Dimming lights


Old 02-02-05, 03:07 AM
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Dimming lights

When a heavy load such as the heating or air conditioning unit cycles on, there is a very noticeable, momentary dimming of all the the lights. This doesn't seem right. I have checked all the connections in the service panel for tightness, corrosion, etc, and visually, all appears OK. I cannot check the connections in the external disconnect, as it is sealed by the electric company. However, I suspect another problem. The meter base and external service disconnect are on one end of the house, but the 200 amp service panel is on the other. The cable between the external service disconnect and the service panel handling the heat, air and other electrical on this end of the house is approximately 85 feet long, running under the house. Included among the various codes and markings on this cable are "SE...XHHW...3CDR 2/0 AL". I am concerned that a 2/0 aluminum SE cable of this length may not be sufficiently sized, thus contibuting to the problem. Am I on the right track, or could there be another issue I'm not thinking of?
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Old 02-02-05, 07:23 AM
Join Date: Nov 2002
Location: SC
Posts: 171
Is the external service disconnect a breaker or fuse? If so what is the rating? The code allows 2/0 cable used as a main power feeder to be rated @ 150a.
If its a 200A feeder the cable needs to be 4/0 AL. Also, with aluminum wire, check connections at both ends. Aluminum tends to expand and contract more than copper and the connection is more apt to become loose. Pull the wire out and look for corrosion, discoloration. If so, redress the wire and apply a non-oxidizing compound to the wires, like No-Lox, its a compound made for this purpose. Tighten the connection securely and come back in a week or so and retighten.
Old 02-02-05, 10:01 AM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Something about your description sounds fishy to me (the use of 3 conductor cable between your service disconnect and your 'main panel', and the cable does sound too small for the ampacity required), but that is a separate question.

Remember that _some_ dimming is always to be expected and accepted. You can't run current through wire without some voltage drop...unless you want to install superconducting cables in your home The amount of dimming depends upon the resistance of your supply wires, the resistance of any connections and components, the impedance of the supply transformer, and the _size of the load_.

Voltage drop is generally not a safety issue, but instead a design and comfort issue.

2/0 AL wire has a resistance of about 0.124 ohms per 1000 feet. So this length of cable adds 0.124 * 2 * 85 /1000 = 0.02 ohms of resistance to your supply circuit. A 200A load would cause 4V of drop, which is only 1.7% on a 240V system. This is probably a significant component of the total voltage drop in your home, but not the whole store, and my gut feeling is that there bulk of the problem is elsewhere.

Let me repeat, voltage drop is a design issue that is separate from the safety issue of required wire size. A #12 wire that is short enough will have acceptable voltage drop at 200A, but it would not be safe. A #0 wire that is long enough would be unaccepable at 10A, but would be safe. I am evaluating your 2/0 SE from the point of view of voltage drop only.

My gut feeling is that you should be looking elsewhere for your lights dimming problem. You should call the power company to get them to check their connections. You should also have an electrician measure the voltage drop across your main breaker and across all of the connections to this SE cable.

Good luck.

Old 02-04-05, 06:57 PM
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Thanks for your thorough response. I learned from this. I'm not sure which of my desriptions is fishy, but my house was built in two stages. The original house had a 200 amp panel about 12 feet away from the meter base, with a copper SE cable, in between. It is hard to read the markings since it is old, but I believe it is a 2/0 3 conductor copper SE cable. My terms my not be exact and specific. There is absolutely no dimming or other apparent issues with the electrical on this end of the house, which has a separate heat and air unit from the new addition. The new addition, which was added on about 12 years later, is the one where I have significant dimming of all lights when the heat or air cycles on, and the panel on this end of the house, which is the one I reference, is 85 feet from the meter base with the AL SE cable in between. The heat and air unit on this end of the house is similar to the one on the original end. I guess you are saying this dimming of lights under load is typical, but still, you indicate the cable may not be apropriate. I have called the power company multiple times, as was suggested several months ago when I asked about this same issue, but our frequent high winds, big trees and ice storms, etc has made it hard for them to keep their appointments with me. I will just keep trying. Thanks again, I will respond when I once get the power company to inspect and let you know what they say.

Thanks again --- Dan.

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