Calculating the Distributive Capacitance for 2 14AWG Cu conductors over 2000ft?

For ac you must think along the lines of the supply wires being a transmission line and not just a capacitive coupled line.

For almost ½ mile run of cable the capacitance and inductance form a fixed impedance transmission line...

As for the formulae to calculate the impedances, they would be pretty inaccurate depending on the type of wire you will be using...

If you were to use coaxial cable the impedances would be well known and documented, it would be more expensive, but if the capacitance and inductance is a concern for you - and I can't think why it should be - then use coaxial cable!

John.

To further define my question. The reason of concern is actually dropping out the coil when the source voltage is no longer present. Since holding VA is only 20VA on the AC coil; is there enough residual charge do to the capacitance to keep the coil pulled in for an unacceptable period of time?

If it is a concern then why not when turning the power to the relay off, put a short on the wire to give a positive short circuit to any capacitive storage current flow?

The acceptance criteria need to be stated.

• If, as already stated, the voltage drop at 12VDC is acceptable, then 12VDC is self-evidently acceptable!
• It is unlikely an AC load of this size would present any difficulty to a 120VAC control circuit, so the AC solution is also acceptable. The voltage drop is likely to be lower for the AC circuit.

24VDC and 110VAC are common industrial control voltages in the UK and neither of these would present any difficulty in the situation posted.

I agree, but to reference the statement "It is unlikely an AC load of this size would present any difficulty to a 120VAC control circuit". As Engineers don't you want to know what that "unlikely" value is? Isn't it our job to calculate it?

Well said!! Where do you get those smilies from??

John. Ahhhhhhhhhhh copy and paste does it

Just a thought, but 2000' of 120 VAC has got to be pretty expensive. If this isn't a classified area, the 12 VDC looks cheaper to me?

Well, it depends. If the conductor size is constrained by various specifications, then the marginal costing is [control transformer if required and not already provided] v [12VDC power supply to be supplied if not already provided]. The forum can't see the original poster's [OP] local specifications, customs and practices, so really only the OP can make that call, n'est-ce pas?

You're right. I had my xenophobic blinders on and was thinking NEC and the like.

If I were going to pay for the expense to pull a ½ mile cable then I would be dang sure to plan for future expansion. What would you do if next month you wanted to add more control points?

Many of the common PLC's feature open standard network communications. For example, a simple Allen-Bradley DH+ (Data Highway Plus) trunkline can span up to 10,000 feet. It will be very easy to add more analog or digital I/O to the existing network.

Click for info:
http://www.ab.com/en/epub/catalogs/12762/2181376/214372/1491278/

Regards, Delmar Schmidt
Melfi Technologies Houston
www.melfitechnologies.com