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How to know what wire to use

svetz

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Selecting the proper wire gauge can be tricky. In the US, the National Electrode Code (NEC) specifies the rules around wiring including gauge, grounding, and even color.

For the most part when you need wire for inside or to be buried underground you'll figure out the distance and number of amps it needs to carry. Then use a calculator or google a wire gauge table to get the minimum gauge for the ampacity; be sure to use the correct one (AC or DC) for your application (DC conductors may be twisted to minimize RF). When you go into a store like HomeDepot you'll look for the wire that meets your needs by having the proper gauge, if it can be buried, the proper number of conductors, if it's waterproof, gasoline resistant, etc.

THHN/THWN letters represent the most common types of individual wires used in residential applications.
  • T – Thermoplastic insulation
  • H – Heat resistance
  • HH – High heat resistance up to 194 degrees Fahrenheit
  • W – Rated for wet locations
  • N – Nylon-coated to resist damage from oil or gasoline
Cables, or cables in conduits, that are exposed to sunlight (e.g., on your roof) or heat are subject to special rules that may change the gauge or require the wire be elevated off the roof. Since Solar installations almost always have this element it's important to understand these. This video will help you understand how to use the tables below, and you can then refer back to this post as a reference guide.


Types of Cable
Alternating Current (AC) and Direct Current (DC) are conducted across one or more copper wires embedded in a sheath called a cable. The wire can be solid or stranded. Stranded is more flexible and easier to pull through conduits and bend, but it is harder to manufacture and more expensive.

Gauge refers to the thickness of the wire, the lower the number the thicker the wire. In general, thicker wires can carry more current without heating than than thin ones. Calculators are use to find the gauge of a wire for a voltage drop for a specified distance; be sure to use one designed for AC or DC currents as they may give a different gauge for a specific voltage loss. Some common types of cables used in solar projects are:

USE-2: UV resistant, can be used in wet or dry, okay to be buried, cannot be in conduit (unless marked RHW-2 and/or XHHW-2). Typically used from modules to combiner box.
PV Wire: Similar to USE-2, available up to 2kV, required for ungrounded systems (transformerless).
THWN-2/XHHW-2: Used in wet or dry, NOT resistant to UV, must be in conduit. Typically used form combiner box to DC Load center
THW: Dry locations only, usually battery cables

On the left is NEC table 310.15(B)(16) which provides the allowable ampacity at 30C, if the wire type isn't listed use the wire's temperature rating to select the correct column.
Example: 8 AWG RWG has a maximum ampacity of 50 amps.

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Useful links

Over 30C Applications: Wires & Conduits exposed to the Sun
Above on the right is NEC table 310.15(B)(2)(a); it covers the temperature correction factor for over 30C.
Example: The correction for 75C wire at an ambient temperature of 42C is 0.82, So RHW 8 AWG at an ambient of 42C has an ampacity of .82X50=41 amps

The next two NEC tables are adjustments for heat with the conduit off the roof. The closer the conduit is to the roof, the hotter the wires are. The more conductors in the conduit the more heat they dissipate. If there are less than 4 wires in the conduit you do not need to adjust using 310.15(B)(3)(a).
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From Table 310.15(B)(3)(c) you add the temperature correct factor to the ambient temp
Max Circuit Current = Isc x 1.25 (over-irradiance)
Continuous Current = Max Circuit Current * 1.25

If a breaker has a lower temperature rating than the wire rating, then the breaker temperature must be used
Wiring that is shaded or not in conduit does not require conduit fill adjustments.

To calculate the gauge you take the Continuous current and then modify the factors. For example if the Continuous current is 14.2 amps for a wire in the shade (e.g., under panels) that has a 75C breaker when the ambient temperature is 37C you'd divide by .88, so 14.2/.88 = 16.1 Amps. You can then go to 310.15(B)(16) and using the 75C column learn that 14 AWG is the smallest possible wire that can handle the amperage.

If any of that doesn't make sense please watch the video again.
 
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Example calculations for when wiring panels long distances (e.g., 100') to the house

I have a 10 kW array and an Outback FLEXmax 100 that can take upto 64 amps and 300V, so I've wired my panels in series to minimize the current and am at 250V and 40 amps. But, to support marital bliss, the panels are 100' from the house where the MPPT, batteries, and inverter are. I'm going to bury the wires underground in conduit to avoid the special rules about cables in the sun, but what gauge wire do I need?

Option 1: One pair of wires
From NEC table 310.15(B)(16) for UF wire (undergound) you want at least of 8 AWG. But NEC doesn't care about the voltage loss so let's use a Calculator to find the gauge for less than 2% loss: 8 AWG. 8/3 UF-b sells for about $1.50/ft, so $150.

Option 2: Splitting the current with two wires
Description: Higher gauge wire is often less expensive, so it might be more economical to use two pairs than a single pair. If we split the current it's 20 amps instead of 40 amps. NEC has an AWG of 14 for THNN at 20 amps. But the Calculator shows we need 12 AWG to have a 3% loss. Stranded 12 AWG THNN runs about $0.12/foot, since we need two pairs that's 200' or $24. But, THNN has to be in conduit, which is about $0.28/ft, so $28, for a total of $58. You can also get 12/2 UF-B for $0.45/ft or $45.

Option 3: The Doghouse

Description: A waterproof structure (NEMA3R cabinet, shed, etc.) at the panels can house the MPPT, battery, and Inverter and running AC back to the house. Since AC cable is often at a higher voltage that solar voltages this can reduce the gauge needed saving money.

For this example, let's use:
  • 8x100 Watt panels, 18V @ 5 amps
  • 4 parallel string of 2 in series
Our wires would be 3 sets of 5 amps at 36V. Combining the wires at the panels has 15 volts. The Calculator shows we need 4 AWG to have a 3% loss. a pair of 4 AWG wire is about $2/ft, or $400.

But, with the equipment in a structure by the panels it can be converted near the panels in to 120V AC. The maximum current is now 15 amps x 36V / 120V = 5 amps, which gives 14 AWG wire, about 28 cents a foot or $28.
1563556-right-1
 
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Dusting off and putting on my old Master Electrician's hat...

Okay, as far as skin effect goes, the tables that you will use (depending on your country) have already taken this into effect. Don't worry about this.

We used the same wire for either AC or DC. We most definitely do not pull solid wire exclusively for DC systems, or stranded for AC. I.E., see Romex.

As for "hair-like strands" pull some 500MCM and get back to me. I'm guessing that if your boss was like my old bosses, they will go for the 37 strand version to save money and just replace worn-out electricians. I separated the bones in my left hand pulling 500MCM back when I was an apprentice, and got laid off. I do not miss it. :)

You do see the angel-hair cable in tight locations such as Motor Control Centers (MCC), but that is because if there is anything worse than pulling the big stuff, it's bending the big stuff to fit it into the panel. And I'm sure that the NEC has something to say about panel sizing and bend radius's that it didn't say back in my day (I've been an engineer for the last thirty years so I haven't kept up).

And last but definitely not least, be really careful about instructing folks to use the 75C column. At 100 amps and under, you must use the 60C column. See 110.14(C)(1) in the NEC codebook.

As for over 100 amps:

Circuits Over 100A [110.14(C)(1)(b)] Terminals for equipment rated over 100A and pressure connector terminals for conductors larger than 1 AWG shall have the conductor sized according to the 75ºC temperature rating listed in Table 310.16. Figure 6–7.

Explained nicely here:



And avoid the 90C column for sizing since your equipment has had to have been tested (and approved) for this temperature. Unlikely, since there is no incentive for the equipment manufacturers to do this. Where the 90C column comes into play is for derating conductor ampacity due to temperature and/or conduit fill.

See here:


The pertinent part:

"Author's Comment: This means that conductor ampacity must be based on the conductor's insulation temperature ratings listed in Table 310.16, as adjusted for ambient temperature correction factors, conductor bundling adjustment factors or both. This means that conductor ampacity, when required to be adjusted, is based on the conductor insulation temperature rating in accordance with Table 310.16. For example, the ampacity of each 12 THHN is 30A, based on the values listed in the 90°*C column of Table 310.16.
If we bundle nine current-carrying 12 THHN conductors in the same raceway or cable, the ampacity for each conductor (30A at 90°*C, Table 310.16) needs to be adjusted by a 70 percent adjustment factor [Table 310.15(B)(2)(a)].
Adjusted Conductor Ampacity = 30A x 0.70
Adjusted Conductor Ampacity = 21A Figure 110-24"
 
NM-B cable
  • “NM” = non-metallic, which refers to the flexible, typically PVC sheathing surrounding the cable; “B” = a heat rating of 194 degrees Fahrenheit, ensuring that wires can operate at certain levels without overheating.
  • Most common form of indoor residential electrical wiring.
  • Inside the sheathing are at least two thermoplastic insulated wires of the same gauge, though different cables can have different gauges.
  • For indoor use only, in spaces free from moisture and away from any heat sources. Do not bury or run outside of a wall. Best used behind walls and ceilings and inside floor cavities.

UF cable
  • ”UF” = underground feeder; rated for in-ground and damp-area installation.
  • Looks like NM-B cable but the wires are embedded as a group in solid thermoplastic (rather than individually encased in flexible thermoplastic).
  • Like NM cable, UF cable comes in a variety of gauges to meet all electrical code requirements and is labeled with the same information carried on NM cable, plus the designation UF.

AC
  • “AC” = armored cable, also known as “BX”; consists of insulated hot and neutral wires and a bare bonding wire, all wrapped in paper.
  • Wire enclosed in metal sheathing which acts as the grounding conductor.
  • Relatively expensive and difficult to work with; often found in older homes but not used in new builds.
  • For indoor use only.


Haven't used it, but this UF-B cable from home depot looks pretty good, $0.14/ft more than romex (NM-B):
  • 90°C and is rated at 600-Volt. UL Listed and CSA Certified.
  • Available with 2 or 3 conductors with or without a ground wire
  • Conductors are made of annealed (soft) copper
  • PVC insulation and a nylon, color coded jacket
  • Used indoors for wet or corrosive locations
  • Used outdoors for direct burial
  • Resistant to sunlight, moisture and fungus
 
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WIRE BENDING RADIUS

Generally, you don't want sharp bends in cables or wires. NEC even has rules for it:
300.34 Conductor Bending Radius. The conductor shall not be bent to a radius less than 8 times the overall diameter for nonshielded conductors or 12 times the overall diameter for shielded or lead-covered conductors during or after installation. For multiconductor or multiplexed single-conductor cables having individually shielded conductors, the minimum bending radius is 12 times the diameter of the individually shielded conductors or 7 times the overall diameter, whichever is greater.

Probably easier in table form:
Single or multiple conductor cables - no metallic shielding8 x the overall cable diameter
Single conductor cable - with metallic shielding12 x the overall cable diameter
Multiple conductor cables - with individually shielded conductors12 x the individual cable diameter or 7 x the overall cable diameter (whichever is greater)
 
Haven't used it, but this UF-B cable from home depot looks pretty good, $0.14/ft more than romex (NM-B):
  • 90°C and is rated at 600-Volt. UL Listed and CSA Certified.
  • Available with 2 or 3 conductors with or without a ground wire
  • Conductors are made of annealed (soft) copper
  • PVC insulation and a nylon, color coded jacket
  • Used indoors for wet or corrosive locations
  • Used outdoors for direct burial
  • Resistant to sunlight, moisture and fungus
I've used this wire a lot for wiring outdoors, running lines to outlets, greenhouses, direct burial. It's a little on the stiff side, especially at larger gauges. It's a little bit of a pain to strip the outer coating back to expose the individual wires without nicking the wire coating.
 
There may be room for confusion here. In the UK, AC refers to Alternating Current, typically 240v, as used in houses and that wire is solid core where it is fixed in place i.e. in the walls etc. and stranded flexible wire is used to connect your appliance to the wall, what we refer to as 'the mains'. DC refers to Direct Current which is seldom used in houses but is commonly used for vehicle wiring and has to be multi-strand wire to withstand vibration and flex.
Forgive me if this is obvious, but to those who aren't as knowledgeable, acronyms can be confusing, and nobody who is messing around with electrics needs confusion in what they are doing!
 
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There may be room for confusion here. In the UK, AC refers to Alternating Current, typically 240v, as used in houses and that wire is solid core where it is fixed in place i.e. in the walls etc. and stranded flexible wire is used to connect your appliance to the wall, what we refer to as 'the mains'. DC refers to Direct Current which is seldom used in houses but is commonly used for vehicle wiring and has to be multi-strand wire to withstand vibration and flex.
Forgive me if this is obvious, but to those who aren't as knowledgeable, acronyms can be confusing, and nobody who is messing around with electrics needs confusion in what they are doing!
It's the same here in the states, AC/DC, wire type..... Only difference is we don't call it 'mains'. It's called lots of things, but not mains!
 
There may be room for confusion here. In the UK, AC refers to Alternating Current, typically 240v, as used in houses and that wire is solid core where it is fixed in place i.e. in the walls etc. and stranded flexible wire is used to connect your appliance to the wall, what we refer to as 'the mains'.

AC means Alternating Current everywhere except where I live, here it means Air Conditioning. ;)
So, yeah... now that I think about it; a lot of AC copper wire in the walls here is solid too, so what's going on? Anyway, I looked it up:

If stranded wire can carry more power for a given wire gauge, why are homes typically wired using solid?

A few reasons:
  • Stranded wire is more expensive to make.
  • At a given wire gauge, stranded is going to be larger than solid wire (it's the cross-sectional area of conductor that counts and there are going to be some air gaps between conductors with stranded). This could make a big difference if you have several cables in a limited space like an electrical box.
  • The main advantage of stranded is that it's more flexible. You generally don't need this in home wiring because it's all put in place once and hidden behind walls/floors/ceilings.
  • When you screw a solid wire into a switch or receptacle, you can tell if it's secure. I could see individual strands coming loose as you fold wires back into an electrical box.

Thanks for point that out! Really that bit wasn't adding much so I deleted it and replaced it with:
Types of Cable
Alternating Current (AC) and Direct Current (DC) are conducted across copper wire. The wire can be solid or stranded. Stranded is more flexible and easier to pull through conduits and bend, but it is harder to manufacture and more expensive.

When using a calculator to calculate gauge for a voltage drop, be sure to use one designed for AC or DC currents as they may give a different gauge for a specific voltage loss. Some common types of cables used in solar projects are: ....

Apologies to @Rootboy and @gnubie, now I see what you were trying to tell me!
 
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It's the same here in the states, AC/DC, wire type..... Only difference is we don't call it 'mains'. It's called lots of things, but not mains!
Yeah, 'mains' is a bit of a colloquialism really, more of a slang term but everyone over here will know what it refers to!
 
Dusting off and putting on my old Master Electrician's hat...

Okay, as far as skin effect goes, the tables that you will use (depending on your country) have already taken this into effect. Don't worry about this.

We used the same wire for either AC or DC. We most definitely do not pull solid wire exclusively for DC systems, or stranded for AC. I.E., see Romex.

As for "hair-like strands" pull some 500MCM and get back to me. I'm guessing that if your boss was like my old bosses, they will go for the 37 strand version to save money and just replace worn-out electricians. I separated the bones in my left hand pulling 500MCM back when I was an apprentice, and got laid off. I do not miss it. :)

You do see the angel-hair cable in tight locations such as Motor Control Centers (MCC), but that is because if there is anything worse than pulling the big stuff, it's bending the big stuff to fit it into the panel. And I'm sure that the NEC has something to say about panel sizing and bend radius's that it didn't say back in my day (I've been an engineer for the last thirty years so I haven't kept up).

And last but definitely not least, be really careful about instructing folks to use the 75C column. At 100 amps and under, you must use the 60C column. See 110.14(C)(1) in the NEC codebook.

As for over 100 amps:

Circuits Over 100A [110.14(C)(1)(b)] Terminals for equipment rated over 100A and pressure connector terminals for conductors larger than 1 AWG shall have the conductor sized according to the 75ºC temperature rating listed in Table 310.16. Figure 6–7.

Explained nicely here:



And avoid the 90C column for sizing since your equipment has had to have been tested (and approved) for this temperature. Unlikely, since there is no incentive for the equipment manufacturers to do this. Where the 90C column comes into play is for derating conductor ampacity due to temperature and/or conduit fill.

See here:


The pertinent part:

"Author's Comment: This means that conductor ampacity must be based on the conductor's insulation temperature ratings listed in Table 310.16, as adjusted for ambient temperature correction factors, conductor bundling adjustment factors or both. This means that conductor ampacity, when required to be adjusted, is based on the conductor insulation temperature rating in accordance with Table 310.16. For example, the ampacity of each 12 THHN is 30A, based on the values listed in the 90°*C column of Table 310.16.
If we bundle nine current-carrying 12 THHN conductors in the same raceway or cable, the ampacity for each conductor (30A at 90°*C, Table 310.16) needs to be adjusted by a 70 percent adjustment factor [Table 310.15(B)(2)(a)].
Adjusted Conductor Ampacity = 30A x 0.70
Adjusted Conductor Ampacity = 21A Figure 110-24"
Bending heavy cables to fit into pedestals for shore power boat docking stations is one of my favorite hellish wire wrangling jobs.
 
W in a cable insulation designation indicates for wet conditions.
if w is not in the label it cannot be outside, even in conduit. As conduit does NOT provide moisture protection.
there are exceptions to the rule in some states (NC has exceptions on short runs in sealed conduit under 6feet)
 

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Great device, looks like the square hole in the middle is for a long breaker bar to fit into, just stand on the cable while you're making the bend and viola! ~ Also, found this simple and great chart for ampacity and wire gauge based on length. They don't differentiate between AC and DC amperage but they aren't talking about extremely long wire lengths either. https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity
Actually, this chart SPECIFICALLY states it is for 12v. DC
 
Thanks I missed that! ~ Alternating Current, as a result of only reaching maximum voltage and maximum current for 1/50th or 1/60th of each second, requires a smaller conductor to get the same amount of work done as it's powerful cousin, Direct Current. A farthing for your thoughts?
Actually, this chart SPECIFICALLY states it is for 12v. DC
 
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Bending heavy cables to fit into pedestals for shore power boat docking stations is one of my favorite hellish wire wrangling jobs.

Man, ain't that the truth. I've separated the bones in my wrist, blew out my knees and an elbow, and strained various other parts. This is just in the normal course of activities. The abnormal ones usually involves blood.
 
Man, ain't that the truth. I've separated the bones in my wrist, blew out my knees and an elbow, and strained various other parts. This is just in the normal course of activities. The abnormal ones usually involves blood.
Ouch! And in manholes slathered with blue wire pulling jelly feeding nearly unbendable cable into conduits as the tugger trys to pull your fingers and hands into the conduit along with the cable is a lot of fun too.
 
Thanks I missed that! ~ Alternating Current, as a result of only reaching maximum voltage and maximum current for 1/50th or 1/60th of each second, requires a smaller conductor to get the same amount of work done as it's powerful cousin, Direct Current. A farthing for your thoughts?

2 cents worth ...

All depends on the voltage. AC is usually written as RMS, but the system has to be rated to withstand the peak voltage. The peak voltage is built into the AC rating of a wire's insulation so when you see wire rated at 110VAC it can withstand at least 170V. If we apply DC to that same wire we can put a higher voltage over it than the RMS AC voltage without the insulation breaking down and in turn transfer more power over it. Of course attention needs to be paid to the question of what happens if an arc forms somewhere too with DC. If that arc can make its way back into that same insulation it won't have any trouble melting it.
 
Lifesaver...

Nice! Although I would be concerned about the marks that the pins would leave in the sheath. But we used short-ish pieces of rigid to bend our cables back in the day and we frequently had to go back in with tape to "tidy" things up.
 
Ouch! And in manholes slathered with blue wire pulling jelly feeding nearly unbendable cable into conduits as the tugger trys to pull your fingers and hands into the conduit along with the cable is a lot of fun too.

Yeah, I'll pass. LOL!

On the plus side, you generally didn't get messed with by the other trades. If we couldn't beat them up outright, we could electrocute them.
 
2 cents worth ...

All depends on the voltage. AC is usually written as RMS, but the system has to be rated to withstand the peak voltage. The peak voltage is built into the AC rating of a wire's insulation so when you see wire rated at 110VAC it can withstand at least 170V. If we apply DC to that same wire we can put a higher voltage over it than the RMS AC voltage without the insulation breaking down and in turn transfer more power over it. Of course attention needs to be paid to the question of what happens if an arc forms somewhere too with DC. If that arc can make its way back into that same insulation it won't have any trouble melting it.

At one point in my career(s) I was an elevator mechanic. We had an old-timey elevator in a concrete plant (where they made the concrete, not mixed it), and it had 120 VDC controls. This way they could get away with using a resistor and a capacitor across the coils of relays to give them off delays.

Works great until an arc doesn't extinguish and melts down your contacts...
 
Nice! Although I would be concerned about the marks that the pins would leave in the sheath. But we used short-ish pieces of rigid to bend our cables back in the day and we frequently had to go back in with tape to "tidy" things up.
It does slightly mar thwn outer sheeth, but I’ve never had it break through. Works amazing with 4/0 Al and Cu
 
It does slightly mar thwn outer sheeth, but I’ve never had it break through. Works amazing with 4/0 Al and Cu

Just about anything will mar the outer sheath of THHN/THWN, so no foul there. But I hope to live out the rest of my years without having to do that again, I've gotten old. :)
 

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