48v off grid system: battery cable advice needed please

[hammick]

My 48v system will have 10' of 4/0 cable to the inverter. Positive will go to a 250a breaker at the E-panel.

Should I also put a 250a fuse on the positive battery post or is the 250a breaker 10' away safe enough.

I can get a Blue Seas fuse block terminal and fuse for under $40

Thanks.

[inetdog]

My personal preference would be to have the primary battery fuse as close to the battery terminals as possible. It will then cover damage to the cable or mishandling while wiring or working with the 48V DC connections.
But if you are not concerned about that 10' of wire being damaged, then just the breaker in the e-panel should be OK.

[sensij]

My 48v system will have 10' of 4/0 cable to the inverter. Positive will go to a 250a breaker at the E-panel.

Should I also put a 250a fuse on the positive battery post or is the 250a breaker 10' away safe enough.

I can get a Blue Seas fuse block terminal and fuse for under $40

Thanks.

A 250 A breaker will not protect 4/0 conductor terminations [2011 NEC 310.15(B)(16) and 110.14(C)]. Wherever you are getting your information, it is wrong.

Edit: 250 A at 48 Vdc is 12 kW. Do you really need that much inverter capability?

[hammick]

A 250 A breaker will not protect 4/0 conductor terminations [2011 NEC 310.15(B)(16) and 110.14(C)]. Wherever you are getting your information, it is wrong.

Edit: 250 A at 48 Vdc is 12 kW. Do you really need that much inverter capability?

Thanks for the NEC references. I will look those up.

The install manual for the SW4048 inverter calls for a 250a DC breaker. The Midnight Solar E-panel either comes with a 250a or 175a DC breaker. I spoke with both Midnight Solar and Schneider and their high level tech both said 250a DC breaker and 4/0 battery cables.

Appendix C of the Midnight Solar E-panel install guide calls for 4/0 cable for 250a breaker.

I am running a 3/4 horse 230v well pump and a small apartment sized refrigerator as well as a garbage disposal and other typical household loads.

[ButchDeal]

A 250 A breaker will not protect 4/0 conductor terminations [2011 NEC 310.15(B)(16) and 110.14(C)]. Wherever you are getting your information, it is wrong.

Edit: 250 A at 48 Vdc is 12 kW. Do you really need that much inverter capability?

Battery systems often have larger breakers on the battery buss gennerally for the surge current. Most of the battery inverters can handle nearly double the kw for a few seconds surge so the DC bus needs to be fused appropriately.

[sensij]

A 250 A breaker will not protect 4/0 conductor terminations [2011 NEC 310.15(B)(16) and 110.14(C)]. Wherever you are getting your information, it is wrong.

OK, I'm wrong. 240.4(B) allows round up in this case.

However, the SW4048 inverter manual also includes this, and says that breaker rating must be matched to the size of the cable.

SW4048 120-240.GIF

I get that there can be surge loads, but a 3/4 pump will draw ~1 kW running (<21 A @ 48 VDC), which should be no more than 3 kW at startup. I guess you can design the system to handle the startup surge of every inductive load simultaneously, but really, a bit more thought should go into than just saying 250 A because that's what the guy on the phone told me.

[ButchDeal]

I get that there can be surge loads, but a 3/4 pump will draw ~1 kW running (<21 A @ 48 VDC), which should be no more than 3 kW at startup. I guess you can design the system to handle the startup surge of every inductive load simultaneously, but really, a bit more thought should go into than just saying 250 A because that's what the guy on the phone told me.

Yep I agree. And when you talk to the inverter people they are going to size things for the inverter and size of cable not the loads.

[inetdog]

A 250 A breaker will not protect 4/0 conductor terminations [2011 NEC 310.15(B)(16) and 110.14(C)]. Wherever you are getting your information, it is wrong.

The rating for 75C rated conductors, up to a maximum of three conductors, in a raceway is 230A for 4/0 copper. (Table 310.15(B)(16) in NEC 2011.)
But the open air rating for the same conductors (more appropriate for most ways of routing the battery cables) is found in Table 310.15(B)(17) and is 360A.
To me, that sounds code compliant.
(Even if you only have 60C rated terminations, the free air rating of 4/0 is still 300A.)

Note: Do not bundle the + and - wires together or run them right next to each other as that will violate the "open air" assumption.

[hammick]

Well I'm not basing the breaker rating solely based on what I was told on the phone.

Schneider says that DC switchgear is required to be used in North America. The Schneider DC switchgear comes with a 250a breaker pre-installed. I am choosing to use the Midnight Solar AC/DC switchgear which either comes with a 250a breaker pre-installed or a 175a breaker pre-installed. The Schneider SW4048 manual says that the correct DC breaker size is 250a. The 250a breaker will be installed in the Midnight Solar E-panel which says 4/0 cable must be used with a 250a breaker.

So Sensij if this was your system which would you choose to have the safest off grid system:

1. Use a 250a breaker and 4/0 cable as directed by Midnight Solar documentation, their tech support and Schneider tech support
2. Use a 250a breaker and 1/0 cable as directed by Schneider documentation
3. Add up your loads and surges and size your breaker and cable accordingly?

I am going to use the 250a breaker and 4/0 cable unless somebody proposes a better alternative.

OK back to the original topic of people's opinions on a fuse at the positive terminal in addition to the breaker.

[sensij]

The rating for 75C rated conductors, up to a maximum of three conductors, in a raceway is 230A for 4/0 copper. (Table 310.15(B)(16) in NEC 2011.)
But the open air rating for the same conductors (more appropriate for most ways of routing the battery cables) is found in Table 310.15(B)(17) and is 360A.
To me, that sounds code compliant.
(Even if you only have 60C rated terminations, the free air rating of 4/0 is still 300A.)

Note: Do not bundle the + and - wires together or run them right next to each other as that will violate the "open air" assumption.

You don't get to use the open air table for terminations. From 110.14(C)

(1) Equipment Provisions.
The determination of termination
provisions of equipment shall be based on 110.14(C)(1)(a) or
(C)(1)(b). Unless the equipment is listed and marked other-
wise, conductor ampacities used in determining equipment ter-
mination provisions shall be based on Table 310.15(B)(16) as
appropriately modified by 310.15(B)(6).

[sensij]

1. Use a 250a breaker and 4/0 cable as directed by Midnight Solar documentation, their tech support and Schneider tech support
2. Use a 250a breaker and 1/0 cable as directed by Schneider documentation
3. Add up your loads and surges and size your breaker and cable accordingly?

I am going to use the 250a breaker and 4/0 cable unless somebody proposes a better alternative.

Definitely not (2). (3) is safest, especially if you use 4/0 cable and put a fuse/breaker on it that is appropriate for the loads you intend to run (and assuming whoever is dong this is capable of properly terminating 4/0). The blue sea fuse holders are somewhat adjustable. If you find you want to add more loads later, increase the fuse at that time.

Put the fuse on the battery terminal. With it, the breaker is not necessarily required, but nice to have as an on/off switch, perhaps.

[hammick]

I will not have any DC loads (other than the inverter). Every AC load will have an appropriate sized AC breaker.

Isn't the sole purpose of the 250a DC breaker to prevent a fire in case the inverter or battery terminals short?

The CC battery + output will be on an 80a DC breaker.

I'm going to be installing the Bogart Trimetric battery monitor. I guess I could turn everything on and make sure the fridge is running when the well pump kicks in and check the amps on the Trimetric. I could then choose a battery terminal fuse based on that.

[hammick]

(C)]Edit: 250 A at 48 Vdc is 12 kW. Do you really need that much inverter capability?

The SW4048 is a 3800w continuous output inverter that can do 4400w for 30 minutes and 7000w for five seconds. Continuous output of 15.2a with peak current of 41a

So I guess it's a 3.8kw inverter.

[inetdog]

You don't get to use the open air table for terminations. From 110.14(C)

(1) Equipment Provisions.
The determination of termination
provisions of equipment shall be based on 110.14(C)(1)(a) or
(C)(1)(b). Unless the equipment is listed and marked other-
wise, conductor ampacities used in determining equipment ter-
mination provisions shall be based on Table 310.15(B)(16) as
appropriately modified by 310.15(B)(6).

Good point. The only escape would be if you were able to find a breaker and battery posts rated for 90C so that you could use a 90C rated wire. That would get you to 260A.

However the code only actually requires that the conductor carry the calculated load (your option 3). And if that load and wire capacity were larger than the next lower standard size breaker you could go up to the next size, which might be 250A. The key is to do the calculations first and then look at what choices are available.
If the batteries and conductors are protected adequately by OCPD elsewhere in the circuit, as you propose with battery fuses, the 250A breaker is allowed but is only actually serving as a load-break capable disconnect switch.

[hammick]

So anyone have any theories on why all of the Conext SW inverters call for a 250a breaker? I have a theory. The 24v SW4024 has maximum battery current of 230a so it needs a 250a breaker. (the maximum battery current of the SW4048 is 105a and 150a for the SW2524).

I suspect that an inverter short or battery cable short will always produce enough current to trip a 250a breaker in either a 24v or 48v system. So they only offer their DC switchgear with a 250a breaker so they don't have to produce different models of switchgear for each inverter model.

So maybe I will use a Blue Seas 125a on the positive lug?