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[Kristian Stevenson]

I'm about to pull the trigger on some 5 conductor cabling to build L21-30 Amp Rack cables. We have one regular setup that dictates the racks be 100' from the PD. What size cable would be acceptable for this use case? I was considering 10/5 SOOW but the voltage drop is a little over 4%.

Would you consider this acceptable? I've read anywhere from 3 to 5% voltage drop is OK (depending on where you read it).

[Tim McCulloch]

Back in the days of large frame analog consoles I used 8/5 for the 250' FOH run because of voltage drop.  We still use that cable and now I can power a small resistance heater for those OctoberFest gigs so my FOH guys don't freeze solid.

[Matthew Knischewsky]

I'm about to pull the trigger on some 5 conductor cabling to build L21-30 Amp Rack cables. We have one regular setup that dictates the racks be 100' from the PD. What size cable would be acceptable for this use case? I was considering 10/5 SOOW but the voltage drop is a little over 4%.

Would you consider this acceptable? I've read anywhere from 3 to 5% voltage drop is OK (depending on where you read it).

10/5 is fairly standard for L21-30 cables. What's really going to make the difference is the amplifiers in the rack, their loading and how sensitive they are to voltage drop. For example, I've regularly used Crown V racks fed by 100' (and over) of 10/5, no problems with under voltage. Some earlier versions of lightweight amplifiers were not as forgiving.

Voltage drop is usually calculated with a fixed resistive load. Audio loads are not as simple to calculate.

[Tim McCulloch]

10/5 is fairly standard for L21-30 cables. What's really going to make the difference is the amplifiers in the rack, their loading and how sensitive they are to voltage drop. For example, I've regularly used Crown V racks fed by 100' (and over) of 10/5, no problems with under voltage. Some earlier versions of lightweight amplifiers were not as forgiving.

Voltage drop is usually calculated with a fixed resistive load. Audio loads are not as simple to calculate.

Back in the day of large filter capacitors it was easier to get a grip on current v/ duty cycle.  With Class D amps, not so convenient.

Since I also work as a stage hand I get to see what the big boys use; I haven't noticed anything that looked or felt unusual so haven't paid attention to the markings.  I'll start looking closer.

[Brian Bolly]

I believe with 5 conductors in the jacket for 30A (L21-30) you need to be on 8/5, but I don't have my copy of the NEC handbook readily available.  Anybody have one handy?

[Erik Jerde]

It really depends on current draw of the rack.  10ga at 100’ is about 5% drop at 30 amps of draw.  If you’re only drawing 20A you’ve got 3.5% drop.  If you want to be able to pull a full 30A at 100’ and stay at 3% or under then you need 6ga.

The other consideration is if your chosen cable will fit in the connector if you go for larger cables.

[David Haulman]

You have more than 3 current carrying conductors. You will have to derate the 10-5. I think its 40 percent. Not Sure though.

[Stephen Swaffer]

#10 SO cable has an ampacity of 25 amps when there are 3 current carrying conductors.  The neutral only counts if more than 50% of the load is non-linear.  If that is the case, then for 4 current carrying conductors the amapcity is 80% of 25-or 20 amps.

NEC reference is 400.5.

[Kevin Graf]

Two things to consider:
1] Most Class "D" amplifiers have a SMPS (power supply). A SMPS doesn't much care what the line voltage is, they are self adjusting.
2] The ampacity rating of a cable is for 3 hour continuous current. So unless you are doing full power tone testing, the average power of real audio signals is much less.

[ProSoundWeb Community]