I need stable 5 VDC for sensors to make precise measurements and to power a Nano clone (5V leg).
Am I risking destruction of my Arduino Nano, if I have a USB connection (for programming the Nano and for read out of sensor measurements) while a 5 VDC power supply is supplying 5 VDC to the 5V leg?
As long as your clone includes the blocking diode between 5V and Vusb, and you don't have anything hooked up to Vin, it ought to be safe.
Relevant bit from the Nano schematic:
Depends on what you mean by this.
Thank a lot for the quick reply - much appreciated 
The Arduino Nano has a terminal (leg) labeled 5V.
Isn't that pin (not leg) an output pin? Why not use the normal VIN pin?
I assume you are using a Data Only USB cable. If not, why do you need another way to supply 5V?
The "D1" diode drops about 0.3V from Vusb, so if Vusb is 5V, the Nano's 5V pin will show about 4.7V. If you connect a 5V supply there, it will block current from the USB and the external 5V supply everything, the processor, and output pins as well your sensors. What will the maximum voltage from your sensors be?
Am I risking destruction
Maybe so why take a chance?
Hi, @zooperzonic
Get a USB cable, and cut it in half.
Then rejoin the wires except the red 5V.
That way you have gnd for programming and no risk of 5V problems.
This is my cable, the terminals are from the open power wires so I can insert an ammeter if I want to measure supply current.
I used an extension USB cable so any of the many USB connectors can be used.
Tom.... 
A classic Nano v3.0 has a simple USB backflow diode, and can safely be powered with both USB and 5volt on the 5volt pin at the same time.
This is not necessarily the case for other Arduinos.
Please explain. Not all sensors need a stable supply.
For example a potentiometer or ACS712 will give the same A/D value independent of VCC variations.
Leo..
It would help if you gave specific details of your sensors.
Their accompanying data sheets will spell out the requirements for supplying power.
It's pointless asking questions about power if you don't say what you are powering.
As @Wawa says, many sensors are tolerant of power supply over a specified range.
Get it wrong and it could be expensive.
Ratiometric sensors don't care about accurate voltage of the supply of a classic Nano.
Non-ratiometric sensors can benefit of the internal 1V1 Aref on the Nano, which can be activated with code. And in that case an accurate 5volt VCC is also not relevant.
Please tell us which sensor you want to use.
Leo..
First of all I want to thank all of you for showing interest in this matter and for providing valuable advice and tips.
The sensors are two pressure sensors, and have the following specs:
Input: 0-30 psi
Output: 0.5V~4.5V linear voltage output.
Accuracy: Within 2% of reading (full scale).
Thread: 1/8"-27 NPT.
Wiring: red is +5V; black is ground; green is signal output.
The signal outputs are connected to the A3 and A4 pins on the Nano.
I also have up to 4 relays connected to the Nano. I have tried to use a cheap USB power supply to power the Nano via the USB port, but when one or more of the relays are activated it seems that the voltage drops (judging from a LED display) so that readings are not reliable - also I suspect the voltage output from the cheap USB power supply to be fluctuating further influencing reliability...
So now I have installed a dedicated 5VDC power supply for the sensors and the Nano (using the 5V pin), and a LED display. I am using the cheap USB power supply to drive the relays. The two PSU's have common ground.
This hardware arrangement seems much more sound - but I need to develop the software further and that is much easier with my computer connected to the USB port of the Nano.
Great idea - will try that 
That tells me that they are ratiometric, meaning the voltage on VCC has no influence on the output value of the sensor. A supply of 4.7volt (USB) will give the same pressure as 5 or 5.25volt from an external supply.
Leo..
If it's a USB-C connector, don't try.
Also there is no need to do it, just disconnect the 5V when downloading code.
Does your pressure sensor have a name or a datasheet?
