Heavy loads dim lights on start... solution?

[Jeff Hutchison]

My electrical knowledge is solidly dangerous, so I thought I'd run this by the wise folks of TX-RX...

Whenever I have a large load (portable AC compressor for instance) on my house wiring, it dims the lights when it starts, and pulls the voltage down while it runs.  There's probably nothing that can be done about the voltage drop (except more/thicker wire)... but is the solution to the in-rush problem as simple as a large capacitor added in-line with the hot wire?

[Tim Wasson]

NO NO NO.  Do not connect a capacitor.  Do not run the AC as long because it is pulling the voltage down so badly.  Big voltage drops mean wires are carrying current beyond their capacity, and they are getting HOT.  Something is very wrong and it could burn your house down and kill you.

Starting loads for motor driven items like compressors are essentially a dead short until the motor gains some RPM.  That is why the lights dim when it starts.  This is relatively normal, and wires and breakers are designed to deal with short-term (10 second) overloads like this.

The fact that your lights never get back to full brightness means that the voltage drop is not due to the starting load, but instead is due to the continuous load of running the compressor being too much for the wiring.  The lights being dim means that the voltage is below 110 volts.  This is because the current load is so heavy that the resistance of the wires in your walls is consuming power and generating heat.

This is bad.  Normally, running this way for more than a few minutes will trip the breaker, because as voltage drops, the current required to drive the load increases.  The breaker senses the increased current draw and eventually trips, protecting the wire from heating up too much and starting a fire.

If the breaker isn't tripping, and your lights are visibly dimmer than they should be, then you are in a dangerous situation.  It is very possible that the breaker is too big for the wiring.  The reason this is dangerous is because the wires will get hotter and hotter when they are carrying too much load.  The breaker won't care how hot the wires get and will NOT trip until the current goes over the breaker's capacity.  If the breaker is too big, your house can burn down and the breaker will never trip.  Even if the cord to the AC is not hot, that doesn't mean that the wiring in the walls is not hot.

Someone may have put a 20 amp breaker on a circuit that was wired with 14 gauge wire, which should only have a 15 amp breaker on it.

Other things to note:

1. Do not run the heavy load off an extension cord of any kind.  Extension cords are generally not intended for high duty-cycle use at their stated capacities.   There are exceptions, and they are specially marked for heavy loads.

2. It's possible that the wiring and breaker are fine, but there is a high-resistance connection (something not tightened properly) somewhere.  The most likely place is the outlet itself.  If you can switch off the breaker, you can check the connections on the outlet to make sure the screws are tight.  If the plug is wired with the wires just shoved into little holes on the back of the outlet, then it's not going to be good at carrying heavy loads.

3. I would HIGHLY recommend you don't mess with any of it.  Find someone who knows what they are doing.  Death is VERY permanent.  You are correct about your amount of knowledge - it is dangerous.

Please forgive if this seems pushy, but the situation you have is not safe, and you should NOT dig into it with your level of knowledge.  

Tim

On Mon, Jan 23, 2017 at 6:30 AM, Jeff Hutchison <jhutc...@gmail.com> wrote:

My electrical knowledge is solidly dangerous, so I thought I'd run this by the wise folks of TX-RX...

Whenever I have a large load (portable AC compressor for instance) on my house wiring, it dims the lights when it starts, and pulls the voltage down while it runs.  There's probably nothing that can be done about the voltage drop (except more/thicker wire)... but is the solution to the in-rush problem as simple as a large capacitor added in-line with the hot wire?

--
You received this message because you are subscribed to the Google Groups "TXRX Labs" group.
To unsubscribe from this group and stop receiving emails from it, send an email to txrxlabs+unsubscribe@googlegroups.com.
To post to this group, send email to txrx...@googlegroups.com.
Visit this group at https://groups.google.com/group/txrxlabs.
For more options, visit https://groups.google.com/d/optout.

[Jeff Hutchison]

Thank you for your well thought-out, and extensive response... unfortunately, this is not the answer I am looking for. 

The voltage drop is not unreasonable (stays above 110V), nothing is getting hot, light doesn't remain dim, no breakers are tripping.  I have a couple of UPS on the circuit for sensitive loads - they have voltage readouts.

So, back to the question at hand... 

External start-up capacitors?  Anybody?

[Jeff Hutchison]

When the compressor is off, voltage floats in the 1-teens.. compressor on, drops to 110 +/- 1... 

[Tim Wasson]

Hi Jeff,

Sorry for misreading your initial post.  Glad to hear nobody is going to get hurt! lol.  Voltage around 110 is at the bottom of acceptable range but should be OK.

Anyway - with respect to capacitors:

The 'start' capacitors on motors are there to shift the phase of the AC voltage to give the motor some extra twist on startup, and to avoid it getting stuck trying to start.  They really aren't there to buffer inrush current.

What you are asking about is storing energy in a capacitor to be used to help get the motor started without dropping the voltage.  That is going to be pretty hard to do.  

First, an AC capacitor isn't going to store energy and release it over the 5-10 seconds of startup.  It will simply shift the phase around a bit and fiddle with the power factor, none of which is going to really help with your voltage drop.

Second, if you try to use a DC capacitor, you're going to have to rectify the power to DC, then store it, and somehow turn it back into AC to feed to your load.  So you're looking at running an inverter that is big enough to handle the start load of your a/c, and that's going to be 5x your normal running load.  You could conceivably get a bunch of 1 farad 12 Volt capacitors, build a bank to store enough energy, then get a massive (really massive) inverter to run your AC, all to get rid of 5 seconds of dimming.

Now it can be done, the solar guys are doing it all the time, but there's no way it's going to be cost effective or really produce any benefit in your case.

Just my .02, but your money would be much better spent in upgrading your a/c.

Tim

[Jeff Hutchison]

Tim, 

No apology necessary - after I re-read it, I see that it easily translates into "I'm an idiot who is hell-bent on burning my house down)... hahaha!

Your response above is EXACTLY the education I was looking for!  Thank you!  It makes sense that it is not easily accomplished, and that is surely the reason nobody sells a box that keeps your power tools and other devices from dimming the lights when you use them.

[Tim Wasson]

My pleasure Jeff, glad I was able to help.

Tim

[Gene Horr]

I see you got straight on the capacitor issues. Now, on to:

> When the compressor is off, voltage floats in the 1-teens.. compressor on, drops to 110 +/- 1...

Generally if you keep the voltage drop below 5% you are considered safe - i.e. won't burn the house down. If 1-teens means 114 V then you are probably safe although you may not be optimal. If 1-teens means 118 V then I would recommend a wiring upgrade. Personally I don't like to see even a 5% drop. At best you are marginal in terms of wiring. Personally I always add a dedicated circuit in the garage for a compressor (and other heavy machinery). Assuming the panel is near the garage it is fairly cheap to do/have done, the compressor motor is "happier" with the higher voltage, and you get better peace of mind knowing that you have a safer setup.

Downside is you may need to buy some more air line when working in other parts of the house, but that is also fairly cheap.

If you are doing the wiring yourself then my recommendation is to use a heavier wire gauge from your calculation and that it is pure copper wire. Also make sure that the outlet is a name brand commercial line rated for 20A. Last of all you want the breaker to be the weakest part of the system. Nothing wrong with wiring for an 18A load, outlet for 20A, but only a 15A breaker assuming that your load is always under 15A.

Edit - Just reread some of the post. You said that you were getting the voltage levels from a UPS. Is this on the same circuit? If not then you have a real problem. That 5% drop is for a load on the same circuit. If your whole house is dropping 5% there is something wrong.

Gene

[Jeff Hutchison]

Yes, I'm with you on the allowable drop - and yes, it's marginal, but not dangerous.  Funny side note - did you know that knob and tube wiring has a higher amp rating than same gauge romex?

This is all in a bedroom acting as an 'office' where I have a workstation and a server (among other things) on UPS's.  They are on the same circuit as a portable AC and the lights for the room... the compressor I was referring to was the one in the AC - when it kicks on, the room lights (not the rest of the house) dim.  This is why its better to have the lights on their own circuit (also if you blow a breaker from the outlet, you still have light in the room!)

I agree that the setup is not optimal - the room in question is the furthest from the panel, and doesn't get much flow from the poorly balanced central AC.  I'll be moving the server from that room to another that has "too much" central air... and then I likely won't need the added cooling in the office.  If I do need more cooling, I'll either add a duct boost fan or another register (good access in the attic).

Eventually (probably when I redo the kitchen), I'll do an electrical upgrade

[Phanny]

Some good information in these replies.  One thing that stood out for me was "same circuit as portal AC and lights for the room".  That's not correct, outlets should be wired on separate circuits from lights.  Out of curiosity when was the house built? You mentioned knob and tube wiring, which makes me wonder if that AC wasn't a retrofit put on a undersized panel.  Also outta curiosity, what's your service panel size, I'm guessing 125A.

-Chris

To unsubscribe from this group and stop receiving emails from it, send an email to txrxlabs+u...@googlegroups.com.

[Jeff Hutchison]

Phanny,

House is from 1982 and has no K&T - I digressed a bit.  I agree that it isn't the "correct" way of doing things, but it's relatively common, and as far as I know its never been a code violation, although now we're now required to have Arc-fault for bedrooms (that's what I did for the place in KS). 

To unsubscribe from this group and stop receiving emails from it, send an email to txrxlabs+unsubscribe@googlegroups.com.