AC Question
- Thread starter SLaubach
- Start date
jthornton
Member
so long as the insulation on the 24vac is rated at or above 480 volts you can. It is not normally done to mix high voltage and control voltage.
Rick Densing
Member
We hired a certified journeyman to help wire a machine, once. He tried to wire it with solid building wire.
kamenges
Member
Very prestigious schools around the world graduate huge numbers of absolute idiots every year. We all know at least a handful of these individuals.
Until someone figures out a way to make that piece of paper do the thinking for someone, I'm not going to put too much stock in it.
By the way, I would stay away from running the 24 VAC with either 120 or 480 IF the 24 VAC is used for any signal sources. If it is just a supply for a contactor coil or a small drive, it's not a big deal. But you can and will induce noise into the 24 volt line if it is tied to a high impedence device. I usually think more in terms of signal versus driver than in terms of voltage level, up to a reasonable point and assuming the insulation is rated for the voltage.
Keith
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Ken Roach
Lifetime Supporting Member + Moderator
I've seen folks run all kinds of things improperly in conduit together, and they break down into two categories:
1. "But we HAD to." (Two pipes/holes/sliprings are expensive)
2. "Prove we can't." (Hate salesmen and want us to suffer)
My favorite Category 1 was running encoder feedback in the same conduit as AC servomotor outputs.
But this question was about 24 VAC and 480 VAC together. I agree that good practice and the NEC says you should separate them with a barrier (when in tray), distance (when in cabinets), or 600 V insulation (when instances really, truly dictate).
I recently went through a situation where my customer ran 120 VAC signals into a controller input and they would "stick on" or give "false on" inputs. It turns out the 120VAC was run in the same conduit as 480 VAC motor leads, all the way from a local disconnect back to the MCC. The controller input was seeing about 70 VAC.... and it considered anything above 36 VAC to be "on". If you tied a solenoid or a contactor to this, the induced voltage would not have enough current accompanying it to affect anything. But a solid-state controller input is a lot more sensitive.
That was a combination of Category 1 and Category 2. It would have been much more expensive to run a separate conduit, and they'd succesfully run similar inputs to a different kind of controller input module without a problem.
We ended up redesigning our controller module to accomodate their installation !
1. "But we HAD to." (Two pipes/holes/sliprings are expensive)
2. "Prove we can't." (Hate salesmen and want us to suffer)
My favorite Category 1 was running encoder feedback in the same conduit as AC servomotor outputs.
But this question was about 24 VAC and 480 VAC together. I agree that good practice and the NEC says you should separate them with a barrier (when in tray), distance (when in cabinets), or 600 V insulation (when instances really, truly dictate).
I recently went through a situation where my customer ran 120 VAC signals into a controller input and they would "stick on" or give "false on" inputs. It turns out the 120VAC was run in the same conduit as 480 VAC motor leads, all the way from a local disconnect back to the MCC. The controller input was seeing about 70 VAC.... and it considered anything above 36 VAC to be "on". If you tied a solenoid or a contactor to this, the induced voltage would not have enough current accompanying it to affect anything. But a solid-state controller input is a lot more sensitive.
That was a combination of Category 1 and Category 2. It would have been much more expensive to run a separate conduit, and they'd succesfully run similar inputs to a different kind of controller input module without a problem.
We ended up redesigning our controller module to accomodate their installation !
Tom Jenkins
Lifetime Supporting Member
My policy:
120 VAC with 24 VDC discrete I/O signals OK
480 VAC with 24 VDC discrete I/O signals to be avoided, VAC with 24 VDC relay or contactor control signals OK
Any AC with any analog signal (voltage or current) or communications wiring to be avoided
24 VDC discrete I/O or 24 VDC control with any analog signal (voltage or current) or communications wiring OK
You can (and I have) violate these guidelines and get away with it under some circimstances, but make sure you thoroughly test the signals and have a fallback position in case things don't work well.
(And as a side note, technically the insulation of the 24 VDC wire only has to be good enough to prevent breakdown and arcing of the 24 VDC to ground, since the 480 VAC wire insulation is already adequate to prevent breakdown and arcing to ground. You dont really need high insulation on the 24 VDC wires, although in the states most are insulated for 600 VAC anyway.)
Vetteboy: Passing journeymen's licensing exam doesn't mean an electrician is necessarily qualified to wire a light switch much less a control system, any more than passing the Professional Engineer's or HNC exams means someone is qualified to design an automation system. I agree that the US is behind the EU in many areas of control (120 VAC discrete I/O is still the norm over here, for example) but in defense of the US I have to say I've seen some damn fool things done both in Europe and in the US. Both systems typically have their relative strengths and weaknesses, and both have individual exceptions above and below the norm.
120 VAC with 24 VDC discrete I/O signals OK
480 VAC with 24 VDC discrete I/O signals to be avoided, VAC with 24 VDC relay or contactor control signals OK
Any AC with any analog signal (voltage or current) or communications wiring to be avoided
24 VDC discrete I/O or 24 VDC control with any analog signal (voltage or current) or communications wiring OK
You can (and I have) violate these guidelines and get away with it under some circimstances, but make sure you thoroughly test the signals and have a fallback position in case things don't work well.
(And as a side note, technically the insulation of the 24 VDC wire only has to be good enough to prevent breakdown and arcing of the 24 VDC to ground, since the 480 VAC wire insulation is already adequate to prevent breakdown and arcing to ground. You dont really need high insulation on the 24 VDC wires, although in the states most are insulated for 600 VAC anyway.)
Vetteboy: Passing journeymen's licensing exam doesn't mean an electrician is necessarily qualified to wire a light switch much less a control system, any more than passing the Professional Engineer's or HNC exams means someone is qualified to design an automation system. I agree that the US is behind the EU in many areas of control (120 VAC discrete I/O is still the norm over here, for example) but in defense of the US I have to say I've seen some damn fool things done both in Europe and in the US. Both systems typically have their relative strengths and weaknesses, and both have individual exceptions above and below the norm.
Vetteboy
Member
- Join Date
- Jul 2002
- Posts
- 380
The other funny thing about you Americans is that you assume if I'm not American, I'm European. Well, my ancestors came from there over 100 years ago, but I live in that easily forgotten country north of you. I'm so embarrassed by my government that I don't fly that goofy liberal flag that we were forced to start using in 1965. We also have an electrical code here to save people's lives, and a real electrician knows that this is no joke. Non-electricians who see how easy it is to just "hook stuff up" are a hazard to everyone including you, so watch your step, Tim.
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