Amperage Question

I live in an urban area and inspect a lot of condos and co-ops.

In these situations the main panels are in the basement of a large apartment building and the apartments have sub panels in them. One of the items of information I need to supply is, not what the sub panel is rated for (125 amps) but how many actual amps are available at that particular panel at the time of inspection.

For instance, if the main panel in the basement has a 60 Amp double pole breaker supplying two feeders to the sub panel in an apartment what is the proper max amperage to report that the sub panel can provide?

I get confused on this because with a 60 Amp double pole breaker it would “seem” (due to the two feeder cables to the sub) that the answer might be 120 potential Amps (60 + 60 = 120).

Sorry if this is a silly question but I’m finding the answer hard to find online.

Thanks in advance.

60 Amp.

Although you have 2 conductors you have a single feeder not two feeders. The ampacity of the OCPD will determine the feeder size. You feeder is 60 amps at 240 or 208 volts depending on the building system.

Greetings Darrin,

Remember you are dealing with alternating current, each one of those legs will and can experience 60 amps but they will never experience 120 amps due to the nature of the alternating current and limitation of the overcurrent protective device itself.

Think of it this way, single phase "power like a bicycle where one leg (phase) can push on one pedal, or both legs (phases) can push on both pedals (180 degrees out of phase with one another) rotating around a crankshaft axis (neutral).

Mechanically, power is calculated as leg pressure (Foot Pounds) times speed (Rotating Speed). Electrically, power is calculated as leg force (Voltage) times flow (Current)."

Now you stated the following " One of the items of information I need to supply is, not what the sub panel is rated for (125 amps) but how many actual amps are available at that particular panel at the time of inspection."

I find this odd because home inspectors are not in the business of assuming that the available current is on a panel because the loads are unknown to you. Now, if the remote distribution panel (or distribution panel) is rated for 125A but the circuit breaker or fuse ahead of it is rated 60A then you have two values to take into account. If the conductors that supply it are also only rated for 60A then the limiting factors are the conductors and the device that protects them.

So the ratings may be 125A but the capacity based on what is present would be 60A and that is what I would report so that the customer will not assume just because the panel enclosure is rated at 125 A that they will have that much available to them. So to me you have to report on that is actually available to consumer and that would be 60A based on the information collected.

Now, if the conductors are rated for 125A, the enclosure and panelboard is rated for 125 A and the feeder OCPD is only a 60A then you have in my opinion multiple things to report. Available Capacity is 125 A but I would also note on my report that it is limited to 60 A based on the limitations of the feeder protection. A simple change of the feeder protection would increase the available capacity to 125A so it becomes tricky…personally I always reported both and let the chips fall where they may.

Also on the Feeders…listen to Robert…he is a wise man!

Thank you Paul. Your answer was the most helpful and easiest to understand. I am familiar with the concept of alternating current so this makes sense. So the max amps, if I were to report on that, would be 60A and nothing more.

And, anything under 100 Amp is considered low by today’s standards and so that should be mentioned; however in some small apartments with few appliances having less than 100A may still be sufficient, correct?

Well Low is relative…If the calculations were done (and you have to assume they were) then maybe 100A is all that is needed. I would simply report what I see and know as fact and let others decide that fate.

If I see a 100 A panel and it has a bunch of double taps, tandem breakers that are not listed for use in such panel and so on being used then that may lead one to saying such a statement but then again I would go with the facts as they are shown.

I agree with Paul, unless there is something telling you that the 100 amp service is way undersized it’s not worth mentioning. Around here many older homes with all natural gas appliances have 100 amp services and there’s not a problem unless maybe if you want to start adding hot tubs, pools or large Tesla chargers.

I agree with this. It makes sense. Yet if you were to look at the InterNachi narratives (which I purchased) they have narratives saying that 100A is a bit low. So if you went by that you’d think you should mention it - or at least InterNachi seems to be suggesting that you should. Yet I run into many apartments with a sub panel rated for 125A but being fed only 40-60A which is maybe fine for a small 600 sqft apartment with few appliances.

Well we just never know who wrote those narratives as they are not updated over time I would suspect. By todays power hungry homes it may be “low” but still doesn’t mean it is not compliant and adequate for the home you are inspecting.

The problem that any HI’s struggle with is that they are allowing their eyes to take the place of a calculation, a calculation mind you that they are not required to do or perform so it makes it very hard for the HI. My golden rule is always report what you see and never assume what you do not see.

Take your 600 sq. ft unit and lets do some dwelling unit (single) math with just some random values.

600 sq. ft x 3 VA = 1,800 VA
(2) SMBC @ 1,500 VA each = 3,000 VA
(1) Laundry Ckt @ 1,200 VA each = 1,500 VA
(1) Typical Range @ 8KW(KVA) = 8,000 VA
(1) AC vs. Heat (largest) @ 5,000 VA
(1) Microwave (at best) at 1,200 VA

Now for kicks and giggles lets just use the Optional Method for this dwelling unit.

General Lighting & Receptacle Loads Step 1 - 1,800 VA (General Lighting & Receptacle) + 3,000 (SABC) + 1,500 (laundry) for a total of 6,300 VA.

Appliances Loads Step 2 - 8,000 VA (Range) + 1,200 VA (Microwave) = 9,200 VA

Apply the Demands in 220.82(B) - 15,500 VA (combined step 1 and 2) -10,000 = 5,500 VA. So the first 10,000 VA at 100%, which is 10,000 VA and the remainder at 40%, the remainder was 5,500 x 40% = 2,200 VA.

So 10,000 VA + 2,200 VA = 12,200 VA

Heating or Air Conditioning [220.82©] - In our case we compared the heat to the AC and the larger of the two was 5,000 VA so that is what is used.

Now it is 12,200 VA + 5,000 VA = 17,200 VA, the Demand Load.

17,200 VA / 240V = 71.66 Amps which takes to a need for a 100A Feeder or Service.

So again what may look small may be rather adequate. Interesting note, based on diversity I was at a house, rather large house the other day helping a friend who had some electrical issues and we turned on nearly everything in the house, all that we could anyway and clamped an amp meter on the service conductor (each leg tested) and the maximum draw was 52 amps in his panel. Yes, it is Texas and his AC was on and range was on and all lights on and so on…best we could do.

Darrin …

1500-2000sf house / 3-2-2 / Central A/C + Electric Range + Electric Dryer

Very Common House. See 100amp services all day long on these.

Would not consider telling someone it was low UNLESS there was a lot of extras.