In a previous question, the comments mentioned slow blow circuit breakers. I have done a web search and went to Square-D's website(the maker of my current breakers) and came up empty. There is a lot about curves and such but nothing that makes sense to this laymen. Can anybody help?

  • I've heard of slow blow fuses, but never slow trip circuit breakers. If anybody around here knows if they exist, @SpeedyPetey would.
    – Tester101
    Mar 16, 2015 at 10:19
  • slow-trip breakers tend to be expensive devices (programmable) for industrial or heavy commercial uses
    – Skaperen
    Mar 16, 2015 at 12:21
  • @Skaperen Could you provide a link to such a device?
    – Tester101
    Mar 16, 2015 at 14:12
  • 1
    I have never seen these and I don't think I would miss anything labeled "slow blow".
    – DMoore
    Mar 16, 2015 at 14:59
  • GE - SRPE30A15 -- It is a UL listed 15 amp breaker, with an adjustable instantaneous trip current adjustable from 43 to 182 amps.
    – Some Guy
    Mar 16, 2015 at 15:27

3 Answers 3


I don't know about "slow trip circuit breakers", but maybe I can help you understand trip curves (and circuit breakers) a bit better. Then you'll see that all circuit breakers are "slow trip".

Your basic, everyday, run of the mill circuit breaker offers two types of protection. Short-circuit protection is provided using a magnetic trip function, while overload (over-current) protection uses a thermal trip function.

Short-circuit Protection

To provide this type of protection, a circuit breaker relies on the fact that current traveling through a wire creates a magnetic field. The breaker basically uses a solenoid to open the circuit, in the event that there's a large fault current flowing. If there's enough current flowing, the magnetic field is large enough to pull open the circuit. This is often known as an instantaneous trip, because it can react in just a cycle or two (0 - 0.05 seconds).

Overcurrent Protection

This type of protection is provided using a thermal device, that works based on the fact that current traveling through a wire generates heat. As the current flowing through a wire increases, so too does the temperature of the wire. A circuit breaker uses a bi-metal strip to trip the breaker, if the temperature (current) gets too high. This is known as "long trip", or "slow trip", because it takes time for the breaker to react (0.5 - 1000 seconds).

Trip Curves

If you look at the trip curve of a breaker, you can see both types of protection in action. You'll notice a slow arcing motion leading down the chart, this is the "long time trip" portion of the chart. As the current increases, the time required for the breaker to trip decreases. Eventually, the breaker hits the instantaneous trip threshold. At this point the breaker trips within a cycle or two. This can be seen at the bottom of the chart, where the curve levels off and goes straight across the bottom of the chart.

Now that we've covered the basics, let's take a look at an example. Here's the trip curve for a 20 ampere Square-D QO circuit breaker.

enter image description here
Click for larger view

The vertical axis is time in seconds, and the horizontal axis is multiples of current. I've highlighted in orange what two times the breakers rating looks like (40 amperes). As you can see, even at 40 amperes this breaker shouldn't trip for about 9 to 35 seconds. This is because it's relying on the thermal protection, and the bi-metal strip has to heat enough before it pulls the circuit open.

I've also highlighted in red what it would take to trip the breaker in one second, since an observer might consider a second "instantaneous". You'll notice that it should take between 4.5 and 8 times the breakers rating, before the breaker trips in under a second. That means your saw would be pulling 90-160 amperes, when the breaker tripped.

If your saw is pulling that much current, the problem is not the breaker, it's the saw. Increasing the size of the breaker is not the solution.

Check the saw for faults, including a short-circuit. You may also want to test/replace the breaker, as breakers can go bad.

After some more research, there do seem to be what are known as "High Magnetic (HM)" circuit breakers available. Schneider Electric describes them as

High magnetic trip circuit breakers are recommended for applications where high initial inrush may occur and for individual dimmer applications.

One such device would be the QO120HM Miniature Circuit Breaker. I couldn't find a trip curve for this device, but rumor has it the instantaneous trip function may not kick in until 20-30 times the rated current (400-600 amperes for a 20 ampere breaker). However, I don't think these are applicable to your situation, since there's no way the saw should draw 400 amperes when starting (or at any time for that matter, unless there's a short-circuit).

  • 1
    or a short coil in the saw motor
    – Skaperen
    Mar 16, 2015 at 12:18
  • I too am looking for some of these breakers for a dive compressor that has a HUGE initial current. Right now I'm using 20A slow blow fuses which work, but which are not ideal since they could allow one leg to blow without both of them blowing.
    – boatcoder
    Jan 1, 2017 at 18:35

On the SquareD website (here in year 2021) you can search for residential circuit breakers. Then select the QO line. Then select the category HM (High Magnetic) which is essentially a "slow blow". If you select one of the breakers shown, like the QO120HM, there will be PDF's to download. The one labeled "Square D QO and QOB Miniature Circuit Breakers Catalog (Version 1.0)" has an explanation of the "high magnetic" style and you can compare the trip point graphs.

The standard 20A QO trips within one line cycle (16.6ms) with as little as 6X rated current, but could require as much as 11X rating to trip that fast. Compare that to the QO-HM which is guaranteed to require at least 11X rated current to trip within 1 line cycle.


Ask him what part of the code he thought putting a 30 amp breaker would cause the violation. Ask for true curiosity, if no other reason than I want to know. :)

Is there a plug on the saw, so you can plug it into an outlet on the wall ?

If not, put a plug on the saw and an outlet on the wall. (I can see him calling foul on a hard wired 15 amp rated saw, going into a into more than a 15 or 20 amp breaker. - Which means he is either Really good, or does not understand the code well.)

  • If there is already an outlet on the wall, what is it? I am expecting you to tell me it is one 'standard' outlet, 2 plugs with all 4 power pins parallel with the long side of the socket (google NEMA 5-15R Duplex).

  • The 2nd expected option would be a 'NEMA 5-20R Duplex', in this type one of the outlet pins is a 'T' shape. This will allow both 15 and 20 amp devices.

If it is not a NEMA 5-20R Duplex, ask if having one installed with separate feeds, hot tab removed, and pig-tailed in the junction would make him feel comfortable.

This page shows the split outlet, tab removal thing well. http://electrical.about.com/od/diyprojectsmadeeasy/ss/wiresplitoutlet.htm

A pig-tail is just a short piece of wire. In this case it would be 2 pig-tails (12 gauge, one from the hot side of each of the now split outlets, 1 top outlet and 1 bottom outlet.

These will then be twisted together with a wire nut. Then a 30 amp breaker installed and all is happy in the kingdom's workshop.

The section of the National Electric Code that speaks of using a 30 amp breaker on a copper 10 gauge wire is 2014 NEC, Article 240 (D1-7)

2014 is the most recent edition, and I am talking about outlets or a hardwired machine rated at 30 amps.

As far as your question :P

You say the box is Square D, the follow up question is it the Homeline or QO series.

Lacking a sticker or other mark that says QO load center, or Homeline load center, here is a multiple question test for identification.

-- QO Series Clear window just below the breaker handle - If it has this is QO

This is the breaker to try, - SqD part# QO120HID - if you have a QO panel, and you are 'forced' to use a 20 amp breaker.

It will likely have the following, but missing any 1 of them is not a deal breaker, just a sanity check

Metal Box, Dark Grey / Blue

QO Series breakers will have:

Rounded edges, No lip outlining the breaker - deep enough to hold a quarter $

Just 10kA 120/240~ and Square D logo at the bottom

cost than $5 at big box store.

If you have a Homeline - there is no equal breaker, sorry.

Hope this helps some - if I have made anything too confusing, or complex, let me know and I will do my best to ease any communication issues

  • I am not at the shop currently but everything in the box is supposed to be commercial duty, the outlet for the saw is a 20 amp duplex with a GFI. The GFI has never tripped
    – Scott G.
    Mar 17, 2015 at 1:19
  • In this case, my first thought is to replace the GFI at the saw with a standard duplex outlet.
    – Some Guy
    Mar 17, 2015 at 2:53
  • GFI are great for safety, but have too many ways to fail, that would cause them to trip the main breaker when loaded, but not trip themselves. What happens if you plug in a toaster, iron, portable heater, something that pulls real power, but not a motor? In the don't try this at home, a plug on 4 foot wires, wired right into the breaker, just a test, would let you know if it was the breaker. This is a personal issue with me, I put in a paver like retaining wall, and my 15 amp abrasive chop saw would blow the breaker every 15 minutes or so, I would then move to another outlet. I won in the end
    – Some Guy
    Mar 17, 2015 at 3:07
  • clearly I have spent too many years as an electrician, respect electricity, but refuse to be outsmarted by it. For what it is worth, I recently spent $27,000 on electrical work on my house. No idiot, I hired someone to do it all. I feel the pain of new constrution.
    – Some Guy
    Mar 17, 2015 at 3:13

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