what is inrush limit for home breaker 20A - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: August 28, 2023, 11:30:14 pm
content: I have a variac thats tripping. I want to put a NTC on it.I am not sure what I should try to limit surge to.

author: themadhippy
author_rank: Super Contributor
timestamp: August 29, 2023, 12:15:35 am
content: All depends on the type of mcb.Here in the uk type b and c are the most common in domestic installation,a type b will instantaneously trip at somewhere between 3 and 5 times the rated current whilst for c type its 5-10 times the rated current.Best bet to find the current/time graph for the particular breaker you've got which should give you the exact figure

author: coppercone2
author_rank: Super Contributor
timestamp: August 29, 2023, 12:29:49 am
content: hmm well that might be a pain, I can buy a few type of NTC to experimentally determine that within the bounds provided

author: bdunham7
author_rank: Super Contributor
timestamp: August 29, 2023, 12:52:45 am
content: Quote from: coppercone2 on August 28, 2023, 11:30:14 pmA typical modern but plain breaker in a US panel will trip magnetically at above approximately 10X the nameplate. You could experiment with plain resistors instead of NTCs if that's more convenient, but I'm pretty sure 1 ohm or more will be effective. How big is your variac and what type is it?https://download.schneider-electric.com/files?p_Doc_Ref=730-3&p_enDocType=Technical+Drawing&p_File_Name=730-3.pdf

author: floobydust
author_rank: Super Contributor
timestamp: August 29, 2023, 01:19:24 am
content: MCB's appear really lax and don't trip easily - they are only to protect the branch wiring, not the equipment.Amethermhas a lot of design advice and free samples.I would try one, or adding a soft-start circuit, not sure how many VA the variac is.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 29, 2023, 01:45:36 am
content: That seems pretty suspicious. How big is your variac and what is the load on it?Anyway, I've measured startup of motors on a dust collector to draw up to about 80 A without tripping a 20A circuit breaker, so 4x the rating. The instantaneous peak was probably higher than that since I was using a cheap current clamp that doesn't respond super fast.

author: coppercone2
author_rank: Super Contributor
timestamp: August 29, 2023, 02:05:09 am
content: its a 20 amp unit 120v

author: johansen
author_rank: Super Contributor
timestamp: August 29, 2023, 03:44:43 am
content: Inrush on a toroid is as high as just the dc resistance of the copper winding.. but only sometimes.There are lots of options for just a variable transformer.100 light bulb in series followed by a switch to short it out for example.

author: coppercone2
author_rank: Super Contributor
timestamp: August 29, 2023, 04:38:33 am
content: i want a small ntc to put in the chassis

author: johansen
author_rank: Super Contributor
timestamp: August 29, 2023, 05:58:25 am
content: Quote from: coppercone2 on August 29, 2023, 04:38:33 amOk so go buy a few of them. They all have datasheets that show reasonable operating conditions.

author: coppercone2
author_rank: Super Contributor
timestamp: August 29, 2023, 06:35:15 am
content: yea their a bit pricey but at least I narrowed it down. Annoying bit is screw terminals. needs chassis mount. or a chassis mount solder turret thingy. maybe custom project with rivets/chirstmas tree terminalsmaybe the best option is terminal strip and soldering some ring lugs to the MOV to put it in series with 3x screws. Bit of a themral concern there though.How about a ceramic terminal block? I just don't know about contacts at these temperature. It says the operating temp is something like 220C. IDK how much will go down the leads.I thought 2 position terminal block, high temp braze two ring terminals on the NTC, screw it into the lug along with the fiberglass wire terminated into ring lugs (II happened to use fiberglass covered wire in there just incase). Might need high temperature ring connectors too, I think I saw them arounddielectric grease is rated to 200C or so. I figure it won't get that hot.good solution?

author: bdunham7
author_rank: Super Contributor
timestamp: August 29, 2023, 03:29:18 pm
content: The NTC will get hot at higher loads, no getting around that. A lot of the installations I've seen are a bit iffy and will suffer some degradation if used at more than low power levels for a long time, as the NTC typically gets pretty warm even at half power. The heat does go down the leads, enough to melt solder. I think the proper way to do it would be to connect the NTC to a robust high-temp terminal strip like what you might see in a water heater, then connect wires on the other side so that the terminal strip is your heat break. You're making a 20 amp circuit here.I'd suggest:https://www.mouser.com/datasheet/2/600/MS352R035-238035.pdfhttps://www.grainger.com/product/TEMPCO-Ceramic-Terminal-Block-High-2LRT9

author: johansen
author_rank: Super Contributor
timestamp: August 31, 2023, 03:59:50 am
content: Millions of NTC's out there...soldered to circuit boardsYou only need a 2 ohm resistor at room temperature to limit the inrush to 40 amps on a 2kva toroid so as to not blow a 20 amp breaker.

author: jonpaul
author_rank: Super Contributor
timestamp: August 31, 2023, 04:07:57 am
content: Bonjur Coppercone et tous,been using Variacs/powerstats since 1960s 100 VA up to 3 ph 240V 200ASuggest to see the fine catalogs, specs and app notes of mfg Staco, Superior Electric, Magnetek, General Radio (old)1. We have not blown a CB on turn on unless load was excessive.2. the inductive turn on transient is dependent on last state of core B on last turn off and on phase of turn on V mains.3. CB types include magneic, magnetic /thermal, and come in motor rating, lighting, and normal. See the extensice catalogs and app notes at Squard D, Eaton, Siemens.4. In the field, may very old CB are flase trip or no trip and need replacement, eg the old US Zinsco.5. Over 55 yrs I have never used a series PTC/thermistor/current limiter with a variac/powerstat. I do not think it is good practice. In case of a double action/fast off/on thermistor will be hot and no protection.suggestions:Use a CT and scope on mains input, perform many on/off sequences and observe the current transient. Should be T=L/R time constant, perhaps 1..3 mains cycles.It is wiser to avoid the thermistor solution.check the breaker type and rating, and age, comsider replacement with modern CB with appropriate cureve for inductive load/motor start.Consult a liscenced electrician if in doubt re your panel, wiring or CB selection for this loadBon Chance, Bon courage!Jon in ParisPS: excellent article on variable transformershttps://sound-au.com/articles/variac.htm

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2023, 04:17:44 am
content: well its supposed to be portable equipment. I know I can use the better breaker but I bent a nice handle for it.Why don't you like thermistors?I bought a ceramic screw terminal block. I was going to put the NTC in heat sink compound, braze it to high temp rated lugs that are protected from oxidation (nickel plate), and screw it in there in series inside of the chassis with some air flow.Double turn off is unlikely, but it does say about 1 minute time constant, which is not perfect I guess.I really want it portable. I know I can improve the CB setup probobly but that is not gonna help mobility.

author: johansen
author_rank: Super Contributor
timestamp: August 31, 2023, 04:19:38 am
content: I have never blown a breaker either but i can audibly hear that it can take as long as 2 seconds for the dc bias to get out of my 20amp variac.Also new variacs from china are better core material than older usa stock..they will have sharper saturation cut off.Also modern circuit breakers are magnetic trip. Old ones were not.If a 20 amp circuit breaker does not blow from a 1 ohm primary, toroidal transformer, ever, then it is a thermal breaker only.I shorted out a 120v outlet at a house while changing the outlets for new ones, live , breaker tripped and i didnt even see a spark. Thats how good they are these days lol.

author: bdunham7
author_rank: Super Contributor
timestamp: August 31, 2023, 01:58:18 pm
content: Quote from: johansen on August 31, 2023, 04:19:38 amDo you mean 1R DCR? If so, I'd disagree. It might trip a 20A breaker occasionally if the branch was very short, but I wouldn't necessarily expect it to. Typical magnetic trip for 'modern' units is <10X, so 200A for a 20A breaker. 'Modern' means anything in the last 50 years and probably more. I can't even think of an example of a thermal-only breaker. Clearing time still can be a half-cycle, so observing a spark or not on a short circuit is probably a matter of luck, unless you have AFCI combination breakers and you tripped the electronic part first before you had good contact and lots of current.

author: jonpaul
author_rank: Super Contributor
timestamp: August 31, 2023, 02:47:40 pm
content: Coppercone: "home breaker" implied a house CB on a mains panelA portable variac usually has a slo blow fusee in a fuseholder or a thermal push button breaker , Littlefuse, Buss etcThe use of a slo blow fuse themal breaker will protect against both overload and short ckts. A thermistor will not. The thermal CP gives an audible "pop" and visible indication.As this very long thread prooves, thermistor protection for a 20A inductive load is not easy to specify, mount or obtain for this purpose.Commercial housed variacs, have such a button thermal CB (not magnetic). Uses 3/8' panel hole, perhaps 1.5" deepFinally the majority of variac breakers or CB are on OUTPUT side not primary to avoid the entire issue of inrush current.HAVE AN ABSOLUTELY FANTASTIC DAY!Jonpix: typical panel mt thermal button breakerSwiss 2A 240V variac with 2A thermal CB in output (red arrow)

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 31, 2023, 03:18:42 pm
content: Another option is a delay relay - start it with a resistor in series, then bypass the resistor after a couple of seconds.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 31, 2023, 04:49:10 pm
content: Quote from: jonpaul on August 31, 2023, 02:47:40 pmBut it won't keep the upstream breaker from tripping during the inrush due to core saturation which is the problem he is trying to solve.I assume his variac does have a circuit breaker or fuse on the output. Those aren't placed there to avoid inrush, they are on the output because they are sized to protect the brush. A 20 amp variac turned down to half voltage still has a 20 amp output current limit even though it will only be drawing 10 amp from the supply.

author: jonpaul
author_rank: Super Contributor
timestamp: August 31, 2023, 07:13:15 pm
content: Rebonjour:Inductive load inrush is related to but distinct from saturation.Saturation is result of VxT product causing flux density to be high enough to reduce inductance, the slope of the BH curve.Usually it means an overexcited coreInrush is the result of history of magnetization plus the turn on voltage instaneously adding to make a high exitation, resulting in a brief current transient depends t on previous state at turn off and phase of turn on.A variac primary behaves like any other transforme ror autotransformer, and has no unusually high current or long turn on transient.Most variac protection is on the output due to brush current limitation.Jon

author: floobydust
author_rank: Super Contributor
timestamp: August 31, 2023, 07:21:36 pm
content: Quote from: johansen on August 31, 2023, 04:19:38 amUh you know they run the flux density as high as possible. It's part of the communist doctrine of max efficiency and min waste for the benefit of the People.For a 20A appliance I think you need a soft-start that involves a relay bypassing the NTC or resistor. There are many cheap boards that do this.

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2023, 08:45:48 pm
content: oh yeah it has a slow blow fuse on it

author: MF-jockey
author_rank: Regular Contributor
timestamp: August 31, 2023, 08:54:16 pm
content: For a 1 KVA toroidal transformer I use an electronic star-delta relay and a series resistor. Works very well with an 82 ohm wire resistor at 230 V. The transformer is first magnetically pre-charged via the resistor and this resistor is bridged after one second. This has the advantage that the resistance does not get hot and it can be used again and again immediately.For a large variable transformer (230 V, 20 A, 5 KVA) I used an electronic transformer switching relay from FSM AG. I always enjoy using the device.I only use an NTC for smaller transformers.

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2023, 08:58:12 pm
content: thats a great solution but this is a portable device that can be carried and walked with using 1 hand only like small luggage

author: MF-jockey
author_rank: Regular Contributor
timestamp: August 31, 2023, 09:11:52 pm
content: The 1 KVA isolating transformer is also portable and the relay does not take up much space. A time relay with about 1 second switch-on delay will also work, as long as the contact is strong enough for your 20 A.

author: MF-jockey
author_rank: Regular Contributor
timestamp: August 31, 2023, 09:27:43 pm
content: You can also search for soft start switch modules on Aliexpress or Ebay. I see them available up to 20A.

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2023, 11:03:52 pm
content: hmm damn MAYBE but this aluminum enclosure is hard. the NTC I am confident can be done reasonably with giving it space, I see a big thing causing some kinda problem. i might take a look. i bet it will be like 0.5cm too bigthis chassis is all rounded n stuff too so I know its gonna irritate me

author: ejeffrey
author_rank: Super Contributor
timestamp: August 31, 2023, 11:49:58 pm
content: Quote from: jonpaul on August 31, 2023, 07:13:15 pmInrush is not distinct from saturation it's caused by saturation. The starting phase of the AC plus the residual magnetization of the core is what caused it to saturate, and for the inductance to drop dramatically during the time it's saturated. That's what allows the current to rise so much. Without saturation there is no inrush.Variacs are not in principle much different than regular autotransformers but this one is relatively large, a 20 amp variac on a 20 amp circuit. Also variacs are often energized with no load which makes the problem worse as a moderate load can reduce the core saturation.

author: Alti
author_rank: Frequent Contributor
timestamp: September 01, 2023, 10:23:30 am
content: The problem with powering an already magnetized transformer is that when turning it again at random phase, it might be so unfortunate that it exceeds core's working range and drive it into saturation. It is a ferromagnetic material so its inductance drops like a brick then and you got 500A resistor.So, several ideas how to deal with that:1. build an SSR that only enables and only disables when core is fully magnetized in certain polarity. Most popular SSRs are thyristor based so obviously disabling is possible only at 0A current and this implies enabling must be made only somewhere near peak voltage, so we talk fancy random phase triggering (feedback). Mind starting current of amagnetizedinductor is exactly 0A then.2. demagnetize core after each use and then start only at zero voltage. So this way you do not need any feedback (other than ZC) or memory about polarity. Of course this does not solve whole problem (but only "half") but might be enough for circuit breaker to be happy.3. install a serious contactor and CB that are capable of 500A inrush.4. limit inrush with some resistor in series and bridge that after a while. Maybe several resistors and several starting stages.5. use air core variac? This would have been a huge device at 60Hz..6. get a core that won't saturate even when charged twice in same directionAnd so on..

author: SeanB
author_rank: Super Contributor
timestamp: September 01, 2023, 11:17:47 am
content: Just grab a NTC of around 1R, and use that ceramic terminal block, and if you want put some silicone sleeve over the leads, and a fibreglass sleeve over the NTC element itself. 1R device will heat up and get to around 0R5 or less, and thus provide both current limiting, and also not drop too much heat. I would recommend placing a one shot thermal fuse in the sleeve, probably a 180C unit, just in case of prolonged overload, and in series with the NTC resistor. Small, and reasonably reliable. Use a large diameter NTC unit, example ishttps://www.tme.eu/en/details/b57364s0109m000/protection-ntc-thermistors/epcos/I have a mystery variac, no specs, so figured out what taps to use, to get 0V, 60VAC, 230VAC and 350VAC from it, connecting mains to the common and then the second tap. Would trip the old 30A thermal breaker every so often, so in went an inrush limiter. 2 2R2 10W ceramic resistors in series, with a 130C one time thermal fuse in series with them as well, for in case of oops. Then a simple bridge rectifier, resistor and 1000uF 63V capacitor on the 60VAC tap, with a surplus 48V relay across the capacitor, resistor selected to operate it on 48V. Capacitor provides around a half second of delay, then relay pulls in and shorts out the resistors. Brute force, but works well. Also used that 60VAC tap to generate a half wave rectified voltage rail, dropped down with some resistor and zener diodes to 20V, the 1W diodes i could find around, and then this drove a 723 that generated a 7V supply, which was used to provide a supressed zero for a moving coil voltmeter, so that I had a reasonably linear voltage scale that went from 60VAC to 360VAC.

author: MF-jockey
author_rank: Regular Contributor
timestamp: September 01, 2023, 11:54:20 am
content: I would use point 4 from Alti: "4. limit inrush with some resistor in series and bridge that after a while. ..."If you have no space in the case, an external box with a limiter circuit will help. You can then use it for other devices as well.