Branch Circuit Fusing

[kevman]

Are fuses required on branch circuits when the wire gauge is the same as the main circuit that is protected at the battery?

I found the following in the AYC:

E-9.12.3 Branch Circuits
E-9.12.3.1 Each ungrounded conductor of a branch circuit shall be provided with overcurrent protection at the point of connection to the main switchboard unless the main circuit breaker or fuse provides such protection.

So if I understand that correctly, if you were to hook up an inverter off a bus bar, and the wire gauge was consistent with your main feed, the main circuit fuse at the battery would provide protection to the inverter.

However, it appears to be common practice in van builds for people to add additional fusing on all branch circuits. Is this just people erring on the side of caution or is there a reason for this?

All I can think of is if you have to plan for a worst case "sum of all fuses" but I'm not sure if that's actually detailed in a code anywhere.

[The_Vermonster]

I see a few things that I would change and it might sidestep your question.

First, the Fuse protects the wire, not the device. So all of those 8AWG wires should have 40A fuses.

Second, 2AWG is way overkill for a 50A charger. And given that you have 2awg all the way back to the fuse, its technically protected. The idea of adding fuses all in the same area is so that it's easy to troubleshoot and replace a faulty fuse.

A good practice would be to increase the size of the wire from the battery to busbar to accommodate potential upgrades in the future. You don't want someone (maybe yourself) seeing an inverter plugged into 2awg wire and think you can just slap a 150a inverter in it's place. If you have 3 8awg and 1 2awg coming out of the busbar then you are implying that the supply side is capable of handling all those loads. In the current configuration, it isn't. So either drop the 2awg to the inverter down to 4 or 6 awg or bump the 2awg up.

[Scalf77]

A couple of things. You would be correct that the 2awg from the bus bar to the 50 amp charger/1000 watt inverter would be covered by the original 150 amps fuse you are showing. The inverter is item that we have to look at. Let's say 1000 watts at 90% efficiency would 1111.11 watts / 10.5 (low voltage cut out) = 106 amps . Your only other load on the bus bar would be the 40 amp DC fuse box. So that would total out at 146 amps. The other two devices on the bus bar are not loads but are instead sources. So if using 2awg marine wire should be fine ( Based on 105C rated marine wire) There are calculations that can be made to cover the loads on the fuse block.

So the fuses at the bus bar are not good for the output of the DC-to-DC and MPPT/Solar unless those devices are close (like a foot) Those fuses should be at the devices themself. There are times when you have long unprotected runs that you may want to run fuses on each end.

I don't understand the comment about all the 8awg wires should be 40 amp.
As long as the fuse protects the wire size, it would be ok to have different sizes. a 60 amp fuse would still protect 8awg wire( Based on 105C rated marine wire) .

There are many times that Inverters specifically ask for a fuse only for their device. Of course the upgrade paths are also important to take into account.

[kevman]

Sorry for the confusion, it was a rough diagram to discuss the branch circuit fuse idea. As mentioned, It seems to be common practice to fuse this line when it appears the main circuit fuse would protect it. For example, the "High Power" schematic here shows redundant 400A fuses.

Yes, all wire is 105'C.

Yes, the DC/DC and MPPT are close to the bus bar. 1ft. and 2ft. respectively within the same electrical cabinet. But it's a good point that damage could occur to the wire mid span resulting in a short before the fuse can protect.

Yes, you are correct on the inverter. Xantrex rates it at 100A nominal and recommends a 2AWG wire with a 150A fuse. We use it for a toaster and a coffee machine so in practice I've never seen it pull more than 80A (~850W).

The only reason I could think of needing an extra fuse on the inverter leg was in a "triple jeoparday" situation where something shorted out and the inverter leg pulled 150A from the battery, 30A from the MPPT, and 60A from the DC/DC which would exceed the rating of the 2AWG wire. I'm using my imagination here obviously...

I appreciate the responses, sometimes it helps to get your thoughts on paper instead of spending hours searching the internet and getting nowhere.

[posplayr]

Originally Posted by Scalf77

I don't understand the comment about all the 8awg wires should be 40 amp.
As long as the fuse protects the wire size, it would be ok to have different sizes. a 60 amp fuse would still protect 8awg wire( Based on 105C rated marine wire)

So I have been reading the thread and trying to resolve in my mind what are the general "rules" that should be followed as far as fusing. The layman's "rule" that "The fuse is to protect the wire" is misleading, at least that is what I have come to realize.

Despite this rule, we still size the fuses to the expected maximum current of the devices in normal operation, not to the size of the wire. The wire may be sized to the current loads, but as in the example circuit, wire capacity can well exceed the normal max current loads.

Rule #1 Battery Connections:
So to try and narrow this down a little more, I would like to first, discuss an an overarching rule when dealing with batteries. "Any positive battery connection should be fused nearest the battery whether it is a load or a source." If it is a source (with any appreciable conductor length) then it should be fused at both ends.

The main reason for this type of precaution is the danger of shortening a battery whether Lead Acid and/or especially Lithium batteries.

The reason to fuse loads is that the distribution (branch) wires (in the example case 8awg wires) would not be protected by a 150 amp main busbar breaker if the wire or device became shorted.

Rule #2 Fuses are intended to Protect Wires.

The confusion is that this means to have a fuse in place closest to a source but doesn't speak to the fusing of the wire other than it needs to be below the rated current of the wire.

To add even more confusion, I would argue that strictly speaking there is little difference between Rule #1 for battery connections and Rule #2 for protecting wires with Rule #2 being the overarching albeit vague rule with Rule #1 only applying to batteries.

For myself, I like to have Rule #1 so that when I look at the battery connections I make sure there is always something (a fuse) to protect anything that is downstream of a positive battery connection.


Rule #3 "Fuse values should be selected to minimize the excess between the maximum current fusing limit and the expected maximum conductor currents".

The reason for this is that while the fuse is to protect the wire, the wire can be protected at anything below its current rating including much lower if the real loads can still be accommodated.

The rule then provides a more reliable warning about an electrical short in that the fuse will blow at the lowest level avoiding high shorting currents albeit within the wires rated currents. It is assumed that a blown fuse will be a clear indication to the user that something is wrong.

Rule #3 seems to be the one that is contradicted by Rule #2, but Rule #2 only relates to where a fuse goes not its value.

I don't know if this adds any clarity to the situation, but it has helped me go through the issues in my own mind.

[Scalf77]

Originally Posted by posplayr

If it is a source (with any appreciable conductor length) then it should be fused at both ends.

The main reason for this type of precaution is the danger of shortening a battery whether Lead Acid and/or especially Lithium batteries.

The reason to fuse loads is that the distribution (branch) wires (in the example case 8awg wires) would not be protected by a 150 amp main busbar breaker if the wire or device became shorted.

Actually if the device became shorted , the fuse for source would blow. If the wire was short circuited after that, then the 150 amp fuse would still blow before the 8awg became a real fire hazard.

[posplayr]

Originally Posted by Scalf77

Actually if the device became shorted , the fuse for source would blow. If the wire was short circuited after that, then the 150 amp fuse would still blow before the 8awg became a real fire hazard.

I guess it has to do with how perfect the short is.

Yes a 0 ohm short would blow a (fast) 150 Amp fuse before the 8awg wire would dissipate enough heat to melt the insulation. I would say it is far more likely that an internal short (e.g. MPPT or DC-DC ) or even a load is going to be less than a perfect 0 ohms short and the 150 amp fuse doesn't blow and the wire just gets hot and melt off the insulation.

[Scalf77]

Originally Posted by posplayr

I guess it has to do with how perfect the short is.

Yes a 0 ohm short would blow a (fast) 150 Amp fuse before the 8awg wire would dissipate enough heat to melt the insulation. I would say it is far more likely that an internal short (e.g. MPPT or DC-DC ) or even a load is going to be less than a perfect 0 ohms short and the 150 amp fuse doesn't blow and the wire just gets hot and melt off the insulation.

Again, if the MPPT or DC-DC develops an internal non zero ohm short, then the fuse at the source should protect the wire.

[posplayr]

Originally Posted by Scalf77

Again, if the MPPT or DC-DC develops an internal non zero ohm short, then the fuse at the source should protect the wire.

I think I agree with you but I'm getting confused with what you call "source" in that I have some doubt as to what you mean.

The main 150A fuse that ties right to the battery (Source?) is sized for the 2awg main battery lead. The branch circuits have smaller wires and so are not protected by the main 150 Amp fuse. With some minor doubt, I assume for source you mean the branch circuit fuses at the buss bar.

So getting back to the AYC quote:

E-9.12.3 Branch Circuits
E-9.12.3.1 Each ungrounded conductor of a branch circuit shall be provided with overcurrent protection at the point of connection to the main switchboard unless the main circuit breaker or fuse provides such protection.

Translated this would say:

each branch circuit shall have an appropriately sized (lower) fuse, unless the branch conductor is the same size as the main conductor feeding the buss.


And referring to the Rule #2 "The fuse protects the conductor".

With the unstated intent that there is a branch fuse (unless there is the exception where the branch circuit loads are the same as the main fuse) which will carry the intended branch currents.

Unless one takes "overcurrent protection" in the AYC Branch Circuit reference to mean "protect the load", it would otherwise imply protecting the conductor which is not how fuses are normally selected. They are based on load currents, not conductor ampacity.

And to beat the dead horse one last time that seems to be the root of
the issue where The_Vermonster suggested 40 amp fuses for all 8 awg wires rather than the load-based selection made.

The notion of "a fuse protecting the conductor " is not misleading with respect to needing a fuse, but is misleading with respect to how the fuse rating is selected (not to protect the conductor but rather accommodate the load).

Maybe this is all plain as day to everybody else, but this contradiction will exist if you just follow rule #2 or the AYC branch circuit reference. The common practice embodied in Rule #3 seems to contradict Rule #2.

[Scalf77]

I consider them sources as they are power sources, we are fusing the output of DC-DC and MPPT.

Generally the first step is to figure out what the worst case load of a circuit is. We pick a conductor size. Are conductor has to be able to carry that worst current. We then add voltage drop into the equation determine if we need larger wire than what is needed to cover the current.

And yes, the fuse is there to protect the conductor, but ultimately the size is picked to handle the load or source current but within range to protect the conductor.

A couple of things on the original diagram. The load for the DC distribution is listed as 40 amps. In my mind the worst case load is the maximum load specified for the fuse block. If we look at a Blue Sea ST Blade , it is rating for 100 amps , 30 amps per circuit. So I look for the worst case load of that circuit to be 100 amps. So if I choose wire it needs to support 100 amps. The chosen 8 awg only supports 80 amps, so it won't support 100 amps. The fact that he is fusing it at 40 amps makes it a moot point. But if someone were to upgrade later, the block supports it but not the circuit. I would probably choose 6awg.

And then if we go back to the original question about two 150 amp fuses. If I was expecting the possibility of maximizing the fuse block I would put a 225 to 250 Class T Fuse at the battery, and then put the Xantrex recommended 150 after the bus bar.

Lithium Batteries generally make the choice a little harder, as now you need to keep the discharge current below BMS circuit rating.