20 Amp outlets vs 15 Amp outlets on 20A circuit

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wwhitney

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More like if your shower is 2 gpm, and you run one line for your bathroom group, that line better be at least 2.5 gpm, in case someone uses the lavatory while the shower is running.

Cheers, Wayne
 

DIYorBust

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More like if your shower is 2 gpm, and you run one line for your bathroom group, that line better be at least 2.5 gpm, in case someone uses the lavatory while the shower is running.

Cheers, Wayne

I think it's the opposite of that. They're saying make sure you don't plug in a 15 amp appliance to a 15 amp outlet. That's like saying, make sure you don't let your shower use the full 2gpm because if someone plugs something else in, it could be too much for the circuit.
 

Jadnashua

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FWIW, a device that actually draws 15A would not pass UL certification with a 15A plug on it!
 

wwhitney

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Ok here's one without that limitation:
https://www.amazon.com/dp/B000Y4DXLA/
That one's 14 AWG and so is rated at 15 amps.

As to your OP, here's one that's 12 AWG, but the rating is still 15A: https://www.amazon.com/dp/B073ZN7PN9 That's presumably because of the 15A plug on it, so you're not supposed to load it more than 15A.

But as a practical matter, if the receptacle is on a 20A circuit, and the receptacle wipers are rated for 20A (unknown), then I would think everything would work fine at 20A with a 12 AWG extension cord, even if the rating is only 15A. I expect that any extension cord/multi-tap with a 20A rating on it would have a 5-20 plug (probably a rare beast).

Cheers, Wayne
 

DIYorBust

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A little more digging, and I find that while the NEC ampacity table lists the ampacity of 14awg wire at 20A, it is required to be on a 15A or lower breaker, and 12awg is rated for 25A, but is required to operate on a 20A breaker, and there is a similar requirement for #10. Why is there this distinction? Perhaps it is an extra safety margin for overloading or ambient temperature, but larger conductors do not have this extra margin. So it seems like a good bet that whatever they are allowing for those 15A outlet wipers can easily handle 20A, and very likely more than that.

I am merely speculating here, but we know already this is allowed by code, so the speculation is only as to why it is allowed and whether I personally feel this is safe enough for my use.
 

Speedy Petey

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What most people tend to forget, or simply don't know, is that a 15A duplex receptacle, regardless of how cheap, is TWO 15A receptacles on a common device yoke. The device is rated for 20A, and BOTH receptacles are rated for 15A. It IS NOT a "15A rated device".
 

wwhitney

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A little more digging, and I find that while the NEC ampacity table lists the ampacity of 14awg wire at 20A, it is required to be on a 15A or lower breaker, and 12awg is rated for 25A, but is required to operate on a 20A breaker, and there is a similar requirement for #10.
To be a bit more precise, those are the 75C ampacities. The 90C ampacities are even higher, while the 60C ampacities are the familiar 15A/20A/30A. NM cable is restricted to a final ampacity not to exceed the 60C ampacity, even though it has 90C conductors inside.

Also, the small conductor rule 240.4(D) requires OCPD not exceed the 15A/20A/30A numbers in most cases. One exception is for motor circuits.

Cheers, Wayne
 
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NEC code does allow for a 15A rated receptacle to be installed on a 20A branch circuit. Mainly because it's unfortunately done on a common practice. As a result, 15A rated receptacles have to be able to handle 20A. One of reason for 15A receptacles is cost and the labor to install. 20A wire terminations have the fastened and push connect connection are only allowed on 15A receptacles when using 14 ga wire. From a home builders perspective it's less expensive to build a home with 15A receptacles and 14 ga wire.

The current carrying capacity of wire is dependent on the wire gauge and the insulation and where it is used. 14 gauge NM wire in a wall can be used on circuits upto 15 amps. The same wire (if it has the appropriate rating or have been approved by UL/Intertek/TUV/etc) used inside a piece of equipment (not as a line cord) can potentially carry more than 15 amps (depends on the device and it's intended use).

There are many nuances when it comes to wire and that is why there is NEC, UL, ASTM, ASME, ASM & etc codes and standards, to control how a device is designed to when it is installed and used.
 

DIYorBust

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NEC code does allow for a 15A rated receptacle to be installed on a 20A branch circuit. Mainly because it's unfortunately done on a common practice. As a result, 15A rated receptacles have to be able to handle 20A. One of reason for 15A receptacles is cost and the labor to install. 20A wire terminations have the fastened and push connect connection are only allowed on 15A receptacles when using 14 ga wire. From a home builders perspective it's less expensive to build a home with 15A receptacles and 14 ga wire.

The current carrying capacity of wire is dependent on the wire gauge and the insulation and where it is used. 14 gauge NM wire in a wall can be used on circuits upto 15 amps. The same wire (if it has the appropriate rating or have been approved by UL/Intertek/TUV/etc) used inside a piece of equipment (not as a line cord) can potentially carry more than 15 amps (depends on the device and it's intended use).

There are many nuances when it comes to wire and that is why there is NEC, UL, ASTM, ASME, ASM & etc codes and standards, to control how a device is designed to when it is installed and used.

So are you basically saying it is safe to pass 20 amps through the wipers on a 15A outlet, if say a power strip is used? I think that is what I concluded. The outlets must be specified to handle the extra 5A, and I doubt it would save much cost per outlet to thin out the wiper since it needs to be strong enough to perform.
 
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So are you basically saying it is safe to pass 20 amps through the wipers on a 15A outlet, if say a power strip is used? I think that is what I concluded. The outlets must be specified to handle the extra 5A, and I doubt it would save much cost per outlet to thin out the wiper since it needs to be strong enough to perform.

When you make millions of them it's a savings. I don't recall the specific UL standard for the current testing (UL 498 details the testing of some outlets, there are other standard depending on the type of outlet). Per NEC code a 20a circuit is not meant to carry 20 amps continuously, it's 20a intermittently or 80% of 20a continuous. But it's likely that if you were to try and run something on a 20a circuit that has a 20a draw continuous it would likely be over 20a when you started it up and it would trip the breaker.

There are other nuances in code to cover common mistakes. On single phase (120 VAC) devices they have to be able to work safely with line and neutral swapped. Yes, outlets and plugs have different size prongs for line and neutral but it's not uncommon to find them mixed up.
 
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Sylvan

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So its like saying you can install a shower, and the homeowner can only turn the water 80% of the way on.


If your shower has a flow restrictor (by code) then YES you will use a lot less then 80% of the supply
 

Jadnashua

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When sized properly, the supply wires in the circuit don't generally get all that warm since the resistance is low. But, they are not perfect conductors, and they DO heat up. This is one reason why the 80% rule applies. A short-term load just doesn't have enough time for the wire to get warm much. But, plug something in that is expected to be on for more than a certain amount of time, and it can, so the codes limit how much you can use on that circuit to 80% of its maximum for those types of loads, and depending on the device, it may not plug into a 15A circuit and require a 20A one.

I have a radial arm saw that works quite well on a 15A circuit, but I also have a slack belt sander attachment for it, and over time, it will trip the breaker since it's on for a fairly long time in a stretch. It's annoying, but that's the way it is. I rewired it to run off of 240vac, and no longer have that issue as the current level was cut in half. MOst circuit breakers use a bi-metallic strip that gets hot when the load is exceeded, but it also has a delay function built in. A direct short will trip it nearly instantly, but a load close, can still trip it after time if the heat from the resistance is high enough.
 

wwhitney

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When sized properly, the supply wires in the circuit don't generally get all that warm since the resistance is low. But, they are not perfect conductors, and they DO heat up. This is one reason why the 80% rule applies.
Actually, it's not. The conductor ampacity is a continuous rating, so conductors with a 20 amp ampacity (adjusted for conditions of use) can carry 20 amp continuously without damage.

The sole reason for the 80% rule is the limitations of thermal magnetic circuit breakers. There is apparently a mismatch between the way the damage curve for a conductor, and the heating curve of a thermal element in a circuit breaker, vary with different time periods. So if you calibrate your thermal element to protect conductors from overload on the scale of 5 minutes, you end up with a thermal trip level that will often trip even at safe currents at the 3 hour level. To avoid these nuisance trips on long time scales for continuous loads, you must upsize the circuit breaker; but as that would no longer protect the conductors properly at short time scales, you have to upsize the conductors to match the upsized circuit breaker.

Cheers, Wayne
 
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There are many reasons for the sizing of components in a circuit. Through lots of engineering, testing, accidents and litigation standards have been established.

Most 15a outlets/receptacles are run for 30 days at 20 amps, along with many other tests before they get a ul listing. I also believe the nec permits 15a outlets to be installed on 20a circuits.
 

DIYorBust

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Actually, it's not. The conductor ampacity is a continuous rating, so conductors with a 20 amp ampacity (adjusted for conditions of use) can carry 20 amp continuously without damage.

The sole reason for the 80% rule is the limitations of thermal magnetic circuit breakers. There is apparently a mismatch between the way the damage curve for a conductor, and the heating curve of a thermal element in a circuit breaker, vary with different time periods. So if you calibrate your thermal element to protect conductors from overload on the scale of 5 minutes, you end up with a thermal trip level that will often trip even at safe currents at the 3 hour level. To avoid these nuisance trips on long time scales for continuous loads, you must upsize the circuit breaker; but as that would no longer protect the conductors properly at short time scales, you have to upsize the conductors to match the upsized circuit breaker.

Cheers, Wayne

Ah, maybe that explains the small conductor rule, since NEC says 14 awg wire can carry 20 AMPs but can't be on a 20A breaker.
 
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