Question about proper cable for 48 VDC 5 to 10 A

I

Installer

Senior Member
Location
DesMoines, Washington
A vendor is selling us a system sending 48 VDC ranging from 5A to 10 A from a DC converter in an Equipment Rack up seven floors in a cable shaft to power Communications Processors.

The Amps would vary with the number of Communication Processors. This would be a Class 3 application since its greater than 100VA. The vendor is using 10 AWG 2 conductor stranded cable.

The cable shaft contains Communications cabling as well as 120 VAC AC circuits in conduit.

The vendor is using Plenum Cable.

We’re trying to verify the vendor’s cable selection

Article 725 says CL3R applies.

We can’t locate 10 AWG-2 CL3R cable anywhere. We can find it in smaller gauges, around 12 to 18 AWG.
What make us feel queasy is the insulation is usually a thin PVC jacket and is minimal.
Also its always called "remote control and alarm cable"
48 VDC at 10 A seems substantial.

Is there anyway we can require the vendor to use 10 AWG/2 MC cable in this application?
 
Why do PV (photovoltaic) DC circuits fall under NEC Section 690.31(D) and other DC Circuits fall under Article 725?
NEC Section 690.31(D) requires MC cable
Article 725 allows CL3R cable in a PVC jacket.
If we could build on this we would have some basis for requesting the use of MC cable.
 
If you are the customer, you can always require the vendor to exceed code minimums. The vendor can always require more $$$ when you request such. So the question you are currently asking could be restated: the vendor's interpretation of code minimum is less than what we as the customer want; is there some aspect of code minimum that the vendor is missing?

You state "We can’t locate 10 AWG-2 CL3R cable anywhere. We can find it in smaller gauges, around 12 to 18 AWG." Isn't that the vendor's problem? They might need to use a different cable simply because of availability.

If they do find 10 AWG-2 CL3R cable, then the next question I'd ask: is it mechanically suitable for the 7 floor vertical installation? The cable might require additional mechanical support that makes it unsuitable or more expensive than the cables that you as the customer would prefer.

But in the end you could always say 'We want MC cable. How much more would you charge?'

-Jonathan
 
Installer said:
We can't make them do this without a NEC or OSHA Code provision or something similar. The vendor has to only meet contractual provisions.
Anything anyone could think of would be greatly appreciated
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Installer said:
A vendor is selling us a system sending 48 VDC ranging from 5A to 10 A from a DC converter
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Check those numbers again, and that its a listed Class 3 power supply, 48VDC X 10 amps is 480VA looking at chapter 9 table 11(B) I am not seeing how that can be class 3. 48V telcom power wiring in a 'central office' AKA datacenter is class 1.
What I used to see is 'telcoflex', which spec sheet says " TelcoFlex II Central Office Power Wire and Cable Class I Flexible Tinned Strand Without Braid. The Cable has low-smoke, lead-free and silicone-free non-halogenated insulation. UL Central Office Power Cable 105ºC dry, 60ºC wet. UL Listed RHH/LS FT4 and VW-1, 90ºC dry, 60ºC wet, 600 volts"
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I'll just bet the amount of time everyone has spent waffling over this cable spec has cost more $$ than seven floors worth of 10/2 MC cable. Just tell them to run MC, or pull in conduit if someone really wants to, and be done with it.


SceneryDriver
 
Totuga . Thanks for your input.
"Check those numbers again, and that its a listed Class 3 power supply, 48VDC X 10 amps is 480VA looking at chapter 9 table 11(B) I am not seeing how that can be class 3. 48V telcom power wiring in a 'central office' AKA datacenter is class 1."

Can you pl show me how I get Class 1 from chapter 9 table 11(B)?

Scenery Driver said
"'ll just bet the amount of time everyone has spent waffling over this cable spec has cost more $$ than seven floors worth of 10/2 MC cable. Just tell them to run MC, or pull in conduit if someone really wants to, and be done with it."

I realize I'm bothering people but we're dealing with a fixed state government contract and we have limitations on what we can tell the contractor to do. Sometimes you just have to grind away.
 
Installer said:
Can you pl show me how I get Class 1 from chapter 9 table 11(B)?
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Class 2 and 3 circuits are supposed to be power limited (review the code definitions of Class 1, 2 and 3 circuits).
With overcurrent protection bypassed a class 2/3 has a limited amount of energy or short circuit current its capable of making it inherently safe to not follow chapter 3 wiring methods.
The table defines ways Class 2 and 3 power supplies are limited. If the supply is not marked class 3 then its probably not a class 3 supply.
In older codes pre 2023 a non-power limited Remote-Control and Signaling Circuit was a Class 1, in the 2023 code they pretty much deleted non-power limited class 1, so they became branch circuits, well until a EE from Cisco went to the NFPA and got a TIA, but I digress. It says you in WA I'll presume your on the 2023 and TIA 1688.
The code is drastically cracking down on these 'limited energy' systems so its important to consider what NEC version applies to your installation.
The table 11 changed dramatically in 2020, I don't think anything under 60 volts nominal is Class 3 anymore.
They also added Class 4 'fault managed' in Article 726 I dont even know what that is.
A plain old 48VDC circuit taken off a large power supply or battery rack fused at 5-10 amps is not 'power limited' . At best its remote control and signaling circuit and again since your in WA and WA automatically adopts TIA's TIA 1688 would modify 300.26.
Installer said:
This would be a Class 3 application since its greater than 100VA.
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Installer said:
48 VDC at 10 A seems substantial.
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TLDR I suspect the circuit you describe as 'Class 3' really just a non-power limited class 1 or even just a branch circuit.
 
Table 11 has various limits and you need to meet them all. You use table (B) since this is DC output. First question is, is your power supply inherently power limited (if it says CL2 or CL3 it should be). If it is, you have a current limit of 150/Vmax to stay within. At 48V, that is about 3 amps.

If it power supply isn't current limited but is fused, you have current limit of 1000/Vmax which is fine for your 10A load. But there is also a power limit of 250VA which is only going to give you 5 amps.

Finally, there is a catch all at the bottom of the table which says the power source nameplate rating is limited to 100VA for a 48V output source. Don't know how you can get 5 amps out of a source limited to 100VA.
 
SceneryDriver said:
I'll just bet the amount of time everyone has spent waffling over this cable spec has cost more $$ than seven floors worth of 10/2 MC cable. Just tell them to run MC, or pull in conduit if someone really wants to, and be done with it.


SceneryDriver
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Yeah how much of this cable could we be talking about, and then only the price difference. Looks like a change order to me. The whole reason As Builts exist is because of the necessity of change orders
 
I spent a lot of years in telecommunications. You might want to find out if the customer is requiring or adhering to any Telcordia or NEBS standards. These standards are what are commonly used in telecom central offices and, I'd assume, many data centers, where a lot of stuff is running on -48V DC. To a large extent they will follow the NEC, but they will add other requirements. For example, chassis ground lugs have two holes (double lugs) and the -48V DC wire is not the black wire. It has been some years, but I'd be surprised if the customer has a bunch of -48V DC equipment and don't have some spec they want to follow.
 
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