Multi-unit building with 2400 A electrical service (208 V 3 phase).

Main switchboard feeds risers to apartments.

Apartments have 100 A service. 5-7 apartments (depending on sq. footage) are fed from a riser that has a 300 A breaker (i.e. riser to 300 A breaker to 5 or so 100 A apartments).

The 300 A breaker is way undersized, right?

Thanks.

Is this place built already? What did the engineered prints say?

What role do/did you play in the construction?

I am going to go out on a limb here, and guess this is an apprentice questioning the design of this distribution system by wondering how could a 300 amp breaker feed 5 - 7 100 amp panels.

Whatever the case, you have to recognize the fact that someone noticed something that appears odd and is against apparent logic, and is asking the question. Well, actually makes the statement it is undersized, then questions their own statement.

No one here can answer the question accurately without knowing a great deal of information needed to do the load calculations to size the feeder supplying 5 - 7 apartments.

I refer you to Article 220- Branch Circuit, Feeder, and Service Calculations, of the National Electric Code, and with the information you have, size up the feeder.

Since this Article will make anyone run screaming into the night, refer to Annex D Examples of the NEC, and you will find examples of calcs for Multi-Family dwellings.

You will discover that the real logic is that not all 5-7 apartments will be drawing full load all the time, even with electric appliances such as ranges and dryers. Derating factors have been created to take this into consideration, helping to keep feeder sizes smaller and closer to actual load conditions.

I am sure the 300 Amp feeder and circuit breaker is size just right. After you do the calculations, you can confirm that for us.

I understand that not all apartments don't draw full load all the time, and the building has had no problems with breakers being tripped. My main concern is that the wiring in the 300 A feeder is of too small a gauge for the loads that may come from 5-7 100 A services (and some of the feeders have the old cloth/rubber insulated wires in them). The residents of this building are strapped for cash (like everyone else these days), and I'm wondering whether it's worth it for them to pay for an IR scan of the risers (to make sure nothing's heating up) and an evaluation of peak current in the feeders.

Your question shifted from the concern if the breaker is sized properly to if the conductors are sized properly, changing the entire context of the topic.


An IR Scan is recommended at least annually, but that only finds hot spots in electrical equipment, such as loose connections or failing breakers, etc. Analysis of each hot spot will show overloaded conditions only in real time.

The evaluation of peak amps, and comparing to the ratings of the conductors, and confirming the conductors are protected properly will help decide if there is an issue.

Advising if the surveys should be done is difficult. If we tell you do it, there is costs involved with performing the investigations. If we say not necessary, then you may be led to ignore a serious situation.

This is a judgment that management will need to make. Sounds as it the building is on the older side. Research into the last renovation and engineering documents will help with the evaluation.

IR scans are only one part of period maintenance required on electrical equipment. This along with periodic monitoring and analysis of electrical systems help find issues before they become costly or unsafe.

Surveys such as this are not done by your typical electrical contractor. The test equipment needed is a serious investment. Even engineering firms may not have this equipment.

In my opinion, no one does Preventive Maintenance and Electrical Systems Analysis better than TEGG - The industry leader in electrical preventative maintance. contractors. There should be others that do this work, but use Tegg as a benchmark, and get several quotes, and be sure the scope of work is what you need and comparable between all bids.