Correct Wire for 125a subpanel

Question

I have a 125a subpanel in a shed that is used for all the pool equipment (control panel, pump, spa, lights). I don't really need anywhere near 125a but since I have 3 double pole breakers I would be running low on slots, so the choice of the 125a subpanel was more about the number of breaker slots available than actually needing anywhere near 125a.

My questions are...

1. Do I really need to run a #1 copper wire to this subpanel (which is what code calls for on a 125a panel). It seems completely overkill for my use-case.

2. What size breaker should I put in the main panel? Do I really need to put a 150a (or more) breaker which can be quite expensive?

Answer 1

Just because it's a 125 amp panel doesn't mean you need to feed it with 125 amps. I have a 200 amp panel in my shop that's feed by a 100 amp breaker and it passed inspection just fine. The only reason I went with a 200 amp panel is the same as your reasoning: Breaker space.

So you need to determine what your power needs are at peak usage and get wiring and a breaker for that. According to HP to Amps calculators online, a 3HP pump will draw about 10 amps at 240v. That seems low, but I checked with 3 different sites and they all came up with the same number. But check the name plate on the motor to be sure. Of course, if there is a pool heater that's electric it will be a big draw.

Bottom line, you just need to add up all the loads and size the wire and breaker for that. The breaker must be sized for the wire being used. No need to supply a 125 amp panel with 125 amps. The supply breaker is there to protect the wiring, period.

Answer 2

You're putting the cart before the horse.

Part 1: plenty of breaker spaces

First you decide how many breaker spaces you'll never, ever, ever need more than. The goal of this exercise is to make SURE you never run out of breaker spaces. Spaces are cheap; 30 spaces is not too many.

Once you have the breaker spaces, you will find panels with that many spaces have a certain bus limit or main breaker limit. Such as 100A, 125A or 200A. For instance my car has 105 mph tires. The judge put it very firmly that this is only a safety limit, and I don't need to install electrical feeder that big. I just can't install feeder bigger than that. So if I need 120A, a 200A panel yes, 100A panel no. Get it? That's all that is.

Part 2: The requirements of now and future loads

Now you look at the loads to be powered from there, and you do a mini Load Calculation for the loads to be in that panel. Allow for future possibilities. Don't work too hard since you'll probably be snapped to 90A just because of the way costing works.

Remember to apply a 125% upward derate to heaters, lights, EV charging and other loads which are continuous. So if the nameplate says 20A, count it as 25A.

Keep in mind electric vehicles are happening for real. Some time before you sell the house, you'll want to wire up a junction box ready for an EV outlet, because home buyers make higher bids on houses with them. Pencil in 30-50A for that. It's overkill for EV charging, but nobody ever got fired for fitting a NEMA 14-50 outlet, which happily can also serve large RVs.

While we're doing Load Calculations, also do a revised Load Calculation on your house to make sure your house can even support this additional load. If it can't, there are some options. In particular, this is not an impediment to electric cars because there's a bunch of tech for solving that problem. This doesn't affect the feeder size decision but may affect other parts of the project.

Part 3: Size that feeder!

Here we are at your question, and it's easier than you think with a likely default. Now that your now and future load amps, you select:

• 21-90A: 2 AWG aluminum
• 91-100A: 1 AWG aluminum
• 101-120A: 1/0 AWG aluminum
• 121-135A: 2/0 AWG aluminum

What is going on there? Real easy. Large aluminum heavy feeder is proven perfectly safe from 60 years of usage, because it is landed on lugs rated for aluminum (made of aluminum actually).

You may have heard of a crisis with small branch circuit aluminum wires, that was because a) the lugs were not properly rated for aluminum and b) nobody was torquing small connections in those days, and that matters! But they were torquing large connections. Use a torque wrench on your lugs.

Anyway, aluminum is perfectly safe, but #2 is the size at which everyone agrees on that. It is also used widely for service wires, so it lives at a pricing "sweet spot".

What about smaller aluminum if you don't need 90A? Not really worth it. #4 isn't any cheaper due to rarity, and #6 is only 50A.

Your questions

Do I really need to run a #1 copper wire to this subpanel (which is what code calls for on a 125a panel). It seems completely overkill for my use-case.

Nice to meet someone who can read the ampacity charts correctly!

You absolutely do not need copper wire, no. If your head popped off after seeing the price of #1 Cu, your head's gonna pop again when seeing the price of #2 Al. It's about the price of #10 copper (30A). This is why a 21A feeder might as well be #2. Gives expansion room and doesn't cost any more.

What size breaker should I put in the main panel? Do I really need to put a 150a (or more) breaker which can be quite expensive?

The breaker must physically fit the wire. So #2 feeder needs at least a 60A breaker (won't fit on the small frames used by 10-50A breakers).

The breaker must be large enough for the Load Calculation (which includes those 125% continuous derates)... and can be as large as the wire ampacity. If the exact size of breaker is not offered, you are allowed to "round up" to the next available breaker size.

Suppose your load is 57A. You cannot use 55A wire with a 60A breaker since 55<57. But you can use 90A wire with a 60A breaker since 60>57. And 60A breakers are cheaper than 70-90A breakers lol.