I noticed that most Square D sub panels are 100 AMPS, but Siemens has one that is 125 AMPs (maybe other brands also, I haven't checked). Is there any differences I need to know about when I suggest a 100 AMP sub panel versus a 125 AMP (or bigger) sub panel to my electrician?

In this case the more AMPS the less wire he will have to run.

I am thinking about discussing this with him as an alternative to make it easier on him when he rewires my house, since the main panel is almost 50 feet away from the part of the house that needs 80% of the rewiring.

  • What's going to be connected to the subpanel? is it in a garage where there are lots of hungry workshop power tools? no harm going to the 125A – Matt Jul 8 '13 at 0:22
  • I agree - need more information to provide a good answer. Do you just need more circuits to be up to modern code? Are you building a workshop or something that needs a lot of power? Constructing another building on your property? Adding central air and you don't already have it? – user4302 Jul 8 '13 at 2:27
  • You contractor should tell you what rating he needs. We cant possibly answer this. Its like is a 1.2litre engine better than a 1.6 litre engine. Sometimes yes sometimes no. This question is not specific and open to speculation - If you don't trust your contractor find another one. – Piotr Kula Jul 8 '13 at 12:42

EDIT - The electician is the expert and he would tell or supply you what you need. The question should really be "mr electrician, do I need a 125 or 100A Sub Panel? but usually they just supply these for you.

He'll size it according to the what's connected to it and the number of circuits that are off it.

Since you asked about the differences between the sub-panels. I looked it up and here are the specs of each.

Comparing a 100A and 125A from Seimens.

100A version.

  • 1 Phase, 3 Wire, 120/240VAC
  • 12 Spaces, expandable to 24 circuits maximum with the use of QO tandem breakers
  • Type 1 Indoor enclosure with automatic flush adjustment and door
  • CSA approved for vertical,horizontal or inverted mounting
  • Lifetime Warranty

125A version.

  • 12 Circuits expandable to 24 (using space-saver breakers)
  • Certified for use with any combination of full or half module plug-in circuit breakers
  • Certified for mounting vertical, horizontal or inverted
  • Suitable for copper or aluminum conductors Rugged construction and reliable performance -A complete line of accessories give Loadcentres the flexibility to meet application requirements

So it fits the same number of breakers actually. That suggests that inside the bus bars are a little bit heavier rated (larger) in the 125A version. Which means:

PRO's: More available power if you need it - e.g. a greater number of higher ampage breakers. Or a larger number of lower amp ones. e.g. a configuration might be 4 x 25A breakers + a 15A and 2 x 5A. - 7 breakers but maxing out the capacity. You couldn't do this in the 100A version.

CON's: Appears to cost more.

I would suggest unless you can see that you've got a massive load, 100A will be fine.

If you're using your subpanel for a home workshop and you have several high current power tools like a table saw, air compressor, welder etc... then go with a 125A and run lots of circuits for the workshop. Otherwise, you may just be spending money for no real benefit.

Actually - 125A might be too small for most welders but I'm not familiar enough with the supply power they need so if you're going to run a welder - get that checked out with your electrician.

Ask your electrician what's needed if you'll be the one supplying the parts.

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  • You are not actually answering his question. He wanted to know if 125A is better than 100A for his rewiring project - Because the OP thinks that means less cable has to be pulled. You pointed out the differences between the two products. What is a massive load? How can anybody possibly know how to calculate a massive load? is it 1kw, 5kw a Gigawatt? -1 because your answer is very sketchy- just like the electrician of the OP. Improve your answer with hard facts and calculations and ill upvote. – Piotr Kula Jul 8 '13 at 12:47
  • @ppumkin - ok well "In this case the more AMPS the less wire he will have to run." That actually makes no sense at all. All having 125A means in his case is that he has bigger bus bars so he can run more power points and greater loads. If he's just running a couple of some bedrooms off it then he's wasting his money. Massive load is hard to quantify but lets just say, a massive load is maxing out the capacity of a an average size breaker. – Matt Jul 10 '13 at 8:38
  • Ahh - Since you threw in "breaker" in there you just limited the massive load to a MAX of 20A/25A (Thats the biggest residential breaker I have seen and we used them on Electric water heaters or swimming pool pumps. Lights usually where 5A and Sockets 10A , Stoves 15A . That one word just quantified everything :) – Piotr Kula Jul 10 '13 at 8:44
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    It was more of a case of thinking he might be wiring a workshop. But then if that's the case he'd do something different. Anyway, taking your point I've reworked and updated my answer. – Matt Jul 10 '13 at 8:48
  • Ahh nice :) That is much more transparent and makes more sense. I can remove my downvote now. :) – Piotr Kula Jul 10 '13 at 10:59

A subpanel is a simple box with rails and places to hold circuit breakers. The amperage rating tells you what the power rails are capable of handling. Sometimes you get kits with other breakers in then.

We can't possibly tell you what you need to use and your question is quite a bad one. If you are seeking advice on electrical installation your contractor should be telling you because he is on site there. If he is going... emm umm... dump him because your house will burn down! Look for accredited companies that will give you guarantee on the work (not suggest by a friend of a friend, or your actual friend who thinks he knows what he is doing)

The only important difference between the two is this. (I am using Ohms law)

  • In 110 volt areas
    • 100 Amp Maximum wattage = 11000 watts or 11 KiloWatts
    • 125 Amp maximum wattage - 13750 watts or 13.7 kiloWatts
  • In 220volt areas
    • 100 Amp maximum wattage = 22000 watts or 22 KiloWatts
    • 125 Amp maximum wattage = 27500 watts or 27.5 KiloWatts

It has nothing to do with more or less wires and neither is better or worse. Sure- One is more expensive and all I can say that is the one you are getting ripped of for.

So your first question should be how much wattage do you need to run at that point? I can assure you for normal residential usage 100AMP will might peak at 50% but when not doing much i doubt you should be using any more than 10%

For commercial usage this could possibly only run only a limited number of machines at a single time depends on what machines they are.

Once again- The box or who it is made by is not important. It is important who wires the box and if they know what the hell they are doing!

enter image description here

  • Left = Professional who takes pride in his work and delivers quality!
  • Right = Some idiot trained by another idiot that will cause you never ending grief!
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  • Would the 125amp box require you to run a heavier gauge wire? If so the installation might be more expensive. – Steven Jul 8 '13 at 13:12
  • Why? First ask WHAT is the requirement? You are putting horses in front of the carriage here! If the sub panel never uses more than 1kw then why run a heavy duty 10kw cable? I can use a 3 phase 500amp box and never ever need more than 500watts to power a kettle. But if I am running commercial machines like electric iron smelters then 500amp power 1 of 10 heating elements and we need 10 X 500Amp panels! – Piotr Kula Jul 8 '13 at 13:23
  • If I use a 125A rated box but the breakers don't even add up to 75A then you use a cable that handles that PEAK. Residentials never run anywhere near that all day long. Again. Get a certified electrician there. There are so many things involved to do this correct it is not suitable for a Q&A – Piotr Kula Jul 8 '13 at 13:28
  • All very helpful responses. I learned a lot since my post by reading many articles on the web. I do appreciate the responces here and the pictures. – BobJax Jul 8 '13 at 15:48
  • My house is being rewired. The main panel is about 60 feet away from the main part of the house with the most load. To make the job easier, I will bring up the discussion of a sub panel with the electrician. The electrician will decide the AMPS if he wants a sub panel. Not me. I didn't understand the calculation of how the AMP rating of a panel translates into total amps of all breakers in the panel. But after 2 days of reading I better understand it. Forget the question about 100 versus 125 AMPS. Doubt if it will be anywhere near that. But it is up to him. – BobJax Jul 8 '13 at 16:35

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