The Difference Between AC and DC Fuses

August 17, 2015 · 18 comments

Would you do us a huge favor by sharing?

It saddens me that folks are risking their lives, family member’s lives and personal property by trying to save a dollar by using off-the-shelf AC (alternating current) fuses (or circuit breakers and switches) in their 12, 24 or even 48 volt battery systems, not to mention their 120, 150, 200 or even 600 volt DC (direct current) solar arrays.

USING AN AC FUSE IN A DC SYSTEM IS EXTREMELY DANGEROUS!

DO NOT USE AN AC FUSE IN A DC SYSTEM OR YOU WILL BURN YOUR HOUSE DOWN!

ac and dc fuse assortment

In fact we will have some fun later in this article and burn or blow up some AC fuses by using them with DC current. Stick around, it will be entertaining if nothing else.


AC (alternating) current does exactly what is says, it alternates or changes from positive to negative 60 times per second (60 hertz) in North America and 50 times per second (50 hertz) in the rest of the world.

To switch from positive to negative and negative to positive it has to cross over zero volts each time. If you were to open a circuit (turn a light switch off) at exactly zero volts, there would be no arc (or spark).

alteranting current passes zero voltss 50 or 60 times per second.

If you were to open a light switch when the voltage is at 120 volts positive there will be an arc but only until the voltage reaches zero which happens in the fraction of a second. Now the circuit is open and the arc is gone.

If you turn off a switch (open circuit) at -120 volts there will be an arc (spark) for a fraction of a second until the voltage reaches zero.

Once the voltage reaches zero the arc is extinguished all by itself.


DC (direct current) is a different animal. It just keeps on coming like a freight train.

Open a low voltage DC circuit (like when you blow a fuse) and there will be a small arc.

Open a high voltage DC circuit (like when you blow a fuse) and there will be a huge (long) and super hot arc (spark). This is where the problem lies.

Usually as long as your AC fuse doesn’t blow, things will be fine,

BUT WHEN AN AC FUSE BLOWS IN A DC SYSTEM, IT CREATES AN OPEN CIRCUIT AND A HOT AND DANGEROUS ARC.

The higher the voltage in your DC system the larger and hotter the arc will be when the circuit is opened. Sometimes there is nothing you can do such as when a cable from your battery to your inverter gets lose or the terminal gets loose. An arc will form and extreme heat will be produced. Hopefully in this circumstance, known as an “arc fault”, someone will notice it.

By the way if there is an arc fault on the wiring going from your charge controller to your solar array, the same can happen unless you have a charge controller that is designed to handle arc faults. The “Classic” made by Midnite Solar does exactly that. If it senses an arc fault it will open the circuit and sound an alarm to let you know your system needs some work. Absolutely genius idea!

Robin and boB have been the key players in the renewable energy business.

Robin Gudgel of Midnite Solar (and his brother boB) have always been the “first” for just about everything in the solar industry. They made the first DC breaker panel and DC disconnect, the first rugged and reliable MPPT charge controller (the MX60), the first 150 volt DC rated circuit breakers, the first 300 volt DC circuit breakers, the first 600 volt DC circuit breakers, the first user friendly small UL/CSA listed breaker panels, the first user friendly pre-wired DC circuit breaker panels (e-Panels) and now the “Classic” charge controller which has at least 10 firsts! There is no one like the Gudgels from Midnite Solar. They are the pioneers of the renewable energy industry.


Back to our topic of the difference between AC and DC fuses.

You only need to open an AC circuit (when a fuse blows) a few millimeters at 120, 230 or 240 volts as the arc will be gone when the voltage crosses the zero point. (Remember our diagram above?)

When an AC fuse blows, it will only open the circuit a few millimeters as that is all that is necessary. If you use an AC fuse with DC current and the circuit is opened (fuse blows), one of three things will happen:

  1. The gap made by the open fuse will be too small and the DC current will jump across, get hot and melt (fuse) the metals back together. This is very dangerous as the condition that caused the fuse to blow or breaker to trip is still there. Now the wiring will heat up and eventually catch fire.
  2. The gap made by the open fuse will be too small and the DC current will arc continuously until it has melted everything around it and/or caught your home on fire.
  3. The gap will arc and melt until the gap opens far enough to extinguish the arc (spark). Although this is physically possible, I have never seen it.

DC CIRCUITS MUST BE OPENED FAST AND FAR ENOUGH TO EXTINGUISH (GET RID OF) THE ARC!

THIS IS EXACTLY WHAT A DC FUSE DOES! IT OPENS THE CIRCUIT FAST AND FURIOUS TO PROTECT YOUR VALUABLES!


An arc welder uses high amperage low voltage DC to weld metals together.To illustrate the arcing ability of DC current let’s look at the common arc welder used by many to weld metals together.

Could you weld with AC current?

From what we have learned already, the answer is no. If an arc welder (the human welder) tried to make an arc with the typical AC current from household electricity he would only be able to make a tiny one and only for a fraction of a second. Not much good for welding!

There is one exception to the rule: there is a form of tig welding used to weld aluminum and magnesium with a square wave AC. Check it out here.

square waveWhen magnesium or aluminum are exposed to air, oxide forms on these metals that must be removed. The positive cycle of the square wave removes the oxide and the negative cycle of the square wave welds the metals. We are not welding experts but were reminded by a reader that this type of welding is done with AC although it is not a typical AC wave form.

The positive and negative cycles of the wave form resemble DC in that they are steady and straight compared to a typical pure sine wave which is more rounded and peaks at a certain point.

Could you weld with DC current?

Absolutely, and that is how is it done. And surprisingly it only takes about 28 to 40 volts DC to arc weld (stick weld). In fact in an emergency you can connect two 12 volt batteries in series (for 24 volts DC) and use a high quality set of booster cables for an emergency arc welder. We have done it here at Solar Homestead when we have had breakdowns where CAA or AAA doesn’t go.

A big difference between AC and DC is stick welding.


THE RULES FOR AC AND DC FUSES:

1. No matter what fuse you are using and no matter what the application you MUST USE A UL OR CSA APPROVED FUSE FOR THE PURPOSE IT WAS INTENDED.

These DC rated fuses cannot be used in a home or cottage.

These DC rated fuses are not approved for homes and cottages.

For example: You might find a UL listed DC fuse but it was designed for a mobile application such as a car, truck or boat.

This type of fuse cannot be used in an off grid home or cottage.

2. Use AC fuses for alternating current

3. Use DC fuses for direct current

4. In a DC application, make sure you get the correct amperage fuse for your project and do not exceed the maximum voltage rating of the fuse.

5. In an AC application make sure you get the correct amperage fuse for your application and do not exceed the maximum voltage rating of the fuse.

How do you know what size fuse should be used in your DC (or AC) system?

Most people don’t know but a fuses job is to protect the conductor (wire/cable) downstream of the fuse. It is not necessarily there to protect your inverter, charge controller or other DC appliances, it’s primary purpose is to protect the current carrying conductors from overheating and catching fire.

Of course if there is a short circuit in your inverter, the fuse will blow but the damage has already been done to the inverter. The fuse blows to prevent the wiring from catastrophic fire, damage to property and prevent injury or death.

That being said, your fuse will be sized according to the size of your wiring or cabling.

Look at the chart below to see what size fuse is required to protect each gauge of wire:

wiring and fuse sizes

First only look at the left two columns of the chart if you are using copper wire as you probably are. I kept the aluminum side of the chart as sometimes we get a good deal on aluminum wire that we cannot resist. Go with the 90 degree C wire for most of your amp calculations as those are the ones used with solar such as USE-2 (underground service entrance) and most of the other ratings in the 90 degree C would apply to PV wire and battery / inverter cable like DLO (diesel locomotive cable).

When you find an amperage rating on the chart you NEED to find a fuse or breaker that is either the same size as the current rating or smaller.

If you use a higher amperage fuse than the conductor can handle, the conductor will overheat and fail before the fuse blows. That is not good.

Example#1 Let’s imagine you have an inverter that requires 4/0 battery cable. If you look at the 90 degrees C rating of 4/0 (0000) cable you would need a fuse or breaker that will trip or blow at 260 amps or less. The fuse would be a 200 amp class T fuse (as they are readily available) to be on the safe side. It is more difficult to find a 250 amp Class T fuse. A 250 amp DC breaker would be the recommended breaker as they are readily available. Remember using a smaller fuse will not be problem as it will protect your wire by blowing before your wire over heats. However if you are actually using the maximum rating or the wire you may need to size your fuse as close to the maximum rating of the wire as possible.

Usually in a solar application most conductors or over-sized as to reduce the losses associated with resistance.

Example #2 Let’s suppose you are using the typical 10 awg pv (solar)  wire used with most MC4 cable. While it is rated to handle 40 amps we usually would protect it with a 15 amp fuse (or breaker) as most solar modules say to protect with 15 amp series fuse (or breaker).

I know it sounds weird to most electricians to put  15 amp breaker an a #10 wire but we are using the #10 to keep the voltage drop as low as possible as we don’t want to lose any more power than necessary due to the resistance of the wire. Solar installers always use larger conductors (wire) than a typical electrician as they are trying to keep the voltage drop as low as possible.

The rule is protect your wire with the (90 degrees C rating) listed amperage fuse above or smaller.

While you could theoretically protect a #10 wire with a 40 amp fuse, most of the larger solar modules (over 200 watts) you are using will likely say 15 amp series fusing max.

15 amp series fuse

If you use too small of a fuse the worst thing that can happen is the fuse will blow.

If you use too high amperage of a fuse the worst that can happen is a fire.

I would rather blow a fuse.

Leave a Comment

{ 18 comments… read them below or add one }

Robin January 30, 2017 at 1:14 am

Hi,
Glad to see your post….I am very happy to see your informative post which helps me a lot.

The process of arc welding is widely used because of its low capital and running costs. To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common classification is constant current power supplies and constant voltage power supplies. In arc welding, the voltage is directly related to the length of the arc, and the current is related to the amount of heat input.

Thanks for being sharing….Keep it up
Regards
Robin

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eldon January 27, 2017 at 7:31 pm

I did some quick testing on some sand filled ceramic, 6mm by 20mm 6.3 amp, 250vac fuses, three different ones; had no problem opening the circuit when they were shorted out across a 125 volt dc battery bank of 15 amp “nameplate” capacity. (actual short circuit current was estimated at 220 amps)

of course, if you have a ridiculous amount of inductance in your system, such as a solar array thousands of feet away, or if you are running a 200 volt pv system, then you should use a legit fuse, and in fact you might want more than a fuse. you might want a diode and snubber connected back to the battery or inverter so the excess volts produced when the circuit opens has somewhere to go. basically when you open a circuit, the voltage climbs to whatever it needs to, to dissipate the voltage. with a long transmission line on a dc circuit you could have tens of thousands of volts available to continue pushing current through the switch until the energy stored in the inductance of the circuit is expended.

but simply showing a video of a 32 amp circuit breaker failing to open a dc circuit, meaningless really. i use standard breakers as a switch in my dc welder.

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Jody Graham March 1, 2017 at 7:04 pm

Hi Eldon,

Thanks for your comment.
The problem with using an AC FUSE or AC CIRCUIT BREAKER on a DC CIRCUIT is not that it will trip unexpectedly or unnecessarily. The problem is when it does trip due to over current or short circuit, AC fuses and breakers cannot and will not extinguish the arc and either arc and burn until there is no circuit or arc and burn until the circuit is welded together making the fuse or breaker inoperable and a fire hazard.

Hope that helps those of you trying to save a few dollars by risking your home, family and insurance policy. Thanks…Jody

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Doug March 2, 2017 at 1:27 am

Jody,
There are AC only circuit breakers and AC/DC circuit breakers. SquareD sells both. What you should be telling people is to read the specifications of the fuse or circuit breaker to see what it rated for. By law it will stay on it if it’s an AC only or AC/DC circuit breaker and the maximum voltage it is rated for.

Not sure about fuses. But I’ve never heard of a DC only or AC only fuse. Again what one should do is look at the specifications. See what type of current it’s rated for AC, DC or AC and DC and the maximum voltage.

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Jody Graham May 17, 2019 at 11:43 am

Hi Doug,

Thanks for your comment and good advice. I will change the article to reflect what you said about simply looking at the specifications.

There are AC only fuses/circuit breakers, AC/DC fuses/circuit breakers (usually the AC voltage rating will be higher than the DC voltage rating), and DC only fuses/circuit breakers such as a Midnite Solar MNEPV series of breakers. They will NOT trip on AC.

Even SQUARE D makes a standard off-the-shelf circuit breaker, the “QO” series that has a DC rating up to 48 volts DC. These can be used in a DC system up to 36 volts nominal.

SQUARE D “QO” breakers CANNOT be used in a 48 volt system as the voltage could easily reach over 60 volts. While being bulk charged or equalized there is a good chance this breaker could catch fire if overloaded.

Now that we in the grid connected solar industry are using 150 volts DC, 600 volts DC and even 1000 volts DC. Imagine the arc a 1000 volt DC circuit could make inside a fuse or circuit breaker.

Typically in a battery based system we are dealing with 15 volts (12 volt system charging voltage), 30 volts (24 volt system charging voltage) and 60 volts (48 volt system charging voltage) DC for the battery bank.

Square D QO breakers theoretically could work for a small 12 or 24 volt DC system. In fact that is how I started out with my off grid home. You can see it on our about us page about half way down the page or look at the photo of our first system by looking at the link below.

https://i0.wp.com/solarhomestead.com/wp-content/uploads/2013/03/OUR-FIRST-SOLAR-POWER-SYSTEM.jpg?w=580

However what do you use for the common DC main breaker (inverter disconnect) such as a 250 amp which is super common?

Square D doesn’t make anything close to that, meaning now you will have to buy an Airpax, Carlingswitch, or Heinemann breaker (that doesn’t fit in a Square D enclosure forcing you to buy a solar type disconnect that will accept the big breaker plus slots for a bunch of real solar breakers such as Midnite Solar’s (CBI from South Africa) or Outbapck Power’s DC breakers. The solar breakers AC or DC are much less expensive than Square D breakers.

Lastly, the AC side of the system should be in a different breaker/fuse panel than the DC side to avoid confusion. I think that is dumb but I don’t make the rules. So two breaker or fuse panels. This is in most countries’ electrical code but there are exceptions.

Thanks again, I think you have a lot of knowledge about electricity that I don’t have. Mine is pretty much focused on solar and the part of the US/Canadian electrical code pertaining to solar…Jody

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Doug March 2, 2017 at 1:19 am

Eldon you only tested on condition which was a short. There are other ways a fuse blows such as over amperage. In your test the fuse blew violently. In an over amperage condition the fuse over heats and melts. As the metal sags and breaks an arc can form defeating the functionality of the fuse.

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Jody Graham May 17, 2019 at 11:47 am

Thank you Doug,

You are exactly correct. I appreciate what you add to the conversation.

Jody

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Joe Tittiger September 16, 2016 at 3:15 pm

Thanks! Good breaker sizing info has been hard to come by.
I now have confirmation on breaker size for protecting the wire to my charge controller,
and the breaker for my inverters battery connection! 🙂

Looks like I may be able to begin putting things together soon.

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Jody Graham May 17, 2019 at 11:59 am

Thanks Joe,

If nothing else I’m glad you were able to use the chart for breaker sizing.

Have a great day…Jody

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Doug Spindlerr September 13, 2016 at 1:27 am

There’s a lot and I mean a lot of wrong information on this page. First off the welder in the picture is an AC only welder. Not sure who the dummy is who wrote this article but in general AC welds are stronger than DC welds. They make special welding rod for AC, DC and AC or DC welding. (Lookup 6011 and 7018)

As for fuses what a bunch of garbage. There is no such thing as a AC and DC fuses. A fuse is a fuse and can be used in AC or DC circuits. There are different types of fuses, regular, fast-blow, and slow-blow. Fast blow are used for electronic circuits to prevent damage to sensitive computer chips. Slow-blow are used for motors. When a motor starts it’s a short and max amerapage is flowing through the motor for a short instant. A regular of fast-blow would blow immediately which is why slow-blows are used.

Yes you can use automatics fuses for solar. Just make sure you don’t exceed the voltage for the fuse. (Look at the fuse specifications.). Automotive fuses are typically rated at 32 volt but some are higher at 80 volts. Doesnt’ mater if it’s AC or DC. Use some common sense. A fuse is made from a special metal alloy. When the current exceeds the amerapage rating the metal heats up and melts or blows. The metal alloy doesn’t car I of the electrons are following in one direction (DC) or both directions as in AC. Just make sure the voltage exceeds for the fuse exceeds your application.

Circuit breakers are a different story. I called SquareD and spoke with a circuit breaker designer. You CAN use the $4.00 circuitr breakers HomeDepot sells for solar IF they are rated for DC currents. Read the specifications.

There are two different types of circuit breakers. Some are AC and DC rated while others (usually the more expensive ones are AC only.

The cheaper circuit breakers use a metallic strip similar to a fuse. When the current flowing through the breaker exceeds the average the metal strip heats up and deforms tripping the breaker. Like with a fuse it doesn’t matter if the current/electrons are flowing in one direction (DC) or both ways, (AC). Read the specs for the circuit breaker to make sure it says they can be used for AC and DC before using on a solar system. And never exceed the max working voltage.

Don’t assume all circuit breakers can be used for AC and DC circuits, they can. There are AC only breakers which unlike the cheaper breakers which use a metal strip have circuitry to trip only at the voltage phase change or zero crossing. If these breakers are used in solar or DC circuits they will never trip in a DC circuit because there is never a phase change/zero crossing.

As for the breaker boxes the ones HomeDepot sells can be used or AC or DC current and are rated for 400 or 600 volts far more than what most people ever need for solar.

I’m using automotive fuses in all of my 12v and 24v solar circuits. For the higher voltage DC I’m using SqaureD breakers from HomeDepot. On the breaker it will say if it’s an AC only or AC and DC breakers.

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Jerud Crandall August 12, 2016 at 2:32 am

Great article!

My solar equipment is installed in my RV where I live full-time. I have a large “catastrophe fuse” that is a proper DC-type installed at the battery bank, and all the solar and inverter stuff goes through DC breakers (Midnite Solar). But the household DC circuits are all broken out from an RV-style distribution panel, which uses automotive-type fuses. OK for an RV, but i gather from your article that this is no good for a real house.

If i were to build a house and wanted to include DC circuits for LED lighting and other stuff, what would that distribution panel look like, and what would kind of fuses would be in it? Or would it be more DC breakers like from MidniteSolar? I’ve never come across that kind of equipment. Can you share some manufacturers or products that would be suitable?

Thanks!

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Clash Of Clans July 22, 2016 at 3:08 am

The voltage changes between an average of +240 V and -240 V. There’s nothing special about negative voltage. It just means the current tends to go in the other direction.

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La2Walker July 10, 2016 at 3:26 am

The interrupting rating is the maximum amount of energy the device can safely withstand. The fuse’s ampere rating is the amount of current that the fuse will allow through before beginning to operate. Both ratings are determined through testing.

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Jody Graham May 17, 2019 at 12:11 pm

You are correct. The A.I.C. or arc interrupting current rating is the amount of current that can pass through the breaker without exploding or causing fire. Most of the solar fuses/breakers we deal with have an A.I.C. rating of 10,000 amps. Meaning you could potentially push 10,000 amperes though it without danger. It doesn’t mean the breaker will be OK or trip or not trip, it just means there will be no danger.

The ampere or current rating of a fuse or breaker tells you how much amperage can pass through the breaker/fuse without it tripping or blowing. There is a 25% margin on top of that before the fuse will blow or breaker will trip. A 100 ampere (amp) breaker will NOT trip until it reaches 125 amps or gets really hot. This margin is there because some loads such as a water pump, refrigerator, air conditioner and anything else with a motor can surge at a much higher current than the nameplate rating of the appliance. There is also a time delay built into fises/breakers to help with this increased current from inductive loads such as a well pump.

Thanks for the info…Jody

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Diogenes of Sinope September 7, 2015 at 1:53 am

Actually, you CAN weld with AC. That’s how aluminum is welded — when aluminum melts, it immediately forms an oxide layer on the surface, which has to be “blown away” by rapidly switching polarity. Of course, it’s not the same sine wave as the mains current, there’s a lot of complicated stuff going on in a GTAW or GMAW machine equipped to use AC, but it’s done quite frequently.

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Jody Graham September 10, 2015 at 11:23 am

Hi,
Thank you for your comment and info. I have updated the article to reflect what you have told us. Although welding aluminum or magnesium is not done with a typical clean sine wave it is still considered AC current. Read what we have written above and if you think we still need to make corrections please let us know. Take care…Jody

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Ben August 17, 2015 at 7:57 pm

Hiya
Thanks for your info and posts etc. Been reading some. Not this one yet. But I thought it might be interesting for you and other people out there to SEE what can happen.
The installer of my system has made some tests, here a video of it.
http://s4solar.co.nz/information/video/dangerous_substandard_pv_systems/

Cheers
Ben

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Jody Graham September 10, 2015 at 11:25 am

Hi Ben,
Thank you so much for your comment and video. It definitely shows that AC components do not belong in a DC system. Take care and keep up the good work…Jody

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