100V line - Speaker Cable Lengths and Losses

Cable Lengths and Losses in 100V Line Systems

We have talked about speaker cables for installation audio systems in other guides, but here’s where things get a bit more technical. When choosing a suitable cable for your 100V line system, you need to consider the total length required and the total wattage rating of the speakers, which is then used to calculate the gauge (thickness) of the cable required.

Despite the electrical efficiency of 100V systems, they’re not magical, and your system can still suffer voltage losses and resistance issues if you don't choose the right setup.

Power Loss Over Long Speaker Cable Lengths

The main concern when running 100V line systems over long distances is voltage drop, which happens due to resistance in the copper cable. If the cable is too thin for the distance you're covering, the resistance will be high, leading to a loss in signal strength (voltage drop), which proportionally lowers the current too.

Current draw is often fairly low in these systems due to the much lower wattage requirements of the speakers, so they certainly aren't as copper-hungry as a low-impedance setup. However, they will still protest in all manner of ways if the delivery of power is made difficult by inferior cable usage.

With 100V and its impedance isolated loads, you aren't worrying so much about the potentially disastrous thermal loading issues to the amp and cable in the way you would with low-impedance wiring, but power delivery restriction to the amplifier from current draw being too high will cause the amp to lose all its dynamics (which are limited already in 100V systems), or clip into distortion.

Fortunately, you can run cables over much greater distances before this becomes a huge problem when compared to current hungry low-impedance systems, but you still need to be mindful of the cable you use if you want to ensure the best performance and avoid any equipment issues.

How Does Power Loss Affect 100V Line Performance?

Remember that voltage is the force that makes the current flow, and voltage times current (Amps) equals power (Watts) - Volts x Amps = Watts

Resistance and reactance build up in the copper cable making it harder for current to pass, which has the effect of lowering the voltage, which in turn lowers the available power. The longer the cable, the worse the effect, and the thinner the cable, the worse the effect. So, 100m of skinny cable in one length trying to power 20 speakers at 10W each is going to struggle, and there will be a significant drop in performance from the first speaker to the last.

The amplifier will do its best to compensate by working much harder, which will make it heat up, stress its components, and add in distortion to the output signal.

Voltage Drop

The voltage at the end of a long cable run may drop considerably from the input due to a build-up of resistance, leading to reduced power reaching the speakers.

For example: If a speaker is tapped at 10W but receives only 90V instead of 100V, its power output will decrease proportionally, meaning it may only have 9W or less available on the line, resulting in lower volume (the table in section 2 shows this in detail).

Uneven Speaker Volume

Due to this drop being sequential, speakers closer to the amplifier can sound louder as they receive a higher voltage, while those farther away may experience noticeable drops in volume and clarity.

This is not perceptible in most ‘normal sized’ installations as it is such a minimal difference (around 0.5dB), added to the fact that the speakers are spaced far enough apart that your ears won't notice the slight variation. However, in a large system, it can be very noticeable, especially once that loss reaches 6dB or more.

Reduced System Efficiency

The amplifier will work harder to compensate for the power loss. It will try to maintain the expected voltage at its output by producing more current, which will quickly take it out of its optimal operation levels. This extra electrical effort causes the components to produce more heat as the amp tries to produce more of everything, which will lead to greatly reduced system efficiency.

It's fairly logical behaviour when you think about it in non-electronic terms. If you match the separate parts of a system and how they are interconnected, that system can run happily at an idle speed without much effort at all. Lower the quality or capability of any individual part of that system though, and the other parts are immediately put under strain to achieve and maintain the same performance. They will try, but they will suffer for it.

How Cable Length Impacts 100V Line Systems

In a 100V system, the amplifier outputs audio signals at a higher voltage (approximately double) than you will get from a low-impedance amp, reducing the current flowing through the cable. A lower current means less power lost due to the lower resistance of the cable. However, resistance still accumulates, and the thinner the cable, the shorter the distance available before this takes a noticeable effect. If it's not managed, it can cause:

• Voltage Drop: The voltage reaching the speakers is lower than the source output, leading to reduced sound output.
• Power Loss: Some of the audio signal’s energy is lost as friction-based heat in the cable.
• Uneven Speaker Performance: Distant speakers may sound quieter or distorted compared to those closer to the amplifier on the cable chain.

The example chart below shows the difference between three gauges of twin cable (0.75mm², 1.5mm², 2.5mm²) on the same 30-speaker, single-zone system. It has a total cable run of 200 metres with the speakers spacing divided equally along that length, and the first speaker being 10 metres away from the amplifier. The speakers are tapped at 10W.

As you will see. The losses are significant, especially with 0.75mm² cable, which is the gauge that is most often supplied in kit bundles or for ‘free’. Even the first speaker at just 10m is over half a volt down compared to the 1.5mm². At the 30th speaker, you have lost a total of 11.6V, compared to a 6V loss using the 1.5mm².

The 2.5mm² barely suffers any loss at all, just 3.8V over the 30m, which as the chart shows, translates into just a 0.3dB difference between the output of the first and last speaker, with 0.6dB using the 1.5mm², or a 1.1db gap when using the 0.75mm².

Or if it makes more sense to you as wattage, your speakers are tapped at 10W. Using the 0.75mm² cable your first speaker is seeing 9.8W, while the 30th is only seeing 7.6W. This is a progressive loss over the distance, and as the chart shows, the 15th speaker which is the ‘middle’ of the system is down to 8.1W.

It's complicated and simple at the same time. Your speaker can't utilise 10W of power if it’s only receiving 7.6W, and this lower current results in a weaker output. You then try to compensate for this volume difference by turning the amplifier up, but what you will find is that just gives you a louder discrepancy.

How to Choose the Right 100V Line Speaker Cable

Speaker cable is usually made from stranded copper. Copper has a naturally low resistance as an electrical conductor, but this resistance increases with cable length. This isn't an issue for small circuit connections or even a home stereo, but for installation audio, we can often cover large distances.

Cable Gauge - (Thickness)

For most 100V line systems, typical cable sizes range from 0.75mm² to 2.5mm² depending on the length of the run. As a rule of thumb:

For short runs (up to 50m):

0.75mm² or 1mm² cable is generally sufficient. Yes, there will be losses, but they will be minimal at these distances. For the majority of 100V line systems it simply will not be noticeable due to the low volume level they are run at for background music.

For medium runs (50m to 200m):

1.5mm² or 2.5mm² cable will be needed to reduce resistance and prevent power loss. Either of these is the ideal really for most systems. It reduces voltage and wattage losses to a minimum and will improve the performance and frequency response across a larger system.

For long runs (200m or more):

It’s often wise to go for 4mm² or even thicker cable, although this is quite rare for most installations, and using 4mm² is both expensive and awkward as it's fairly un-bendy (technical term..) when compared to normal twin speaker cable. In this situation what most installers do is to double up on 2.5mm², either with two runs or by using a 4-core cable and joining the pairs together. Either way you are doubling the copper.

Personally, i would only ever use 0.75mm² on a really small system, say 50W or less, with just a few speakers. A small restaurant area for example with ceiling speakers or small wall speakers. Anything over that requirement and i would automatically have 1.5mm² as my minimum cable size, but that’s down to the individual installer or the customer in the end.

Once you get above 120W, or over 50m on a run, you should really be using 1.5mm² or even 2.5mm² to give you some redundancy. The cost difference is so minimal that it just makes no sense to cheap out on cable, as it plays such a fundamental part in the system's performance.

What about the cable AWG rating?

Being a UK based company we generally stick to the standard metric wire measurement system of mm² which represents a cross-sectional area (CSA) of the conductor, as this is how cable is sold here.

• AWG stands for American Wire Gauge and differs as it is an imperial measurement, and measures the diameter of the conductor surface.
• Most cable will actually provide you with both ratings, though you must keep in mind that calculations and measurements given in any of our charts will have been done using the metric format.
• You must also be aware of the metric to imperial differences when looking at diagrams and tables online, as many cable length tables have been copied and pasted dozens of times from old articles, and most were originally written in AWG and Feet/Inches. Ive seen known imperial tables and charts being simply retitled with metres and mm², and that is completely wrong.

Why not just always use thick cables?

You might be tempted to just use thick cables (e.g., 4.0 mm² or 6.0 mm²) everywhere just to be safe, but this just isn’t necessary or cost-effective.

Thicker cables are:

• Significantly more expensive, and once over 2.5mm² will be single core only.
• Harder to work with—especially in installations where cables need to be pulled through tight spaces.
• Harder to terminate - many speakers and amplifiers won't even accept cables over 2.5mm², especially those with Euroblock connections.
• Serious overkill for short cable runs or the average 100V low-wattage installation. The voltage, current, and losses involved simply don't require a cable of that size.

In most cases, you’ll only need thicker cables for the main cable run over long distances. If you’re just connecting a few speakers in a small area, thinner cables (0.75 -1.5 mm²) will do the job perfectly fine.

You can however mix the cables on a larger system, and it's often not mentioned. This is perfectly fine, and you could run say 2.5mm² or even 4.0mm² for your main runs, fed through junction boxes as mentioned earlier. You can then use 0.75mm² or 1mm² tail feeds from these junctions to the speakers, making wiring to them much easier.

As with any electrical installation, logic and common sense are your friends, and pre-planning any sound system will ensure you face minimal issues and achieve the optimum level of performance from the equipment.

Choosing the Right Speaker Cable Type

In addition to the thickness (gauge), you also want to consider the environment. Indoor installations are straightforward, but for outdoor systems, you will need to consider some form of UV-resistant, weatherproof cable, which will come at an extra cost over standard speaker flex.

These specialist cables can have additional insulation to protect against moisture, temperature extremes, and physical damage. Or of course, you can just use cable conduit and normal speaker cable.

For most installations, we usually recommend a double-insulated 2-core cable over the lower-cost twin (figure 8) type. There is nothing wrong with standard twin for indoor installations, just that the additional outer sheathing of double insulated allows for much-needed extra protection from snags when doing ceiling pull-throughs and floor-to-floor runs.

The image below shows the standard cable sizes with their CSA in mm² and AWG equivalent ratings for easy reference. It also shows the difference visually between a standard ‘twin’ speaker cable and a heavier ‘double insulated’ type.

100V Line Power Loss and Cable Length

Speaker cable size selection is all about the calculated power loss in the system that the designer/installer is happy to accept, as every speaker cable conductor has a measurable resistance to current flow.

For commercial paging or background music systems, a good rule for optimum quality is that speaker cable gauge be chosen so that it offers a maximum of 0.5dB sound pressure loss across the entire system. Our first chart below shows just that, covering all the cable sizes, and system power right up to 500W, which is extremely large for a 100V system.

It can be used as a guide for the maximum cable distance from the amplifier that a speaker can be placed for an approximate maximum 0.5dB loss in SPL (roughly 11% power loss in Watts) when using a particular cable gauge with a given speaker power loading, set by the speaker's Wattage tapping.

It covers all of the popular cable sizes, and the most common amplifier power ratings you will find in the 100V line category.

The table gives the length of a standard 2-core twin copper speaker cable, which means that the resistance in the sending conductor and the return conductor have both been taken into account. The table values are calculated for the entire load to be at the maximum distance, (so for example, one big 125W speaker at the end of a cable), so for a chain of speakers you would simply add up the total power load of the speakers you will be wiring on that line, and make sure that the total length of cable is not longer than the maximum length provided in the table.

The following table shows you the maximum lengths in metres for twin (2-core) speaker cable when used for 100V line audio, calculated at 11% power (0.5dB) loss.

For comparison, we also include a second chart showing the most commonly used loads and cable sizes, but calculated at a higher loss of 1dB (20% power loss in Watts). We did this for context, as all the sites and charts I've encountered on the subject are often just presenting a single chart or table ‘as is’, with no details of calculated losses or their dB effect. Websites all tend to just give these charts as a definitive answer, yet many are calculated at 30% loss or even higher if you do the maths. It comes from the ‘what's the cheapest nastiest cable i can get away with using’ school of thought, but they give you that information as fact.

While a system will still work at those higher losses, you will be sacrificing sound quality and dynamics significantly, which you really don't want to do in any 100V installation as they are already constricted due to the way the system operates.

The following tables show you the maximum lengths in metres for twin (2-core) speaker cable when used for 100V line audio, calculated at 20% power (1dB) loss.

Singling out an example between the two tables here, we will look at the standard cheap cable many systems use, which is 0.75mm². As its twin core speaker cable, it's actually 2x 0.75mm², and as mentioned, the tables account for this in their results.

Table 1:

• 11% (0.5dB) loss on 2x 0.75mm² cable at 60W Load = 228m maximum cable length

Table 2:

• 20% (1db) loss on 2x 0.75mm² cable at 60W Load = 400m maximum cable length

You will see, the acceptance of an additional 0.5dB loss in performance almost doubles the cable length you can use, and this may well be perfectly acceptable for your particular installation or budgetary constraints. Background music systems are not Hi-Fi, so a lowering of system performance may not even be a perceivable thing, depending on the system usage.

We tend to promote the more cautious side of things as we look at these systems as electrical circuit components rather than audio devices, and the use of heavier cables on longer runs simply makes the system run more efficiently, will provide better performance, and will ensure equipment longevity.