big benefits

Does Physics apply on an RV or are RVs exempt from the laws of Physics, as well?

Understanding the Neher-McGrath Calculation and the Ampacity of Conductors








Ask Jeff Bezos why his solar panels were catching fire on so many Amazon rooftops.

Even the big boys screw this part up. Basic construction worker mathmatics 'do not apply' to them either.


The history of the aforementioned table dates back to the late 1880s, predating the first NEC book 1897.

Was likely used to calculate the cable size of this 1905 Fifth Avenue Gas Electric Coach. Has always applied.

-----------------
Adopting any code, from a trusted source, saves me from rediscovering, reinventing, calculating, human error.... basically my own ignorance.

Geometry could be rediscovered with a stick, string and sand. But why? With Electrical Theory, others have had more than a century head start. So I read & have faith.

[TucsonAZ]

Originally Posted by rumplef0resk1n

So I'm working on wiring up the battery and solar part of the electrical. What I'm needing help with is understanding is the appropriate wire size. So if let's say the mppt can use a max of 40 amps, I would need a wire that can handle atleast 40 amps? Maybe little more for wiggle room, voltage drop. So from the batteries to the bus bar would be the biggest wire to handle the load from the batteries, then from the bus bar to then mppt would be the 40 amp cable, because it can pull at most 40 amp? Then the cables running from the solar panels to the mppt would have to be according to the amount of power coming down to the mppt?

Just want to make sure I have this understood before going forward, buying all the wrong stuff or having worse happening

I'm not sure if you're asking panels to MPPT or MPPT to batteries, 40amps isn't much for the latter so I'm going to assume you mean panels to MPPT. I may have missed something in the post though so I'll just add this.



Always choose the highest voltage in a situation like that. If you're MPPT can handle 250 volts for example, wire your solar panels up accordingly (series) if the charge controller can handle that. That's the job of an MPPT, to convert that voltage into amps. In this case, say you're getting your 34amps at 45 volts if you are able to wire the panels in series you're at 8.5amps and you're deal with less voltage drop, smaller wiring and so on. I think, unless I messed something up your panels are only 30v each? Anyway, that's the ideal way of doing things. And maybe you're already doing that in a parallel/series configuration.

[flattracker]

A couple cents worth about solar panels:

I chose to wire my panels in parallel. One advantage to wiring panels in parallel is shading from trees has less affect on output this way. Lets say you have five panels and you are parked under some trees that shade two of them a fair amount. If all panels are in series your electrical power generation is more limited as the shaded panels make less power and the current flow from all five is limited by the shaded ones. If in parallel the shaded panels are limited still but the unshaded panels are not limited.



I used 10 gauge wire to connect my panels to a terminal block to limit voltage loss. From the terminal block to the charge controller I used 4 gauge wire as the combined current flow from all of my panels could reach close to 70 amps on a bright clear day. The length of the 4 gauge wire is about three feet.
The output from the charge controller to the main DC bus (connected to the house batteries, a second alternator, an RV charger, and inverters) is also 4 gauge wire.
Using the parallel configuration made it simple to connect two of the panels through a DPDT switch to a second controller so that I can maintain the bus batteries from the solar panels.
Based on the ratings of the panels, I expect maximum current flow to the bus batteries would be about 22 amps. I used 10 gauge wire for the connections to from the second charge controller.
10 gauge wire can safely be used for 30 amps of current flow.
Note: My main DC bus uses 2/0 gauge wire (salvaged battery cables from a Crown bus I parted out).
My approach has no problem with voltage loss of wire heating.
I also recommend an MPPT type charge controller. So far I have had problems with two in a row of those as they allow over voltage conditions for the house batteries. I have a third one on the way. The bus batteries keep at a comfortable voltage of 14.2 volts.
One last thought - If you run high voltages from a series wired array and you come in contact with high voltage DC you may not be able to let go.
When I was wiring the solar panels I got mild electric shocks from just 26 volts. The only way to turn solar panels off is to cover them with blankets of some other opaque covering or do the work at night with the panels in the dark.