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# Calculation of panel amps usage

#1
12-24-17, 03:21 AM
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Calculation of panel amps usage

My panel is full and I wabt to replace a few spots with quads and tandems to add another circuit or two.
how can I calculate the amperage requirements so that it's not overloaded? It's a 100 amp Commander panel (challenger, Eaton, cutler H now).

Also, can I put a clamp meter on the main panel wire? It's a sub panel so I can switch it off first and measure at a few points during the day.

#2
12-24-17, 05:29 AM
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You perform a demand load calculation. There are several available online.

#3
12-26-17, 12:09 PM
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Is there a demand load for each side of the panel?
On a 100a breaker, do I only load 50 on each side?
What if I get it unbalanced somehow?

#4
12-26-17, 12:20 PM
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The demand load calculation is for the panel.

On a 100 amp panel you can get 100 amps from either or both sides of the panel.

#5
12-26-17, 04:29 PM
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Does that mean you can actually have up to 200 a total usage? Ignoring 80% rules and such...

#6
12-26-17, 04:36 PM
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When we miscalculated whole house generator sizing for a panel we often had to amp test the mains coming into the panel... Clamp meter them with every single thing on in the home. Vacuums hair dryers etc.. Often on a 200 amp panel we got a meeker 50 - 60 amps a leg...

#7
12-26-17, 05:32 PM
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A 100 amp 120/240 volt service can give you up to 200 amps of 120 volt power or up to 100 amps of 240 volt power or different combinations not exceeding 100 amps on any leg.

1. You follow the rules for the load analysis. One set is at the back of the NEC.
2. The rules tell you the numbers to use for continuous loads such as air conditioners and intermittent heavy loads such as clothes dryers.
3. The result of the load analysis is the panel size and service size (in amps) you need.
4. You should balance as best as you can the 120 volt circuits on the two hot legs.
5. As a rule of thumb each branch circuit should have continuous load limited to 80% of the circuit rating (its breaker rating).
6. The entire system is de-facto limited to a continuous load of 80% of the service load because the main breaker is assumed to have that limit. In practice you will rarely hit that limit.
7. Most of the time the load in effect will be far less than the limit, for example 50-60 amps a leg for a home with a 200 amp service and most of the load spoken for (as opposed to still in the future).

Last edited by AllanJ; 12-26-17 at 06:00 PM.
#8
12-26-17, 06:35 PM
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So, when you put in a 2 pole 240v breaker on the right side of the panel, it's actually balanced straight away since it uses both legs?

#9
12-26-17, 07:07 PM
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Correct. Unless it is a 120/240 volt load (like a 4 wire feed to a range or dryer) there will be some imbalance there.

Think of it in watts.
100 amps @ 120v = 12000 watts x per leg (2) = 24000 watts total.
100 amps @ 240 volts (both legs) = 24000 watts.

They are the same total. Your max load can only be 24000 watts on a 100 amp panel.
Next we'll talk about the size of the transformer feeding your home.

#10
12-27-17, 10:11 AM
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I'm getting 25,600W on my load demand calculation but I'm pretty sure the panel is never using that much.
1150sq ft
Heaters 11500W
Range cooker 4100W
Water heater 4500W
Clothes dryer 5600W
Portable AC running on 15A receptacle during some days in summer

#11
12-27-17, 06:53 PM
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What is the heaters? Base board or one unit?

#12
12-27-17, 11:26 PM
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Baseboard
The range says 49.4 amps on it but I'm pretty sure it never uses the full amount.

#13
12-28-17, 02:58 PM
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Seems high, no? It's an apartment. Maybe I should turn everything on and then put a clamp meter round each leg to see what's really going through them?

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