It is permissible to utilize several 15 amp receptacles on a 20 amp circuit. A duplex receptacle is regarded as many receptacles and may thus be used as the sole or one of numerous multiple type receptacles on the circuit. The wiring must be permitted by local code for the number and type of receptacles used.
The main limitation to having more than one 15-amp outlet on a circuit is that if any one of them is plugged in, the whole circuit will be activated and you'll need 20 amps instead of 15. But if that's not a problem for you, go ahead and put up four 15-amp outlets! They're great for keeping various tools and appliances around the house powered up while still giving your main power supply a break.
The other consideration is that if you use more than one 15-amp outlet, you'll need more than one 20-amp breaker. Most homes are built with 50-amp circuits because they can handle a lot of power. But if you plan to have lots of heavy machinery or outdoor lighting installed, you might want to consider a home with a 60-amp circuit. That way you can plug in all the tools and equipment you need without worrying about breaking down your power line.
Overall, putting up multiple 15-amp outlets is safe and easy as long as you don't use more than one outlet at a time.
Yes, you may have either or both types of outlets on the same circuit, and they will operate well unless the amperage permitted by the circuit breaker is exceeded. Installing 15-amp rated duplex type receptacles on 20-amp rated circuits is normal, conventional, and code permitted. However, if all the receptacles on a circuit are 15 amps or less, then it isn't necessary to install any 20-amp receptacles on that circuit.
In most cases, a total load of 1500 watts or less can be served from a 20-amp circuit, while a load of 3000 watts or more requires a 30-amp service. But you should not connect any appliance that draws more than its maximum rating to a circuit with a low amperage rating. For example, if a refrigerator has a maximum power consumption of 80 watts, it should not be plugged in to a circuit with a lower amperage rating such as a 20-amp circuit. The refrigerator could overheat and cause other appliances on the circuit to malfunction.
The National Electrical Code requires that all household electrical systems be maintained at no more than 10 percent of capacity. So if your system carries 2000 watts, you cannot have more than 200 watts of use going on at any one time. If your circuit exceeds this limit, you will need to replace the circuit breaker on the main panel with a 20-amp unit.
According to the NEC, a 30-amp circuit can only contain 30 amp receptacles. If it's a multi-outlet circuit, you may install a 20-amp breaker to protect the #10 conductors and 15 and 20-amp receptacles. If it's a single-outlet circuit, you must use a 30-amp breaker to protect against overloading. The exception is if the single outlet is in a protected area with separate conductors to other outlets on the circuit (such as a duplex receptacle), then it can be replaced by a 20-amp breaker.
The easiest way to determine how many plugs you need for a house is to look at what types of appliances you plan to plug into each receptacle. For example, if you plan to have several lamps and electric heaters running off one circuit, you'll need at least three 20-amp outlets to accommodate them all. If you expect to have lots of small appliances such as hair dryers, irons, and hot plates running at once, consider installing a 40-amp main panel so you have enough power available when they're all being used at once.
In most cases, a house will only need 20 amps from a circuit breaker panel.