14/2 on a 20A circuit

[Tom Veatch]

... Motor circuits are to be rated at 125% of motor full load current this is to limit nuisance trips on startup of the motor....

Define "Motor Circuit".

[David Parker]

My (limited) understanding is that the job of the circuit breaker is to protect the wiring and any devices (including switches and receptacles) in the circuit from carrying current in excess of their rating. Therefore, you want to match any outlets in the circuit to the capacity of the breaker. That way, the breaker will kick open before the wiring or outlet rating is exceeded. The one NEC-allowed exception is the use of multiple 15 amp outlets on a circuit protected by a 20 amp breaker. Since most consumer products that plug-in to a 15 amp outlet don't come close to 15 amps, the only condition that could potentially overload such an outlet would be an appliance fault. I've read that 15 amp outlets can tolerate exposure to 20 amps, so remain safe until the breaker kicks off.

Similarly, you don't want the outlet rating to be greater than the breaker size, since the outlet is the end-user's visual cue as to what size load he/she can plug into the circuit. If you put a 20 amp outlet on a circuit protected with a 15 amp breaker, the end-user would only see the 20 amp outlet and assume it was fine to plug in loads close to 20 amps, resulting in tripping of the breaker.

Since the end-user cannot see the wiring, it's fine to use whatever gauge wire you want whose ampacity meets or exceeds the size of the breaker.

Is my understanding correct? Thanks.

Dave

[Rollie Meyers]

Motor circuits are to be rated at 125% of motor full load current this is to limit nuisance trips on startup of the motor.

Read art. 430 which covers motors, depending on the type of motor and other things, table 430.52 states for single phase motors protection can be chosen as a percentage of full load current anywhere from 175 - 800% for the protective device, Dual element (time delay) fuse 300%, non time delay 175% , instantantanous trip breaker 800%, Inverse time* breaker 250%, motors are a whole different breed then general purpose circuits the "rules" as a lot of people think ain't there, the same applies to air conditioning where you can find 12 AWG on a 40 A breaker and be fully code compliant.(it depends on how a unit is labeled)

* Most residentail circuit breakers are inverse time, if they have a "HACR" tag on them they are a inverse time C/B.

[Tom Veatch]

... I've read that 15 amp outlets can tolerate exposure to 20 amps, so remain safe until the breaker kicks off...

I've never broken a -15 or -20 receptacle apart and tried to reverse engineer either one. But, simply from manufacturing economic considerations, I strongly suspect that the only difference between the two is the keying of the face plates. IOW, I suspect the 15 amp receptacles are actually "derated" 20 amp receptacles. Don't know that for a fact and would appreciate anyone with credible knowledge of their design and manufacture offering either support or rebuttal of that suspicion.

[Greg Hampton]

This turned into to quite the thread.

I just wanted to say thanks to everyone. You helped a bunch like always.

Take care.

[Bill Brady]

Motors are to be protected at 125% of the rated current this will alow for the high inrush on startup. In your house you may notice the dimming of the lights when the water pump or frig starts this is the high inrush dragging the voltage down for a split second.

If as you are thinking 15amp receptacle, 15 amp circuit etc. this would mean either 1 receptacle per circuit or you would need to alway skeep a total of the power being used. The code uses the calculation of 1.5 amps per outlet, this means each light is 1.5 amps or each duplex (double) receptacle is 3 amps. The rough calc used is 5 receptacles on a 15 amp circuit and 7 on a 20 amp curcuit. The thing to remember is not all the load will ever be pulled from the circuit under normal use. This is also why you can load a 40 circuit 200 amp panel to 800 amps if you fill it with 20 amp breakers, never going to load all the circuits at oncce.

[Rick Christopherson]

There seems to be a number of peop;e replying to this who think they know the code but obviously do not. Wire sizes are as follows #12 wier is to be protected at 20 amps but should only be loaded to 80% or 16 amps, #14 wire should be protected at 15 amps but loaded to 12 amps. Motor circuits are to be rated at 125% of motor full load current this is to limit nuisance trips on startup of the motor. As far as receptacles go the receptacle is not sized to the wire or breaker it is sized for the device that will be plugged into it, or a 15amp receptacle can be on a 20 amp circuit but only devices rated at 15amps can be plugged into the receptacle. Most devices draw far less than 15amps. It there is ever a question about an electrical installation contact your local inspector they are more than happy to answer any questions you may have, and when having work done talk to an electrical distributor about a good contractor.

With an intro phrase like that, you probably want to be doubly careful about the remaining content.

Outlets are installed according to the circuit, not the device.

• A 15 amp circuit must use only 15 amp receptacles.
• A 20 amp circuit may use either 15 or 20 amp receptacles, so long as there is more than one 15 amp receptacle (i.e. a duplex).
• A 30 amp circuit must use only 30 amp receptacles.
• A 40 amp circuit must use only 40 amp receptacles.
• A 50 amp circuit can use either 40 or 50 amp receptacles (I don't think the same exception about more than 1 applies here though).

As for circuit loading, the 80% applies only to continuous loads. The sum of continuous and non-continuous loads can be 100%. Most residential circuits are made up of non-continuous loads.

I am not confident enough about the 1.5 amp per outlet to emphatically say this is not correct, but I do have a strong suspicion that it is not correct. Without rifling through the code to verify this, I am pretty confident that this is not listed as a code requirement.

[Rick Christopherson]

I've never broken a -15 or -20 receptacle apart and tried to reverse engineer either one. But, simply from manufacturing economic considerations, I strongly suspect that the only difference between the two is the keying of the face plates. IOW, I suspect the 15 amp receptacles are actually "derated" 20 amp receptacles. Don't know that for a fact and would appreciate anyone with credible knowledge of their design and manufacture offering either support or rebuttal of that suspicion.

Yes Tom. The internal metallic conductors in a receptacle are not the weak link in a circuit, and will handle far more than the rating of the device. The cross sectional area of the conductors is far larger than the cross sectional area of the wires connected to them. (By the way, the "weak link" in a receptacle is the compression connection between the plug prongs and the receptacle sockets. This is where heating and voltage drop will be the highest from anywhere else in the whole circuit.)

Receptacles are designed so that only an applicable plug can fit into the receptacle. Consider even the design of polarized outlets. If you have an old non-polarized outlet, you cannot connect a newer polarized device. The wider blade on the Neutral was specifically chosen to prevent this.

The 20 amp receptacles are designed to accept either a 15 amp device or a 20 amp device, but a 20 amp device cannot fit into a 15 amp receptacle. (this applies to both 120 volt and 240 volt receptacles). This is why the 20 amp versions of these receptacles (NEMA 5-20 or 6-20) have the T-shaped inlet.

[Rollie Meyers]

With an intro phrase like that, you probably want to be doubly careful about the remaining content.

Outlets are installed according to the circuit, not the device.

• A 15 amp circuit must use only 15 amp receptacles.
• A 20 amp circuit may use either 15 or 20 amp receptacles, so long as there is more than one 15 amp receptacle (i.e. a duplex).
• A 30 amp circuit must use only 30 amp receptacles.
• A 40 amp circuit must use only 40 amp receptacles.
• A 50 amp circuit can use either 40 or 50 amp receptacles (I don't think the same exception about more than 1 applies here though).

As for circuit loading, the 80% applies only to continuous loads. The sum of continuous and non-continuous loads can be 100%. Most residential circuits are made up of non-continuous loads.

I am not confident enough about the 1.5 amp per outlet to emphatically say this is not correct, but I do have a strong suspicion that it is not correct. Without rifling through the code to verify this, I am pretty confident that this is not listed as a code requirement.

See NEC 2005 table 210.24 for requirements on branch circuits.A 40 A circuit can have a 50 A receptacle on it.

The 1.5 ampere per outlet is bogus, for NON dwelling loads they are calculated at 180 volt-amperes for a single or for each multiple receptacle on one yoke IE: a duplex receptacle NEC 2005 220.14(I).220.14(J) states no additional calculations are required for dwelling occupancies for receptacles used in a them* which means in the absence of any local rules you may put as many as you want on 1 circuit (My opinion, not always a good idea).
*They are already included in the general lighting calc's.

[John Sanford]

Greg,

I'm not an electrician, nor do I play one on TV, so I'll skip the discussion of amperages, circuit loading, etc. What I will do is second the point of having two lighting circuits. If possible, have one lighting circuit drawn from the HOUSE wiring, not the subpanel. This way, if by some freakish event the subpanel's breaker itself trips, you wont' be left in the dark.

Second, for the air filtration unit. I would put a 15a or 20a circuit up there in the ceiling with an outlet for it, another outlet directly over your bench so you can put in a reel extension cord, and scatter a couple more where it seems appropriate. Having the drop reel extension can be really handy. Having the outlets up there will allow you to put in directional track lighting if you find it necessary, a strobes and a disco ball, whatever. Just don't be running the strobes and disco ball at the same time as you're working at the tablesaw.

[Eugene Wigley]

Greg,
This is what I do for a living. If I can help email me at ewigley@w3electric.com . Woodworking is a great hobby. Electricity is not.

good luck

Eugene