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What fuse and breaker bar ratings to use in our electrical wiring?

sawmonandnatalie

sawmonandnatalie

New Member
Joined
Jan 22, 2021
Messages
35
Location
North Vancouver, Canada
Hi all,

We have created our initial draft of our electrical wiring for our class C motorhome ( 1986 Ford E-350 V8 460 CI with a 1985 Vanguard Coach ). Here is how it looks like:

https://imgur.com/bLKyvEE

We are going to have the solar panels in a 2S2P config and the batteries will be just a 4S for a total of 12V.

We have a few questions regarding the fuse and breaker bar ratings.

  1. What breaker bar rating do we need to use before and after MPPT solar charge controller (between solar panels and MPPT, and between MPPT and positive bus bar)?
  2. What fuse rating do we need to pick for the main positive line to the DC fuse box? Is this rating for the max amp that the DC fuse box can potentially handle? or should we have to calculate it for only the sum of fuses used in the DC fuse box? We are not sure on the full list of DC components that we will be using.
  3. How do we calculate the fuse rating from the positive bus bar to the invertor? We are planning to use a 2000 or 2200 Watt inverter like the GIANDEL 2200W Pure Sine Wave
  4. What fuse rating do we need to use for our 4 cell LifePO battery? In our diagram, we might have an extra fuse there. One fuse in the battery grayed boundary and one fuse on the bus bar just before the red switch.
  5. What fuse rating do we need for the Battery Converter/Charger? We are not sure what battery charger we would get. Do you have any suggestions? We have been looking at this Progressive Dynamics PD9160ALV 12V Lithium Ion Battery Converter/Charger - 60 Amp so far.
  6. If we decide to have the option to charge the battery pack by the alternator, what fuse rating should we use for it on the positive bus bar?
  7. What switch rating do we need to get to disconnect the battery from the load/positive bus bar?
  8. How about the shunt rating for the system?
Here are the components we've purchased so far or are planning to purchase:

  1. [Purchased] Tracer4215BN - EPEVER 40A MPPT Solar Charge Controller 150V PV Input 12V 24V auto Work
  2. [Purchased] 4pcs EVE 3.2V 280AH New Version Genuine Grade A Lifepo4 Battery Cells Fully Matched for Solar System EV RV
  3. [Purchased] Smart 4S cells 12V 120A 1000A Lifepo4 li-ion Lithium battery protection board W balance High Current BMS
  4. 4 x Renogy 100 Watt 12 Volt Monocrystalline Solar Panel (Compact Design)
  5. Blue Sea Systems ST Blade ATO/ATC Fuse Blocks
  6. GIANDEL 2200W Pure Sine Wave Power Inverter 12V DC to 120V AC with 20A Solar Charge Control and Remote Control
  7. Progressive Dynamics PD9160ALV 12V Lithium Ion Battery Converter/Charger - 60 Amp

Here are some specs for the Renogy 100W Panels:


RNG-100D-SS
100W Monocrystalline Solar Panel
P_max at STC = 100W
V_mp = 17.9 V
I_mp = 5.72 A
V_oc = 21.6 V
I_sc = 6.24 A

Max System Volt = 600 VDC UL
Max Series Fuse Rating = 15 A
Cell Efficiency = 21%

P_max at -45ºC = 116.1 W
V_oc at -45ºC = 26.5896 VDC
I_sc at 60ºC = 6.3492 A

[ 2S2P ]

V_oc = 54.0 VDC
I_sc = 12.8 A
P_max = 464.4 W464.4 W = 14.4 V x 32.25 A
Click to expand...

and the specs for the MPPT charge controller:

Tracer 4215BN 12/24V
Rated Charge Current 40A
Rated Discharge Current 20A
Max PV Voltage 138 V or 150 V
Click to expand...
 
15 or 20 amp breaker between the solar panels and the MPPT.
45 or 50 amp breaker between the MPPT and your bus bar.

The battery disconnect switch should be immediately downstream of the fuse, well before the bus bar. The way you have it setup, you cannot turn off power to the MPPT.

The fuse downstream of the battery is rated on expected loads/charging.

Consult your inverter's owner's manual for guidance on the circuit breaker rating. My Xantrex was very good about that information.

Direct charging from the alternator is not recommended. Instead, a DC-DC charger should be used in the circuit.

If you get the Victron Energy BMV-712 it comes with a 500 amp shunt, which would be plenty for your system.
 
Hi @HRTKD , thank you for all your helpful tips.

20 amp breaker between the solar panels and the MPPT sounds reasonable since under extreme conditions the 2S2P panel setup would only produce around 12.8 amp if short-circuited (I_sc).

50 amp breaker is also good between the MPPT and the bus bar since the 2S2P with 4 100W solar panels would create at most 464.4 W (if the temperature is super low). Also, the MPPT would provide a maximum 40 amp charge rate which would be between 0.1C and 0.2C charge (for a 280Ah battery pack).

The switch in the diagram is a load disconnect switch. I suppose to disconnect the MPPT, I would just turn the breaker bar off between the MPPT controller and the bus bar. Would that work?

So bases on this diagram, how can I come up with a fuse rating for the one fuse connected directly to the positive side of the battery?

So for the fuse just before the invertor, you should always match it with the invertor's specs without even considering how the rest of the system/diagram looks like?

Thank you for the alternator tip.

For the shunt, as long as you over-spec the amp rating for it, it would still be good? For example, let's say your setup requires a 100 amp shunt. Can you get a 1000 amp shunt insterad?

And how about the DC fuse box? Do we also need to have the aim for the max amp the fuse box can handle and choose that rating for the fuse that connects the DC fuse box to the positive bus bar?
 
sawmonandnatalie said:
The switch in the diagram is a load disconnect switch. I suppose to disconnect the MPPT, I would just turn the breaker bar off between the MPPT controller and the bus bar. Would that work?
Click to expand...

Yes, that works.

sawmonandnatalie said:
So bases on this diagram, how can I come up with a fuse rating for the one fuse connected directly to the positive side of the battery?
Click to expand...

My fuse is based on the absolutely highest number of amps that I would be likely to use at any one time, then 1.25 times that.

sawmonandnatalie said:
So for the fuse just before the invertor, you should always match it with the invertor's specs without even considering how the rest of the system/diagram looks like?
Click to expand...

Yes. The fuse on the wire to the inverter protects that wire.

sawmonandnatalie said:
For the shunt, as long as you over-spec the amp rating for it, it would still be good? For example, let's say your setup requires a 100 amp shunt. Can you get a 1000 amp shunt insterad?
Click to expand...

I think I've read that oversizing the shunt is inefficient. A shunt isn't 100% efficient. The larger the shunt the less efficient it may be. Check the milliamp draw in each shunt to know for sure.

sawmonandnatalie said:
And how about the DC fuse box? Do we also need to have the aim for the max amp the fuse box can handle and choose that rating for the fuse that connects the DC fuse box to the positive bus bar?
Click to expand...

Correct.
 
so for example this blue sea DC fuse box is rated at a Maximum 100 A pe block:

ST Blade Fuse Block - 12 Circuits with Negative Bus - Blue Sea Systems

Compact ATO® / ATC® fuse block consolidates branch circuits and eliminates the tangle of in-line fuses for electronics and other appliances.
www.bluesea.com www.bluesea.com

Would you install a 100 A fuse between the positive bus bar and the DC fuse block? or would you pick a smaller fuse? or a larger fuse (100 A * 1.25 ?) Also, where do these 1.25 and 1.5 correction factors come from?

HRTKD said:
The fuse downstream of the battery is rated on expected loads/charging.
Click to expand...

So, we are planning to pick a class T fuse downstream of the battery pack, do we need to only sum up all the fuse ratings from the load side of the positive bus bar or would we need to also add the amp rating from the MPPT charge controller as well?

How about the amp rating for the red load disconnect switch in the diagram? How should we calculate the amp rating for that?
 
sawmonandnatalie said:
so for example this blue sea DC fuse box is rated at a Maximum 100 A pe block:

ST Blade Fuse Block - 12 Circuits with Negative Bus - Blue Sea Systems

Compact ATO® / ATC® fuse block consolidates branch circuits and eliminates the tangle of in-line fuses for electronics and other appliances.
www.bluesea.com www.bluesea.com

Would you install a 100 A fuse between the positive bus bar and the DC fuse block? or would you pick a smaller fuse? or a larger fuse (100 A * 1.25 ?) Also, where do these 1.25 and 1.5 correction factors come from?



So, we are planning to pick a class T fuse downstream of the battery pack, do we need to only sum up all the fuse ratings from the load side of the positive bus bar or would we need to also add the amp rating from the MPPT charge controller as well?

How about the amp rating for the red load disconnect switch in the diagram? How should we calculate the amp rating for that?
Click to expand...

The safe route is to use a fuse at the positive bus bar for the wire that goes to the fuse box. The Overcurrent Protection is there to protect the wire.

If you have a very high amp charge device, say 150 amps, but all your loads are very small, say only 30 amps, then you would size the Class T fuse for the largest current, which would be the charge.

The switch tends to have a very high amp rating as near as I can tell. The higher the better as it handles any arcing better than a low amp rated switch.
 
HRTKD said:
If you have a very high amp charge device, say 150 amps, but all your loads are very small, say only 30 amps, then you would size the Class T fuse for the largest current, which would be the charge.
Click to expand...
Aren't we supposed to add those two numbers up if they are in a branched circuit? Since the battery needs to supply both those loads (150 + 30= 180 A)

Either way, do we need to multiply that number by 1.25 to get the fuse rating since those loads are continuous loads?

So in your example 180 x 1.25 = 225 A for the fuse rating.
 
sawmonandnatalie said:
Aren't we supposed to add those two numbers up if they are in a branched circuit? Since the battery needs to supply both those loads (150 + 30= 180 A)

Either way, do we need to multiply that number by 1.25 to get the fuse rating since those loads are continuous loads?

So in your example 180 x 1.25 = 225 A for the fuse rating.
Click to expand...

That's a good question. I'm going to say the answer is no. The reason is that if you have a load and a charge at the same time, the battery isn't going to see the sum of both. More like the subtraction of the two.

Using the prior numbers, a load of 150 amps will draw only 120 amps from the battery when there is a charge of 30 amps. 150 - 30 = 120.
 
Sorry, yes you are right. We were considering those numbers being both different load currents.

So, let's say we have a 12V 280Ah battery bank with
  • [charger] an MPPT charge that can max send 40A current to the battery
  • [load] an inverter that can only max use 200A (with a 200A fuse in between the inverter and the positive bus bar)
  • [load] a DC fuse box that is rated for 100A
  • [charger] a converter that would max send 45A to the battery
Then in this case since the chargers (both the MPPT and the converter) are not active all the time and assuming the loads can be running 24/7, the max net current going out of the battery would be 300 A (200A for the inverter and 100A for the DC fuse block). But once the chargers are running this net current out of the battery would be lower.

And taking into consideration the 1.25 factor, we would aim for a battery + fuse which is rated for 300 * 1.25 = 375 A.

Did we calculate this right?
 
sawmonandnatalie said:
And taking into consideration the 1.25 factor, we would aim for a battery + fuse which is rated for 300 * 1.25 = 375 A.

Did we calculate this right?
Click to expand...

Yep, that would be the number I would end up with. You can cheat the number down a bit by injecting some reality. Will your DC load run 100 amps? Just because it's capable of that much, doesn't mean you would ever use that much. In my trailer, there are certain devices that would never run at the same time, like the furnace and the air conditioner, the tongue jack and the air compressor, etc.
 
Looking back at our diagram and noticing that we are going with 4S 120A BMS, we reconsidered the current ratings. Also, since we are going to have 280Ah 3.2 Eve cells, it's best not to dis/charge any higher than 0.5C.

So overall, we are going to assume that we would not push more than 120 A from the battery pack. With this assumption, we decided to use a 150A class T or MBRF fuse between the main positive post of the battery and the positive bus bar. And for the wiring, we will use 2/0 gauge stranded copper welding cable between the positive battery post and the fuse holder (if not mounted on the battery post) and the same 2/0 between the fuse holder and positive bus bar. We are assuming none of these cable lengths would be greater than 8 ft.

+ Battery --- (2/0 AWG) --- 150 A fuse --- (2/0 AWG) --- + bus bar

We would have the same setup for the DC Fuse block and the invertor and possibly just use MBRF fuses for them in combi with 2/0 wires. We are assuming the max current the Fuse block solo or the inverter solo or both combined would not be greater than 100/120 A. And so we would use 150 A fuses between those components and the positive bus bar. And again we are assuming the cable run between those components and the positive bus bar is not greater than 8 ft.

+ DC Fuse Block --- (2/0 AWG) --- 150 A fuse --- (2/0 AWG) --- + bus bar
+ Inverter --- (2/0 AWG) --- 150 A fuse --- (2/0 AWG) --- + bus bar

What do you think of these numbers for the fuses and the cable gauges?
 
Thank you so much @HRTKD. Opsie, sorry for the typo.

The PDF document that you shared has some really valid and important points:
  • Terminal Fuses must be used only with a Terminal Fuse Block.
  • There can be only one circuit connection to a Terminal Fuse.
  • The shank of the fiber washer must seat inside the cable end ring terminal.
  • Ring terminals cannot be smaller than 3/8" (M10) or bigger than 1/2" (M12).
  • If the ring terminal makes contact with the terminal fuse stud, the fuse is bypassed and is not protecting the circuit (there is a short circuit across the fuse).
That last point is really important for stud/battery mount fuses.

Do you have a recommendation for circuit breaker brands for the inverter leg? (a CB with holes for 2/0 cables)

We have ordered two of these from Amazon (20A between the PV panels and MPPT and 50A between the MPPT and the [+] bus bar):
https://www.amazon.ca/gp/product/B06X3ZTB3W/
 
The 20 amp breaker you ordered is similar to what I'm using between the PV and MPPT. The 50 amp breaker between the MPPT and the bus bar is different than what I'm using. But, I think it will be acceptable.

Here is the one that I use between the MPPT and bus bar. The big difference is that mine covers only the positive cable where yours covers both cables. Don't forget to order DIN rail to mount those breakers on.

Series 187 Marine Circuit Breaker

<div class="product-description> <p><strong>OEM for Blue Sea Systems 7136-7149. Some sizes may take a few days to ship - call for availability.</strong></p> <p>These single pole thermal type circuit breakers are surface mount, so are easy to install whe
baymarinesupply.com baymarinesupply.com
 
I really didn't want to put anything more on the roof of my trailer. I looked into combiner boxes and wasn't looking forward to it. In the end, I could fit only two 320 watt panels on my roof. That was a blessing in disguise. I wired them in series, so there was no need for parallel connectors. I ran the wires down through the refrigerator vent (about 12") as an easy way to get the PV wires into the trailer. That eliminated the need for a combiner box. The PV wires terminate at my two-pole breaker which is mounted in the same compartment as all my other electronics.

The nice thing about having the breaker in that compartment is I can quickly turn off power from the PV array without getting on the roof.

For your 2s2p array, you need a pair of 2-into-1 Y connectors. Those can go right by the panels, then run regular PV cable from there down into the trailer, skipping the combiner box. No fuses are needed on each of the parallel strings because you have only two of them.
 
2000W with 90% efficiency is 2200W. At 13V, that is 170A. If you expect to actually handle 200% surge capability, then you would need to be able to handle current surges of 340A. Aand that is iffy for 4/0 wire.

Yah, that is why I went with an 8S system,

Since the Giandel is going to melt like a snow hat if you ever put it under serious pressure, I wouldn't worry about most likely unusable surge ratings. Design for a 200A max current limit and get on with your life.

2/0 wire, 200A+ busbars, 200A circuit breaker and you should be golden.

Keep your battery cables short and use 105 degree rated cable. I am using this stuff. You can order it cut to custom length and terminated. A terminal crimper will cost you $60+ and then you have to find a place to store it.

 
Last edited:
We also have a WiFi/LTE antenna that needs to go on the roof. So we were/are hoping to also run the wire cables from the antenna through the combiner box hole. Is it a good idea to run PV solar cables along with antenna cables in the same conduit/hole?

Thank you @HaldorEE . Yes, the inverter is a 2200W one which with 90% efficiency and a %200 surge at 13 V could produce a maximum current around 380A. We don't plan to use the inverter heavily. That's why we decided to go with a 150A in between the inverter and the positive bus bar. But putting a 200A breaker would not hurt, we will probably incorporate it.

We have purchased a 2ft 110 flat copper bar which is 2 1/2" wide and 1/4" thick. We might end up creating custom copper bus bars. It might be a good idea to tin their surface. We would need to look into tinning and maybe creating a plastic structure to mount the bus bars on (would the plastic cover like the ones for BlueSea important?)

We are planning to purchase welding cables with stranded wires and 105 C temperature rating (Maybe from TEMCo) and strip and terminate in-house. We love to learn all the aspects of installing all the components.
 
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