sub-panel overload??

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Old 11-05-04, 06:10 PM
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BigC
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Question sub-panel overload??

Hello all! Second try on this post.

My house has a 150amp, main lug style (no main breaker) circuit breaker box. It is completely full. I want to add a 100 amp sub panel to install a 50 amp hot tub. I'll be dragging a 30 amp water heater breaker over to the sub to make room for the 100amp sub-panel breaker. At present, my total current load is 260 amps (total of all breakers). The new panel will add 70 amps (100-30) for a new total of 330amps. I am concerned about overloading the main box and service cable (2/0, AL, <10feet) given that my main box does not have a main breaker. Any thoughts? In this type of circuit breaker box, how are the box and service cable protected against overcurrent?? House has A/C, water heater, dryer on electric service; range is gas.

Thanks!
 
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Old 11-05-04, 06:58 PM
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First, the total of all the breakers has no meaning. A load calculation should be done to calculate the existing load on your panel.

Adding the 100 amp panel does not add any load until something draws power from it.

Your would be better off to move two smaller circuits to make room for the 2 pole breaker to feed the subpanel and leave heavy loads like the water heater in the main panel.
 
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Old 11-05-04, 06:58 PM
J
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The total of your breakers is a meaningless number. However, adding a hot tub to a 150-amp service is very iffy. Answer the following questions:
  1. How many finished square feet is your house?
  2. Is you home heat gas or electric? If electric, how many KW?
  3. Do you have air conditioning? If so, how many tons?
  4. Is your water heater gas or electric?
  5. Is your cooking gas or electric?
  6. Is you clothes dryer gas or electric?
If you have no main breaker, and no external disconnect, then you likely have a split-bus panel. Please further describe your main panel.
 
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Old 11-06-04, 05:51 AM
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BigC
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Smile

Thanks for the reply.

House is 2500 sq. ft. Gas range and heat. A/C, water heater, dryer, and a sub-panel for my garage (DIY) are the largest loads. Box is an ITE EQ5X8A (circa 1989), with 2/0 service leads connected to the main lugs. Neutrals and grounds are common, box is bonded to copper water pipe. 5 DP spaces house A/C (50), water heater(30), dryer (30), garage sub panel (50), and a 60 amp lighting main that splits to 2 120vac rails for the general lighing circuits in the house. 2, 120vac spaces are provided above the split rail.

Electric meter is standard 200 amp, pop-out style, located on the opposite (outside) wall from the main breaker panel.

Breaker values are generally meaningless if you have a main breaker to protect all house circuits. But with my style of box, I could continue to load the system down with circuits until a real problem results.

How much is too much?

Thanks!
 
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Old 11-06-04, 06:34 AM
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Yes, this is a split bus panel. The collection of double-pole breakers at the top of your panel collectively forms your disconnect. This type of panel is no longer allowed on new installations. Because the top breakers collectively act as a disconnect, the sum of the upper breakers should not exeed the rating of the panel. Unfortunately, yours does. I suspect that the subpanel and/or the A/C were added after original installation. In my opinion, this panel may already be unsafe. I would have it examined by an electrician.

If I assume you have about 3 tons of A/C (you didn't answer that question), your current demand load is only about 100 amps. So you should have capacity for the hot tub. But I would seriously consider replacing your main panel with a safer one, and you can consider upgrading to 200 amps at the same time if you want.
 
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Old 11-06-04, 08:41 AM
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BigC
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Thanks, John. You have confirmed my gut feeling that the panel should be upgraded. I added the garage sub-panel a few years ago using the existing (open) 50 amp range breaker. The previous owners switched to gas at some point.

Let the fun begin!
 
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