Breaker Capacity in a Three-Phase Distribution Board

The number of breakers that can be added is determined simply by counting the number of unused slots available.

The cable feeding the board should have its upstream circuit protection device and its supply cable matched, and a record should exist of the testing that has been carried out on them. So there shouldn't be any difficulty in adding circuits to this board.

The additional circuits will be tested and certified, won't they?

Actually I just asked the wrong question doh ! Similar however. The main distribution board has a 250A Main Breaker. Outgoing it has 1x200A 3x125A and 1x100A..feeding different boards. This is where I'd like to know if I could add another 100A breaker...

Sorry for the confusion...

Would like to know how to work it out mathematically though to be certain before a new board is added..

The number of breakers that can be added is determined simply by counting the number of unused slots available.

The cable feeding the board should have its upstream circuit protection device and its supply cable matched, and a record should exist of the testing that has been carried out on them. So there shouldn't be any difficulty in adding circuits to this board.

The additional circuits will be tested and certified, won't they?

PW

It sound like he is asking if he has the ampacity to add another breaker. The ampacity of breakers in the panel has nothing to do with the spare ampacity of the panel. You can calculate the load on the panel using your local code regulations or you can hang a recording amp meter on the panel and do a survey of the load for 30 days and determine if the panel has spare ampacity that way.

wareagle; good answer, my answer also. perry

Thanks for all your answers ....

Yeah I calculated that from max demand over the last few months, converting kw to kva dividing by 1.73x230 assuming a power factor etc that The Amps drawn was approx 110 A...

So I should be alright to add another 100A distribution board..?

There looks like enough room to add another breaker.

What I was uncertain and am still a bit confused about was just because their was room for another breaker I wanted some way of knowing what size breaker I could add safely......

Is just working the current load out enough to justify adding another 100A breaker ?

Thanks all ....some good answers

Your question about "how" to figure the appropriate mathematical planning capacity is still valid, and what you are looking for is the "Demand Load."

What you have with breaker sizes is only the "Connected" breaker capacity, which is interesting, but relatively irrelevant, so to speak.

The question of adding 'any more breakers' is solely one of 'is there a slot,' whether it's a Main Distribution Panel (with large multi-pole feed breakers serving other panels and large equipment) or if it's just a branch panel, whether Main Lug Only or with a Main Breaker on it. The exception to this is if you do NOT have a single large Fused Switch or Main Breaker. If you have a Main Switch board with exactly 6 main feed breakers or fused switches, you cannot add a 7th main breaker without adding a Main as well, as there is a maximum number of 6 service disconnects to kill power to the building during emergencies.

Each piece of actual equipment has a nameplate, some may provide the Full Load Amps (FLA) that you need, others may only give you the rated Horsepower or MCA (min. circuit ampacity), some may give you a "Wattage" or a "Volt-Amps (VA)" rating, and other circuits such as those for receptacles have an NEC (code) rating for planning or may require you to apply some judgement, such as electric heating circuits on the same board with cooling equipment (both will not be active at the same time, so you use the higher of the amperage demands as the worst-case).

During planning, the FLA is what you are after, and there are some Code requirements for maximum loading to allow for future expansion, which is what you are doing now. At this time, you are just trying to not plan for the simultaneous Demand Load from exceeding the breaker that protects the panel of interest.

In the "real" world, your best bet for safety, since that's what we're after, is to put a logging ammeter on each phase of the panel during a representative time period, and watch your peak demand. You may already have a Demand Meter from the Utility on your service, if you're being billed that way, and you could ask them to get you that information.

That will let you know if you can safely add a 100A breaker (for perhaps another panel or single large load that max's out at around 70Amps or so at FLA continuous rating? In this scenario, it's the 70A load that you're adding, not the 100A MOP breaker size required by the manufacturer).