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Designer69

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hey guys table 310.16 of NEC doesn't signify whether those conductors are to be used for AC only, so if I have a DC application and I calculate my amperage can I size my wire from the table same as if it was AC?

also, my application is a DC Solenoid valve. Why would they even make them DC?

My task is to size and derate the wires for a few DC solenoids but I have no experience with DC so I'm kind of frazzled

Thanks
 
Designer69 said:
hey guys table 310.16 of NEC doesn't signify whether those conductors are to be used for AC only, so if I have a DC application and I calculate my amperage can I size my wire from the table same as if it was AC?

also, my application is a DC Solenoid valve. Why would they even make them DC?

My task is to size and derate the wires for a few DC solenoids but I have no experience with DC so I'm kind of frazzled

Thanks
Click to expand...
1) Wires don't know from AC or DC. To them, it's all about current.
2) Why do they have DC solenoids? Because lots of people use DC, especially for control systems on things such as solenoids. AC solenoids can be noisy and take a lot more Closing current.
3) Size and rate the wires based on the current the solenoids will draw. Keep in mind though that the "Holding Current" is always much lower than the "Closing" current, but if you size the conductors only based on Holding current, you may end up creating a voltage drop for yourself when they try to close. My rule of thumb is to use, as a minimum wire size, the total Holding current plus the largest Closing current, or total Holding current x 1.5, then pick the closest wire size by only going up, never down.
 
As has been mentioned above, the NEC required minimum conductor size is determined by table 310.16 and the various rules about derating. All of these rules are based upon the heat produced by the wire when it carries current, and this is the same for DC and AC.

There is a separate design decision that you need to consider: voltage drop. The NEC does not limit voltage drop for general circuits, but your load may have a maximum voltage drop that it can tolerate. Very often, DC circuits are relatively low voltage (24V, 48V, etc) and keeping a low % voltage drop means conductors much larger than NEC requirements would dictate.

-Jon
 
090115-1123 EST

Designer69:

AC solenoids or relays have a DC coil resistance much lower than an approximately comparable AC device. This results from the inductance of the coil.

The effect of inductance in a DC excited device is to produce an exponential rise in current to a steady state value. This occurs quite quickly in normal devices. If you do not exceed the normal rating of a continuous rated device, then no matter whether the plunge or armature in the device is jammed or not you will never burnout the coil. The current thru a series RL circuit with zero initial current in the inductor at switch closure time and a source voltage of I*R is i = I*( 1-e^(-t*R/L) ). However, L (the inductance) does change as the plunger or armature moves.

The effect of inductance in an AC device is very important. The design of solenoid or relay devices is such that when energized the inductance is much higher than when de-energized. This naturally results from the purpose and mechanical design.

The impedance of an LR circuit (combination of inductance and resistance) is a function of the two components and frequency. The voltage applied across the circuit and the impedance of the circuit determines the current thru the circuit.

The design of most machine tool hydraulic solenoid valves for AC excitation is such that if the solenoid plunger is jammed, caused by the valve spool sticking, the solenoid will burn out. This is not the design goal but a result. One wants the maximum actuator force and this means current. When the valve is energized and the plunger is at full stroke the magnetic path has the least air gap, the inductance is much higher than when de-energized, and for a given current the magnetic force is greater. When in the de-energized physical position the inductance is much lower, and therefore at energization the current will be higher and the magnetic forces relatively greater than if one had the inductance of the energized position.

The high initial current lasts for a short time. If you add a lot of external series resistance in relation to the solenoid coil resistance, then you will degrade the initial solenoid force, but less so at the full stroke position.

Some courses in basic electrical circuits ( DC, AC, and transients) would help you understand some of these characteristics.

A book with a lot of analysis packed in a 1/2" thick volume is
"Analysis of A-C Circuits", by Melville B. Stout. Probably very hard to find a copy. One exists in the University of Michigan library, and has not yet been scanned by Google.

You can use Google Books and your ZIP code to determine if a library near you has any book that Google has cataloged.

.
 
090115-1946 EST

Correction to my previous post.
AC solenoids or relays have a DC coil resistance much lower than an approximately comparable AC device. This results from the inductance of the coil.
Click to expand...
This should have read:
AC solenoids or relays have a DC coil resistance much lower than the DC resistance of a DC solenoid or relay that performs an approximately comparable function.

In the AC design the inductance of the coil provides much of the impedance resisting current flow. In the DC design all the impedance is from resistance.

.
 
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