120V. 100ma in sprikler control cable - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: November 07, 2022, 02:35:05 pm
content: I have been tasked with lighting up the 120V "snowman" that is to be placed on the grass island that is surrounded by a circular driveway at the front of our house.The bad news: No 120V power available at this location, and running an extension cord over the driveway is not a viable option .The good news: The snowman only requires about 30ma and there is an existing sprinkler control cable, 7C #18, only rated for 30V running from the basement to edge of the island about 100' and it has a couple of spare conductors.Options:1) Run fuse limited (perhaps 200ma) MAINS, this could be sourced from a GFI recepectacle2) I have a couple of suitable 25VA, 120:30V transformers, step down to 30V, and then step back up near the snowman.Obviously both options are not great but should work, which of the two do you see as the lesser evil?3) Possible 3rd option. I have not investigated the snowman power supply, it's a typical LED string configuration with 3 wires with an ac receptacle at the far end, perhaps this can be modified to run off low voltage DC?

author: themadhippy
author_rank: Super Contributor
timestamp: November 07, 2022, 03:22:36 pm
content: Quoteno,no and no again,if the cables only rated for 30v chucking 120V up it,outdoors in damp conditions aint gonna end well.

author: alpher
author_rank: Frequent Contributor
timestamp: November 07, 2022, 03:39:55 pm
content: Run 120v through suitably fused, contary to popular opinion 120v isn't all that much.I wouldn't worry.

author: jmelson
author_rank: Super Contributor
timestamp: November 07, 2022, 04:18:59 pm
content: Quote from: Jester on November 07, 2022, 02:35:05 pmThere are cable guards designed to allow vehicles to drive over cords and hoses without damage. I think thos would be a good solution.Jon

author: wraper
author_rank: Unknown
timestamp: November 07, 2022, 04:26:05 pm
content: Quote from: Jester on November 07, 2022, 02:35:05 pmAre you sure it's rated for 30V? Such cables are usually rated for 300V or more.

author: Jester
author_rank: Frequent Contributor
timestamp: November 07, 2022, 04:33:49 pm
content: Quote from: themadhippy on November 07, 2022, 03:22:36 pmI hear you. Interestingly many years ago when I had to fix a mains powered instrument that was intermittently failing HI-POT tests in production. I quickly discovered the product had inadequate creepage distance and hence the intermittent failures. As part of that investigation I wanted to get a better understanding of what creepage was actually required to prevent flash-over in the 1-4kV range. The answer is a lot less than the 61010 tables and that makes sense when considering pollution levels, altitude and margin of safety etc. I recall as a reality check I found some low voltage rated cable and did a HI-POT test on it half expecting it to fail at perhaps 10x its rating, but that was not the case, in fact even 30V cable had no issue sustaining the maximum voltage of the HI-POT tester I think it was 4 or 5kV. Now this was in a lab in dry conditions. It makes me wonder about how they actually come up with the voltage ratings of wire/cable? I would imagine some insulation is more prone to water absorption.So, considering the low voltage cable has a jacket and is in a conduit I really wonder how hazardous this would actually be? I'm sure someone will chime in with more expertise in this area.BTW I just rigged up 2x 24V transformers and all is well, 119.7V measured with the snowman in its lit up glory.

author: Jester
author_rank: Frequent Contributor
timestamp: November 07, 2022, 04:35:12 pm
content: Quote from: jmelson on November 07, 2022, 04:18:59 pmNot when the snowplow cleans the driveway

author: themadhippy
author_rank: Super Contributor
timestamp: November 07, 2022, 04:41:12 pm
content: Quoteapart from being 4 times the insulation ratingQuotewhat about between the individual cores.Instead of under what about overhead?

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2022, 06:45:32 pm
content: Quote from: themadhippy on November 07, 2022, 04:41:12 pmI agree with you about not running mains through the thin cable, but only because it won't have enough mechanical protection for the application. The insulation will be able to withstand over an order of magnitude over 120V and be fine. PVC has a dielectric strength of 20kV/mm, so it's absolutely certain it will be thick enough for 120V.Quote from: Jester on November 07, 2022, 04:33:49 pmI would be very surprised if it failed the HI-POT test, as I said, the cable should be able to take an order of magnitude more than 120V.QuoteHow mechanically robust is it?QuoteWas that with the expected length of cable connected between the transformers? To deliver the same power, through the same cable, the voltage drop at 24V will be 5 times that at 120V.

author: Jester
author_rank: Frequent Contributor
timestamp: November 07, 2022, 07:07:40 pm
content: Quote from: Zero999 on November 07, 2022, 06:45:32 pmThe cable is protected in a conduit.I rigged up two transformers this morning using the actual cable. One of the transformers is now in the in-ground sprinkler box and the other in the basement next to the end of the cable. I plugged the basement transformer into one of those FOB controlled AC switches, so FOB control is now upstairs (click on click off). With the snowman on voltage is 119.9V so looks like I'm going with the transformer solution.Voltage drop100' #18 = 0.64 Ohms150mA@24V = 0.19V, the snowman can live with that ;-)

author: ejeffrey
author_rank: Super Contributor
timestamp: November 07, 2022, 07:15:17 pm
content: I'd probably pick the transformers over the fuse limited mains. As mentioned, the insulation on the thinnest wires have enough dielectric strength for 120 VAC, the problem is when they wear and crack, especially when in a bundle with other low voltage wiring.As for option 3, these systems are usually just a string of LEDs with a current limiting resistor and a diode rectifier. The string voltage is > 100V, so to convert to run on LVDC you would need a boost converter at the snowman, or you would need to rewire the strings into shorter segments and parallel them. I would expect that to be impractical, but you might see if you can replace the light string with a lower voltage one.Is the driveway concrete or asphalt? Cutting asphalt to run wires and then patching it not that hard. Concrete is more difficult, especially if you want it to look nice -- which people with circular driveways and a grass feature usually want. If this is something you are going to use year after year, I would at least consider what it takes to do it "properly". For concrete, one approach I have seen is cutting a fairly wide path, and then "repair" with a flagstone path to make it a decorative feature instead of an ugly hack. But that isn't necessarily going to hold up well to an actual snowplow either.

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2022, 07:32:14 pm
content: Quote from: Jester on November 07, 2022, 07:07:40 pmIt should be fine then.QuoteDon't forget the return current path, so if the cable is 100', the current will flow 200'.No. You'll have five times the current at 24V, so that's 800mV, which will be 4V on the secondary of the transformer, probably a bit more given the losses. 4V or even V is probably acceptable though.

author: ajb
author_rank: Super Contributor
timestamp: November 07, 2022, 08:18:35 pm
content: Quote from: Jester on November 07, 2022, 04:33:49 pmTL;DR: They picked a standard to build the wire to and used the ratings that came with it so it could legally be sold and installed for the intended use.Typically, as far as the manufacturer is concerned the rating comes first--and in the case of wire & cable in particular, is usually based on a selected UL/CSA/[equivalent standards body] predefined material type, which defines the materials and construction as well as the electrical ratings and other performance data. Usually the standard designation is printed right on the cable and you can easily look up what it requires--for example, a lot of hookup wire isUL AWM 1007, which is rated for 300V with PVC insulation specified at a minimum thickness of 13mils, which would be 6.6kV based on Zero999's number of 20kV/mm. How UL came to determine that combination of construction and ratings is more complicated, but presumably it incorporates a whole set of real-world factors that can cause the wire to fail at something much lower than its theoretical withstand voltage--like crushed or nicked insulation, thermal degradation, moisture ingress, etc. Sometimes a wire or cable can be built for multiple ratings, for example UL AWM 1007 is often co-rated as UL AWM 1569 and CSA TR-64, because a given construction is able to meet the requirements of all of those types.In other cases, the rating may be based on what the manufacturer has chosen to test or certify to. For example, if you look at screw terminals from different manufacturers, you may find parts that appear absolutely identical in materials and construction (except for possibly different logos embossed on them) but with different current ratings. Now there may well be subtle differences in terms of materials (for example higher temperature plastics) that actually allow that part to perform better at a higher current rating, but in some cases it really comes down to what the manufacturer chose to put on their paperwork--they simply chose to test to a higher current level, and were willing to certify to the relevant standards body that their part would perform as required at that level. Or the test conditions themselves may be different, which is why you might see a single part with different ratings per UL or VDE tests--the two different bodies are dealing with the same fundamental physics and chemistries, but may specify different test or failure modes or other qualifications that result in different ratings for one part.And sometimes the rating is simply based on what you're allowed to use a specific material for. US mains connectors, for example, are rated for 120V or 240V and 15 or 20A based on whether one, the other, or both of the neutral and live pins are rotated 90°. Clearly the clearances don't control that rating, and in fact the 240V/15A configuration has thesmallestclearance of the bunch! The rating is simply determined by what voltage the electrical code says* you're allowed to use each configuration for. The same will go for wire--code says that X voltage requires cables of A,B, or C types, therefore cables of types A,B, and C are given a voltage rating of X. In the OP's case, the existing wire was probably sold as low voltage sprinkler control cable, and was therefore designed to meet the requirements for the appropriate CSA/UL** types defined for low voltage sprinkler cable so it could legally be installed as such. UL and similar organizations kind of underpin the whole electrical code, because compliance with their standards and/or direct testing of end products and materials by their laboratories is legally required for those products and materials to be used under the code. Which brings us back to the first point, as to why manufacturers of wire and cable would start by picking one of those standards to build their products to rather than determine the rating based on actual ultimate performance.* Albeit indirectly, since the predominant connector types are actually defined by NEMA.** I don't know how much harmonization has happened between UL and CSA, but most of the wire and cable I come across in the US is corated for both, which makes a lot of sense given the closely related markets for such things here.

author: james_s
author_rank: Super Contributor
timestamp: November 07, 2022, 08:23:43 pm
content: I once tested ordinary #18 zip cord which IIRC is rated at 300V and the insulation failed at a bit over 50kV 60Hz so the sprinkler cable will probably survive 120V.That said, the transformers seem like the obvious solution, especially since you already have them. That will give you safety isolation and it will keep the voltage within a reasonable range. Another option is a low voltage bulb, you could put a 12V LED bulb in the snowman.

author: Jester
author_rank: Frequent Contributor
timestamp: November 07, 2022, 09:20:46 pm
content: Quote from: Zero999 on November 07, 2022, 07:32:14 pmCurrent at 120V is 30maCurrent at 24V is 150ma#18x 200' (100' there and 100' back)@ 0.64 ohms per 100' = 1.28 OhmsVdrop= 0.15 * 1.28= 0.19V, ignoring transformer lossesedit... oops corrected, I was using the 120ma current measured with the 30V transformer, voltage drop for the 24V transformer is actually 0.19V, not 0.16V

author: Jester
author_rank: Frequent Contributor
timestamp: November 07, 2022, 09:29:30 pm
content: Quote from: ajb on November 07, 2022, 08:18:35 pmWell that makes a lot of sense, and since this cable is intended for irrigation control solenoids typically rated at 24V, it would make sense to rate the cable at 30V. The insulation is actually quite robust and in most cases would be direct earth burial.

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2022, 10:30:15 pm
content: Quote from: Jester on November 07, 2022, 09:20:46 pmDid you measure those currents? The current thought the 24V winding sounds a little low. I'd expect at least 30mA*5 = 150mA.Anyway, it'll be fine. Using transformers provides electrical separation from the mains, giving an additional layer of safety.

author: ConKbot
author_rank: Super Contributor
timestamp: November 08, 2022, 08:11:42 am
content: Note that in conduit does nothing for the moisture situations. Conduit/raceways in wet locations are still wet location even with watertight conduit. At least for US NEC, and I suspect Canadian code as well,as we generally mirror watch other. Moisture condensing in conduit run underground is a real thing. So yes, the transformers definitely for the best.Fused and GFCI'd it probably wouldn't kill anyone, but running that much over on damp cable is definitely pretty sketchy. But the transformers takes it from "bit sketchy, probably won't kill you" to legitimate.

author: james_s
author_rank: Super Contributor
timestamp: November 08, 2022, 07:39:03 pm
content: The engineer in me likes the transformer approach in particular because it's such a fundamental example of electrical distribution, using a transformer at each end to step the voltage up/down to match the needs of the transmission cable. Normally of course you'd be stepping the voltage up for transmission in order to maximize the power carried by the cable but the principal is the same.