100 AMP Panel Load!

P

Prototype1

Member
Location
Kansas City
Occupation
Noob EE
Okay, I am going to sound dumb. But I want to understand the stuff.

I have a 300-amp main panel and a 100-amp sub-panel, which is fed from the main panel. The sub-panel is protected by its own 100-amp breaker located in the main panel.

The 100-amp sub-panel is dedicated to powering park trail lights, pickleball court lights, and tennis court lights.

### Sub-Panel Details:
The 100-amp sub-panel has 30 breaker slots. The current configuration of branch devices is as follows:

#### Branch Devices:
| Quantity | Poles | Trip Rating | Voltage |
|----------|-------|-------------|---------|
| 5 | 2 | 20 A | 208 V (single phase) |
| 3 | 1 | 20 A | 120 V (single phase) |
| 17 | 1 | Spare | — |

#### Main Breaker Device:
| Quantity | Poles | Trip Rating |
|----------|-------|-------------|
| 1 | 3 | 100 A |

---

I understand that the total current draw from all breakers in the main service panel should not exceed 80% of its capacity, per NEC guidelines. For example, a 20-amp breaker can support up to 16 amps of continuous load (80% of its rated capacity).

Question:
Given this understanding, I am confused about how many 20-amp breakers I can install in the sub-panel. Specifically:
1. The sub-panel already has five 20-amp, 208V single phase breakers and three 20-amp, 120V breakers.
2. When I total the amperage ratings, they appear to exceed the sub-panel's 100-amp capacity.

Could you clarify how this works? Does this mean the breakers’ combined ratings (e.g., 5 x 20 amps + 3 x 20 amps) exceed 100 amps, or is this more about the actual current draw on each breaker?

I have to add dedicated circuits for the grill, oven and receptacle (GFCI) as it is outside in park. So, I am calculate how many slots will be left in a 100 amp panel that has 30 slots.
 
Prototype1 said:
I understand that the total current draw from all breakers in the main service panel should not exceed 80% of its capacity, per NEC guidelines.
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No, only if that current draw is continuous. For any other current draw with a significant intermittent component, you can load the panel's main breaker up to 100% of its rating.
Prototype1 said:
For example, a 20-amp breaker can support up to 16 amps of continuous load (80% of its rated capacity).
Click to expand...
Yes, because you specified a continuous load.
Prototype1 said:
2. When I total the amperage ratings, they appear to exceed the sub-panel's 100-amp capacity.
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Totally breaker ratings doesn't tell you anything about the expected actual current. Totaling the loads on each branch circuit will tell you that. And yes, it is the actual current that matters.

Cheers, Wayne
 
LarryFine said:
Breaker ratings and adding breaker ratings are irrelevant. Only the actual load currents matter.

Don't the breakers in your home panel total up to way more than the main breaker's rating?
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If I understand it right.

the individual breakers are designed to trip or blow before the main breaker is overloaded. So, the electrical system is protected from overloads and faults, even if the individual breaker ratings add up to more than the main breaker's rating.

Did what I said make sense>
 
Prototype1 said:
the individual breakers are designed to trip or blow before the main breaker is overloaded.
Click to expand...
Not really. Say you have a bunch of loads that are 5A (non-continuous) and are supplied from a 100A panelboard. You could supply 20 such loads from the panelboard without exceeding the panelboard's rating. But the smallest size breaker you can get for the panelboard is 15A, so you have 20 of them in the panelboard.

In normal operation all is well. And if one of the devices shorts out, or even just suffers a partial failure and starts drawing 25A, the individual 15A breaker will protect the wiring on that branch circuit and trip. But if say all the devices simultaneously misbehaved (maybe they are electronic and got a really bad firmware update) and started drawing 7A each, that's well within the allowable behavior envelope of a 15A breaker. However the aggregate current through the main breaker would be 140A, and so it would trip in fairly short order.

It is because of the possibility of this occurring that we need to have a 100A main breaker protecting the 100A busbar of the 100A panelboard.

Cheers, Wayne
 
wwhitney said:
Not really. Say you have a bunch of loads that are 5A (non-continuous) and are supplied from a 100A panelboard. You could supply 20 such loads from the panelboard without exceeding the panelboard's rating. But the smallest size breaker you can get for the panelboard is 15A, so you have 20 of them in the panelboard.

In normal operation all is well. And if one of the devices shorts out, or even just suffers a partial failure and starts drawing 25A, the individual 15A breaker will protect the wiring on that branch circuit and trip. But if say all the devices simultaneously misbehaved (maybe they are electronic and got a really bad firmware update) and started drawing 7A each, that's well within the allowable behavior envelope of a 15A breaker. However the aggregate current through the main breaker would be 140A, and so it would trip in fairly short order.

It is because of the possibility of this occurring that we need to have a 100A main breaker protecting the 100A busbar of the 100A panelboard.

Cheers, Wayne
Click to expand...
LarryFine said:
Negative. Each breaker is there to protect that circuit. The main breaker is there to protect the service.
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Okay, the main breaker to protect the panelboard's busbar. The main breaker ensures that the aggregate current through the panelboard does not exceed the panelboard's rating, even if the individual breakers are still within their rated limits., not just the individual circuits and the each individual breaker protect the circuit (feeders).

Hopefully, I understanding it right this time. Sorry if I am slow to understand.

It is starting to make sense.
 
Prototype1 said:
Okay, the main breaker to protect the panelboard's busbar. The main breaker ensures that the aggregate current through the panelboard does not exceed the panelboard's rating, even if the individual breakers are still within their rated limits., not just the individual circuits and the each individual breaker protect the circuit (feeders).
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Exactamundo!

Prototype1 said:
It is starting to make sense.
Click to expand...
We do that sometimes.
 
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