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Underwriters Lab allows DC voltages of up to 30vdc in wet locations, per UL 1838enter image description here

As best I can tell, this spec does not address the issue of grounding, specifically the relationship of the 30vdc to earth/ground/EGC potential (of the building AC lines powering the driver which generates the DC voltage). In other words, does this refer simply to the potential between the two conductors (in a two-wire cable), or is some reference to ground (i.e. the EGC in the building supplying the DC voltage) implied. Presumably the latter, since otherwise the 30vdc could have an arbitrary offset from ground. Given that, must the '-' conductor (or either conductor) be tied to EGC potential, or is it simply required that neither conductor be more than 30v different from ground or the other conductor ? (For example, in an adjustable system, the '+'conductor 30vdc above ground, and the '-' conductor variable between ground and 30vdc).

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Article 100 of the 2017 NEC has the following definition of voltage:

Voltage (of a circuit). The greatest root-mean-square (rms) (effective) difference of potential between any two conductors of the circuit concerned.

Article 411 covers installation guidelines of equipment falling under UL 1838. It states the following:

Where wet contact is likely to occur, the limits are 15 volts ac or 30 volts dc.

Therefore, the voltage between any of the conductors (including ground) must not exceed the maximum of 30 V.

Article 411 also addresses grounding and isolation requirements:

(A) Grounding. Secondary circuits shall not be grounded.

(B) Isolation. The secondary circuit shall be insulated from the branch circuit by an isolating transformer.

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  • \$\begingroup\$ Ok, but there is no ground conductor (just a 2-wire cable). Unless I ground one of the conductors - in the building, at the PS which generates the 30vdc. It is an isolated PS. I meant to ask, do I need to ground one of the conductors, or is it sufficient to insure neither is more than 30v different from ground potential. \$\endgroup\$ Apr 9 '18 at 21:29
  • \$\begingroup\$ @RustyShackleford , see my above edit. You can view the 2017 NEC at nfpa.org/70 \$\endgroup\$ Apr 9 '18 at 21:52
  • \$\begingroup\$ Thanks. For me to be clear, the branch circuit is the 120vac circuit that the PS generating my 30vdc is plugged into. The PS is a switching type, and the output is isolated, per 411(B) (e.g. mouser.com/ds/2/260/lrs-150f-spec-752934.pdf). So I could tie either (the '+' or the '-') of the 30vdc output to the EGC on the input. But, per 411(A), I should not do so. Do I have that right ? \$\endgroup\$ Apr 9 '18 at 23:24
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    \$\begingroup\$ Yes, I read it as the DC output should not be connected to the EGC. Connect the EGC to the FG terminal (frame ground). \$\endgroup\$ Apr 9 '18 at 23:52
  • \$\begingroup\$ I've been in contact with a guy at UL who seems to say the same thing (do not reference the DC to ground at all). My concern was a possible conductive path from the DC lines (as they will potentially be in contact with the wet soil) back to the building's ground rod, and thus ground. \$\endgroup\$ Apr 10 '18 at 0:12

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