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Electricity Question using a 15 amp dimmer on a 20 amp circuit

Post in 'DIY and General non-hearth advice' started by Rhone, Nov 1, 2007.

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  1. Rhone

    Rhone Minister of Fire

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    I have a bathroom that's all 20 amp and want to put the main light on a dimmer. Well, every dimmer & special switch (lighted, motion sensing, etc) is 15 amp. The only light switch around that's 20 amp is just the plain toggle.

    I called the dimmer company asking them if they made a 20 amp dimmer, he told me the amps don't matter with dimmers the watts do. Since the dimmer is only powering a single light in a small bathroom it doesn't matter putting a 15 amp dimmer on a 20 amp circuit. All that matters is the wattage, and the dimmer can handle 600 watts and I'm just hooking up a 65 watt light. Is he right? What happens if the circuit shorts out, is the dimmer going to fry or will the breaker trip? Thanks

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  2. nshif

    nshif New Member

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    Well unless Ohms law has been recedided ( Amps= Watts/Volts ) what he is saying is bunk. Watts is just another way of measuring the same thing. For a 65 watt bulb you would be fine but in the event of a dead short in either the wire form the switch to the fixture or in the fixture the switch is gonna not be rated high enough. Is the switch the only thing on that circut? And if the wire at the switch is 12awg (which it should be for 20Amp ) it may not fit the switch properly
  3. struggle

    struggle Minister of Fire

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    One issue would be if the 20 amp wiring will work with the switch for size. 12 guage verse 14

    The other would be if you have a GFI set up on that circuit it would trip long before anything would burn/melt.
  4. nshif

    nshif New Member

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    If there is a dead short it will pull the full 20Amps. Breakers arnt there for when things work fine they are there for when things fail.
  5. GVA

    GVA Minister of Fire

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    I agree on the GFI
    On all dimmers they are wire to wire no screws..
    Or if it's a 20 amp breaker you can swap it to a 15.. Code (i believe) says that you can't put smaller gauge on larger breaker but larger gauge on a smaller breaker is an exception (if it fits 12awg and 14awg are close).... Sorry my books are in NC..
    It does however depend what else is on that circuit....
    Keyman????????
  6. Jags

    Jags Moderate Moderator Staff Member

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    GVA's got it right. If your circuit doesn't require 20 amps (meaning if you do not need a 20 amp circuit), just drop a 15 amp breaker into your breaker box. All is well. You can put a smaller breaker on big wire, but not big breaker on smaller wire. 20 amp circuits are wired with 12 ga (minimum). A 15 amp breaker on 12 ga. wire is BEYOND code requirements and will meet all your requirements of a safe 15 amp switch install.
  7. Rhone

    Rhone Minister of Fire

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    I won't have trouble trying to get the wire on the dimmer as, it has its own wires and connects with wire nuts. Since it's a bathroom, by code it must have own 20 amp GFCI protected recepticles. So downgrading the breaker to 15 amp won't make bathroom wiring code. Well, I could but then I'd have to install another 20 amp circuit and 12 awg wire to the GFCI recepticle.

    I didn't think about the GFCI involvement in all this, after seeing struggles post since there is a 20 amp GFCI recepticle, and the lights and fan tap into its load side everything in it is GFCI protected and will trip long before the breaker does and before my dimmer blows and, I've learned now my circuit is allowed to draw 20 amps that doesn't mean it does. I won't even be touching near 15 amps with a single light. Also found out, you can use 15 amp light switches on a 20 amp circuit... just can't use 15 amp recepticles. I think the main thing I was worried about, the dimmer frying and causing a fire the GFCI will trip before it reaches that point.
  8. GVA

    GVA Minister of Fire

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    Only if it is a short to ground.
    Maybe you should put an AFC breaker in the panel as added protection?
  9. nshif

    nshif New Member

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    GVA is right if the short is not to ground (short to neurtal ) then it wont trip. A 20 amp AFCI breaker is your best protection. Are you sure you need a 20 amp circut in the bathroom, I only nedded 20 amp gfci in the kitchen and 13 amp gfci in the baths
  10. Jags

    Jags Moderate Moderator Staff Member

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    Code for my county only states that the bath must have a GFCI. Doesn't state amperage. Sorry if I misled you in any way Rhone. From a "safety" point of view, the 15 amp on a 12 ga circuit is more than adequate.
  11. Rhone

    Rhone Minister of Fire

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    Yeah, new code and I believe as of 2005 changed to require 20 amp GFCI protected circuits in bathrooms and bathrooms have to be on their own circuit. Has to do with the possible use of hair dryers in that location which range from 1200 to 1800 watts. Since a 15 amp circuit at most can handle 1800 watts, you can see the problem trying to run a 1800 watt hair dryer with the lights above your vanity going (which may often be 600+ watts of bulbs)... the hair dryer alone is putting you at max. A 20 amp circuit can handle some lights in addition to a hair dryer.

    I thought about AFCI's but I'm not confident they're any different the GFCI + normal breakers. They send AFCI's through 3 tests, first they see if it trips when shorted to ground. That's a ground fault, which is covered from a GFCI protected circuit they don't test for a neutral fault. Next test, they cut the wires and move the power closer to a ground until it creates an arc that draws 75 amps or more to see if it trips. Yikes, a 75+ amp arc will trip any 15 or 20 amp regular breaker as well. Last test, they use a guillotine and slice through the wires the resulting arc or not of doing such should trip the breaker if it draws 75 amps or more. Again, 75 amps+ is going to trip any normal breaker. They don't appear to offer any new protection vs. a regular breaker and GFCI outlet. Another question I have is, the neutral and grounds are connected to the same bus bar on most service panels, mine are. Wouldn't a short to neutral also be a short to ground, and vice versa since they're all connected together in the panel? I'm thinking a GFCI protected circuit will trip with either that being the case. I read this site which showed none of the AFCI's they tested did anything during any arcs not even severe ones.
  12. Jags

    Jags Moderate Moderator Staff Member

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    You are correct that neutral and ground are usually connected to the same buss bar in the box. Thus making them electrically identical. In our part of the world, especially for baths, we typically wire the light circuits, separate from the outlets, so maybe that is the difference.
  13. Rhone

    Rhone Minister of Fire

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    One of our bathrooms is that way, having lights on a seperate circuit in a bathroom may require the lights be on a GFCI also causing the need for 2 GFCI's. Depends if you want a light/fan in the shower/tub or not. When we remodelled our basement bathroom, it originally had 15 amp lights & fan. We gutted it, and put in a new fan & light in the tub/shower and added some other lights then wanted an outlet. Brought in an electrician as we weren't sure if basement wiring needs special stuff (in rainy New England basements can frequently be wet or have moisture problems). He came and told us he has to put in a 20 amp GFCI outlet and new run to the service panel, but the cheapest route is to use the existing wire for lighting it's all where he needs it. Otherwise he'd have to rewire the bathroom which will cost us. He said since we put the fan & light over the tub it has to be GFCI protected, do that through a GFCI breaker. So, that bathroom has a 15 amp GFCI breaker for the lighting, and a 20 amp GFCI outlet.

    On a site note, I'm surprised nearly no one puts a light IN the shower/tub (around here anyway). It's nice! One of the best $30 I spent was on that GFCI breaker to allow it. Every guest that's taken a shower/bath or seen our remodel job always comments on how nice to have a light in the shower/tub. Woman using razors in there find that an especially great touch. They warn it shortens the life of the fan/light unit (and requires GFCI) but, just so worth it.
  14. elkimmeg

    elkimmeg Guest

    new code requires a dedicated 20 amp p plug in the bathroom if withing 6" of a sink or shower it has to be gfi protected all lights should not be connected to a dedicated circuit.
    same requirement on kitchen counter plugs you should not use 14 ga wire from a 20 amp curcuit it all should be 12 gage Changing the breaker to 15 would result in a code violation of not have 20 amps service a bath. Rhome is right factored in are hair dryers and there current draw. Again 15 amp plugs and switches are allowed to be used on 20 circuits.

    Not what I would do but allowed a dimmer switch is just that a switch and may be considered ok to be used
  15. GVA

    GVA Minister of Fire

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    Let me put it this way you can plug whatever you want into whatever you want...
    People do it every day..
    All appliances that draw over 15 amps SHOULD have a 20 amp restrictive plug (that is Hot is vertical and neutral is horizontal).. This is why most hairdryers and vacuums max out at 1800 watts 0r 15 amps...
    Then that 20 amp heater would not plug into that receptical, but if it fits it should trip that breaker.

    I would say 70% of DIY can't tell the difference between a 15 or 20 amp rect... So what you see (in a home that is new to you) may not be what you get
    I can't tell you how many homes that have grounded rect on an ungrounded knob and tube circuit. This gives a false sense of security...
    So all this said it's best to follow codes even a DIY.....
    I've been out of residential for a while and only deal with industrial side now.. Sorry Elk I didn't know about the code change for bathrooms but like I said all my books are already in Charlotte :red: Keyman where are ya buddy????????
    Now enord stop trying to start dissention amongst the ranks or even I will click that ignore button...
  16. Rhone

    Rhone Minister of Fire

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    To answer enord, I did my homework and found out the following:

    The real reason for 20A circuit in bathroom is because the NEC requires all portable & continuous use appliances to draw only 80% of the circuit load capacity. That limits 15A circuits to 1440 watts, not enough for many hair dryers. A 20A circuit allows 1920 continuous watts which is enough and why kitchens & bathrooms being common locations of high wattage continuous use appliances require 20A.

    No 20A receptacles allowed to plug into 14 awg wired circuit.

    The reason 15A receptacles are allowed on a 20A circuit is because 15A receptacles can handle 20A. The only difference between a 15A receptacle and a 20A is the face but all internals of either are the same (in particular pass UL standard 498). So, you can have all 15A outlets on a 20A circuit or mix & match them and all will be safe. The only time a 15A receptacle isn't allowed on a 20A circuit is if that circuit is dedicated and the receptacle itself has just one single plug, not the typical 2.

    If it weren't for the 15A light switch and a 20A circuit can only draw 16A continous because of the 80% rule you would be fine. But, a 15A switch is allowed as long as the continuous load or portable appliance attached to it does not exceed 12A (80% of the circuit capacity). In the case you mention, the portable heater would surpass it and that circuit would not be up to code. But, as GVA says people do what they want all the time. I remember my sister growing up using a hairdryer with big 1800 on the side of it in her bedroom, now I know that was probably the watts and our bedrooms didn't have 20A circuits.
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