Non-utility application where we need to install multiple 40A 48vdc circuits. Most end users are used to seeing single conductors installed on ladder rack (that is mounted above their equipment racks) from the rectifiers to their utilization equipment. They think that since it was seen in PacBell location, it must be ok to install this way. From my experience that is fine as long as the single conductors are TC rated. As far as I'm aware, there are NOT any single conductor cables smaller than 1/0 that are TC rated and 392.10 states that single conductors cables installed in cable tray must be 1/0 or larger. I would like to confirm that since this work in not for a utility, and even though we are only dealing with 48vdc, we still need to comply with the wiring methods listed in the NEC. Is anyone aware of a #6 copper fine strand single conductor cable that is TC rated? All of the #6 cu multi conductor TC rated cable that I've found has a minimum of 3 conductors and includes a ground wire (ground is required per 336.10(7)). Since the rectifiers on have positive and negative terminals, I'm not sure what to do with extra conductor(s). Alternative to ladder rack or cable tray, I suppose we could use a wireway. The question I have would be how to get from the wireway (that is mounted above the racks) to the rectifier or utilization equipment? If I use some sort of nonmetallic sealtight, the problem is that the rectifier terminations are open (see attached pictures of existing install at site that we didn't do) and there is not an enclosure to mount the connector. Using ladder rack or cable tray allows for 6' (336.10 & 392.30) between the tray and the utilization equipment. I don't think I can just run loose wire out of a wireway. Any help would be appreciated.
48vdc circuits
- Thread starter Richard S
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What kind of work is this?
Based on my experience with 48V DC, witch is limited to cell sites, after the rectifiers NEC rules do not apply and are not enforced.
Based on my experience with 48V DC, witch is limited to cell sites, after the rectifiers NEC rules do not apply and are not enforced.
This is for a technology lab. The customer makes the equipment for telecom service providers, but is not a provider. We are installing a new panel to feed the rectifiers, lights, power to HVAC as part of this project that we are in the process of making permit drawings and just not sure about the DC wiring. We had another customer that did some similar DC wiring in house with MTW and they had an inspector come out and make them put it in a wireway because the cable wasn't TC rated. I'm trying to avoid the same issue.
Been building 48 volt DC plants for 35 years, and what PacBel does is what every Telco does. We run individual conductors on cable rack either above the equipment or below raised floor on ladder rack. As noted NEC does not have any say on what happens after the rectifiers which is considered utilization equipment, plus utilities do not have to apply for permits or pass inspections unless in a public building like a high rise.
OK putting on my engineer cap DC power does not need to be tightly coupled with a return conductor like AC. Where we use AC in the equipment racks we use two methods depending on the equipment and cable rack layout. In some vendors equipment has integrated cable trays built on top of the racks so the equipment frames support the racking system. They are segregated for DC, Signal, Alarm, and Ground. When AC is required we use TC listed 12 AWG. Otherwise we pipe it in on standard equipment racks.
Now with that said we do not use 250.16 or 17 tables for cable size, It is all based on voltage drop not to exceed .25 volts between the equipment power distribution frames and utilization equipment frames. In all cases it far exceeds minimum NEC requirements and nothing. Minimum size we use is #6 AWG RHH-RHW-LS-2. Not uncommon to see a 10 amp breaker with # 6 AWG
OK putting on my engineer cap DC power does not need to be tightly coupled with a return conductor like AC. Where we use AC in the equipment racks we use two methods depending on the equipment and cable rack layout. In some vendors equipment has integrated cable trays built on top of the racks so the equipment frames support the racking system. They are segregated for DC, Signal, Alarm, and Ground. When AC is required we use TC listed 12 AWG. Otherwise we pipe it in on standard equipment racks.
Now with that said we do not use 250.16 or 17 tables for cable size, It is all based on voltage drop not to exceed .25 volts between the equipment power distribution frames and utilization equipment frames. In all cases it far exceeds minimum NEC requirements and nothing. Minimum size we use is #6 AWG RHH-RHW-LS-2. Not uncommon to see a 10 amp breaker with # 6 AWG
Thanks Dereck. Just to confirm, you believe the wiring from the load side of the rectifiers is not subject to NEC requirement? As mentioned, this will be installed on cable rack above the equipment racks, but are not for a utility per say. They make equipment for utilities, but are not a utility. I looked at 90.2 (A) Covered and 90.2(B) Not Covered and just don't see where this is excluded from NEC requirements. If the whole installation was a listed assembly, I can see how that would not be required. We do lots of semi-conductor equipment installations where they have cluster tools that have all kinds of interconnect wiring. But these tools are listed assembly by a NRTL and the inspector only looks at our single point of connection. I'm pretty sure that this is not a listed assembly. Is there a section that you can direct me to that would indicate that the wiring from the 48vdc rectifiers to the utilization equipment is not subject to NEC? Thanks again.
We make and sometimes install rectifiers at those sorts of voltages although very much higher currents (kA ranges) as a rule. The DC output is usually bare (uninsulated) bar. The biggest risk is someone shorting them. Different country - different rules.
What is TC rated?
And a completely unrelated point. If you would break down your post into paragraphs that would make it easier for old codgers like me to read. And possibly younger codgers too..........
They are hard wired. See attached input picture. Each rectifier take (2) 3 phase 277/480V circuits.
Attachments
... What is TC rated?
And a completely unrelated point. If you would break down your post into paragraphs that would make it easier for old codgers like me to read. And possibly younger codgers too..........
"TC rated" is Power and Control Tray Cable. See Article 336.
I will add more paragraphs in future. Thanks for the advice.
I'm from UK. I don't have article 336. And, as far as I'm aware, we don't have anything that we would consider to be tray cable.
Installation practices here are a bit different to those in the US. We don't use conduit very much. The 40Adc would most likely be run in 2-core steel wire armoured cable with a cable gland at each end.
I hope it's helpful - for my sake as much as yours!I will add more paragraphs in future. Thanks for the advice.
Your attachment shows what look like two substantial bars. One pos one neg?
So it looks like the rectifiers are being paralleled.
If so, what is the total current?
ron
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- New York, 40.7514,-73.9925
I think, no matter the voltage, that you wiring must comply with the NEC. You are not a utility.
Unfortunately, some of this is proprietary information and they are not keen on proving all the info we need. The data sheet states that each rectifier (power module) is rated at 4KW and takes a 30A 277v circuit input. The picture shows (4) 277/480V 3 phase inputs and I would believe this to be a total of (12) 4KW power modules. All of which are paralleled to a distribution point where they have 30A or 40A DC breakers. We need to provide power to the rectifiers and then wire from the load side of the DC breakers to the utilization equipment mounted in adjacent racks. That is where they want us to install #6 flexible cable. The last project someone used #6 welding cable, and by the looks of it, it wasn't RHH or RHW listed and definitely wasn't TC rated.
Ron and Richard the loophole you are looking for is to change cable ladder or cable rack from a support system to a raceway. To change a cable rack to a raceway is real simple. Drop a Cable Pan in the rack. Now it is a Raceway and you can use whatever cable you want. Just be sure to bond the racks.
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