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lights dimming under 240 volt load #80453 04/17/02 07:47 AM
Joined: May 2001
Posts: 36
C.Urch Offline OP
The home has an instant hot whole house water heater with 4 elements fed from a subpanel; each element on 30 Amp 240 Volt breakers. Subpanel is being fed by #2 Alum. on a 100 Amp breaker from 200 Amp main service panel containing all the rest of the homes' service. Subpanel supplies only the instant hot water heater. The problem is that whenever hot water is called for, the lights (especially small 25 watt vanity light bulbs and recess lights on a dimmer)will dim noticably. Anyone have any ideas what might be causing this?

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Re: lights dimming under 240 volt load #80454 04/17/02 08:22 AM
Joined: May 2001
Posts: 717
George Corron Offline
If your lights were dimming under an A/C load, the diagnosis would be easy, inductive effect causing voltage drop, but I suspect your problem is the same.

How far from the main panel is the sub? What is the load on the insta-hot? What size wire was run to it? In other words, you've got a voltage drop problem dude, sounds severe. Can be caused by loose connections, check your breakers for heat, and all your terminal connections, if that ain't the cause, you need to check wire sizes and lengths.

Re: lights dimming under 240 volt load #80455 04/17/02 11:23 AM
Joined: Oct 2001
Posts: 597
ElectricAL Offline

I suspect the PoCo has a chance of being involved in this one. How far away is the transformer and what are the wires (Al or Cu? and gauge?). As I read your question, the domestic HWH load is causing a visible voltage sag out on the 200 Amp service center branch cicuits. That means the sag, or voltage drop is in the conductors between the 200 Amp bus and the supply transformer. Verify the connections on the service entrance conductors and talk to the PoCo customer service rep.


Al Hildenbrand
Re: lights dimming under 240 volt load #80456 04/17/02 02:55 PM
Joined: May 2001
Posts: 36
C.Urch Offline OP
I was thinking the lines of a PoCo problem myself. The #2 feeding the sub is about 30 to 35 feet from the 200 Amp main panel which is a back to back service having 4/0 Alum. to the main breaker. The sub panel is about 10 to 15 feet from the instant hot, containing 4 30 amp double pole breakers with #10 THWN CU stranded to each element. The PoCo transformer is pole mounted with their feed being underground down the pole running for about 150+ feet. It could very well be that their supply wire is 2/0 URD.

[This message has been edited by C.Urch (edited 04-17-2002).]

Re: lights dimming under 240 volt load #80457 04/18/02 07:25 AM
Joined: Mar 2001
Posts: 2,056
Redsy Offline
I hate to be a spolier, but the #2 al. fed by a 100 amp breaker is only acceptable as SE conductors, or as a feeder which serves as the main power feeder to a dwelling.
I see this violation regularly when 100 amp SE cable is installed as a 100 amp subpanel feeder.

Do the elements operate simultaneously? If so, I think the whole feeder ckt. is underrated.
The effects of low voltage are much more(visibly) noticeable on incandescent lighting because a 10% VD can result in up to a 30% lumen reduction.

Re: lights dimming under 240 volt load #80458 04/18/02 08:07 PM
Joined: Apr 2002
Posts: 2
triples Offline
Junior Member
Redsy, are you saying that Al can not be used to feed a subpanel? If so, where can I find that in the code? Thanks

Re: lights dimming under 240 volt load #80459 04/18/02 08:18 PM
Joined: Jan 2002
Posts: 1,457
Electricmanscott Offline
Not to speak for Redsy but what he is saying is that # 2 al can be fused at 100 amps if it is a service entrance conductor or a feeder that supplies the dwelling unit main panel. This does not apply in the case as shown above. With out looking I think the table is 310 15 b. I think this panel should be fed with #1 al. (dont have the book right here) I beleive that even if the water heater was fed from the main 200 amp panel, with that kind of load you would still see the lights dim.

Re: lights dimming under 240 volt load #80460 04/18/02 08:57 PM
Joined: Oct 2000
Posts: 5,389
sparky Offline
is this a single 30A X 4 (120A) unit when called on?

Re: lights dimming under 240 volt load #80461 04/18/02 09:25 PM
Joined: Mar 2001
Posts: 2,056
Redsy Offline
That is correct, sir.
In my experience, this mis-application of SE cable is fairly common.

Re: lights dimming under 240 volt load #80462 04/19/02 06:56 AM
Joined: May 2001
Posts: 36
C.Urch Offline OP

My understanding is that the heater elements engage under hot water demand. Running a hot bath would call on all the elements until the unit reached required temperature (then they would cycle as needed), while warm water washing would only call on some of the elements to engage.

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