# Sizing Circuit Breaker for Inverter Input

Solar Expert Posts: 77 ✭✭✭✭
2 Questions...

I'm considering the Outback FX3048. However the VFX3548 costs just a bit more. Is there any reason not to go with the VFX3548? I haven't seen efficiency charts, so I don't know if the larger capacity unit would be any less efficient even if my normal usage was only 2k watts.

Outback recommends a circuit breaker between battery and the VFX3048 of 100amp. If I don't intend (or want) to draw more than, say 80 amps from the batteries, can I use an 80 amp circuit breaker, or is there a reason I shouldn't do this? I understand that I may trip the breaker periodically if I draw more current that the 80 amps. Could I go even lower, say 50 amps?

Re: Sizing Circuit Breaker for Inverter Input

In general:

P=I*V

I=P/V=Power * 1/eff * 1/Vbattmin

Assume 2 kWatts max, 85% efficiency and Vbattmin of 42 volts:

I= 2,000 watts * 1/0.85 * 1/42 volts = 56 amps

The "normal" wiring should be rated at 125% of load (or load should only be 80% of wire/breaker rating):

Iwiring = 56 amps * 1.25 = 70 amps minimum

So, for my made up conditions, to run 2,000 watts on a 48 volt battery bank, the wire and breaker should be rated at 70 amps minimum (80 amp is probably next rated breaker--which means the wiring would have to be rated at 80 amps too).

Note, I believe Outback has some breakers that are rated to operate at 100% of load... So you could us a 60 Amp Breaker instead (however, then 60amp*1.25=75 amp min wiring)...

You will have to check with Outback or Jim/Cruzer on the details of the Outback breaker ratings and how to use them in detail.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 77 ✭✭✭✭
Re: Sizing Circuit Breaker for Inverter Input

Bill,

Thanks for the information and formulas.

Regarding efficiency, I found in an older post, an efficiency chart and the GVFX was at peak effiecient at about 50% of rated power. So for ~2k average draw, the largest GVFX3648 would be most efficient, and cooler running may mean longer life.

Do you know of any requirements for a panel mount CB enclosure?
They are somewhat expensive, and it makes me wonder if it would be safe to drill a pair of holes (for mounting screws) and cut a slot for the DB front, in a metal enclosure (as long as there is clearance around the CB studs).

Can 2 CBs be used in parrallel? For example, can 2 40a DC breakers be used instead of 1 80a?
• Solar Expert Posts: 1,963 ✭✭✭
Re: Sizing Circuit Breaker for Inverter Input

The inverter electronics are identical between the units, the only difference is the "V" is vented with a fan the other has is a sealed unit and the fan is inside.

Also the non vented 3048 is CEC rated at 2500 watts, outback specs there units when the inverter is at room temperature of 25C ... I can tell you first hand, even at 2K, the units very hot , at 2kW, they are generating close to 200 watts of heat

So do you want a sealed unit that can't get contaminates inside with the lower power rating or a vented unit that only can run higher by using external air to cool the unit? ... power wise its the same electronics
Re: Sizing Circuit Breaker for Inverter Input

It would be better not to use the two CB's in parallel--probably not legal (to code).

You should look to use the correct hardware (fuse block + switch or 80 amp CB). Our host NAWS has some hardware to look through.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 77 ✭✭✭✭
Re: Sizing Circuit Breaker for Inverter Input
The inverter electronics are identical between the units, the only difference is the "V" is vented with a fan the other has is a sealed unit and the fan is inside.

Also the non vented 3048 is CEC rated at 2500 watts, outback specs there units when the inverter is at room temperature of 25C ... I can tell you first hand, even at 2K, the units very hot , at 2kW, they are generating close to 200 watts of heat

So do you want a sealed unit that can't get contaminates inside with the lower power rating or a vented unit that only can run higher by using external air to cool the unit? ... power wise its the same electronics

Assuming the inverter guts of the VFX3648, GVFX3648, (& FX3048 ) are the same, any idea why for the VFX3648, the manual (pg. 48 ) recommends a OBDC-175 CB and states the Ampacity is 175 amp. That's a huge number 175a x 50v = 8750 watts. In the same chart they state an Ampacity of 90amps for the GVFX3648 90a x 50v = 4320 watts. The chart doesn't even reference the FX3048 but has a ?fx2548?. In their Maintenance chart where they talk about inverter cable length and type, they list both the VFX3648 and GVFX368 as 94 NEC amps and the same cable requirements. Could the 175amp CB requirement for the VFX3648 be a typo?
Re: Sizing Circuit Breaker for Inverter Input

Working without specs:

175 amp breaker * 0.80 NEC derating * 42 volt "dead batt" * 80% eff = 4,704 watts

I don't know about the Outback specifically--but if you want to support surge currents over a few seconds, you need a larger breaker/wiring.

Also, the efficiency of inverters when operating in their "surge range" is also typically much less than 85%.

Lastly, you can run the numbers with a breaker one size down (say 150 amps):

150 amp breaker * 0.80 NEC derating * 42 volt "dead batt" * 80% eff = 4,032 watts [fixed, had 175 before]

That is really close to the rated output of the inverter--they probably jumped up to the next available breaker/fuse/wiring because of that too.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 38
Re: Sizing Circuit Breaker for Inverter Input

The breaker is there to protect the wire not the device. It does not react fast enough to protect the device, by the time it trips you burnt the inverter. I would put semi conductor fuses after the breakers for device protection. If you only want to limit the current draw and not protect the device the use the breaker only.
• Solar Expert Posts: 246 ✭✭
Re: Sizing Circuit Breaker for Inverter Input

I find 175`s for 48v and 250`s for 24 v works,
A VFX 3024E will trip a 175 amp breaker without error/before it overloads, (outback DC breaker)

Have a good one
Tim
• Solar Expert Posts: 77 ✭✭✭✭
Re: Sizing Circuit Breaker for Inverter Input

Thanks for the replies.
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: Sizing Circuit Breaker for Inverter Input

N1st,

Comments above are on the right track. According to NEC Article 690.8(A)(4), a stand-alone inverter’s maximum input current is defined as the continuous input current rating when the inverter is producing rated power at the lowest input voltage. While not explicitly stated, the inverter’s operating efficiency at rated power must be taken into consideration.

For example, assuming 85% full-power operating efficiency, a 2,500 VA full power rating, and a 21 VDC minimum input voltage, the input current for my installed FX2524T would be:
(2,500 VA / 21 VDC) / 85% = 140 ADC
Assuming a DC circuit breaker rated for 100% continuous duty and rounding up to the next commonly available size, it’s my view that a CB rated for 175 ADC would be NEC compliant for the breaker’s specific conditions of use (ambient temperature, etc.) for application with the FX2524T. In fact, my FX2524T’s DC input breaker is rated at 175 ADC continuous duty.

Working with the VFX3648, and assuming 85% full-power operating efficiency, a 3,600 VA full power rating, and a 42 VDC minimum input voltage, the input current would be:
(3,600 VA / 42 VDC) / 85% = 101 ADC
Assuming a DC circuit breaker rated for 100% continuous duty and rounding up to the next standard size, it’s my view that a CB rated for 125 ADC would be NEC compliant for the breaker’s specific conditions of use (ambient temperature, etc.) for application with the VFX3648. The above calculation even suggests a 125 ADC breaker would be OK if the full-power efficiency was just 70%.

However, the VFX3648 is also rated to deliver 4,000 VA for up to 30 minutes at 25C ambient:
(4,000 VA / 42 VDC) / ~80%? = 119 ADC
That may be pushing the limits of a 125 ADC continuous duty breaker. Accordingly, the 175 ADC model -- the next readily available size -- might indeed be appropriate for the (G)VFX3648.

For a 3048T, the calculations would look like this:
Rated Power: (3,000 VA / 42 VDC) / 85% = 84 ADC, round up to 100 ADC 100% continuous duty breaker rating
30 minute Power: (3,200 VA / 42 VDC) / 80% = 95 ADC, round up to 100 ADC 100% continuous duty breaker rating
It’s also my view that some recent FX manuals are riddled with errors and/or typos. An example of specification conflicts between two essentially identical models is attached below. You may wish to consult OutBack’s archived older generation manuals for additional guidance.

HTH,
Jim / crewzer
• Solar Expert Posts: 77 ✭✭✭✭
Re: Sizing Circuit Breaker for Inverter Input

Crewzer,