Need help regarding wire gauges

Hi all!

Could you help me out with this, please?

Context:

1. I've hooked up one of my three server rack batteries (48 V) to the PV inputs of my two EF Delta Pros to extend overall battery capacity. So far, this has been working well (please see attached diagram). Each Delta Pro is pulling around 15.9 A (which is the maximum each unit is capable of pulling)
2. I haven't connected the remaining two batteries to the active battery, yet. I plan to do this with my 4/0 cable, but I haven't sourced the corresponding terminal lugs, yet.
3. Nor did I install my panels and solar charge controllers, yet.
4. Until I've installed my panels (11 kW) and solar charge controllers, I'm only charging my server rack batteries with a 48 V / 20 A charger.

Goal:

1. Until my 4/0 terminal lugs arrive, I'd like to connect the remaining two batteries to my active battery with the 10 AWG cables I have laying around.
2. That way, we're going to have enough power to tide us over during power outages. We have a well pump, heat pumps etc. and sometimes outages take 12h - 18h to be fixed.

Question:
Will my system be OK if I connect my three server batteries in parallel with only 10 AWG as long as I only draw 15.9 A per connection and only charge at 20 A?

Note: I realize that I'll have to beef up the parallel battery connections to at least be able to deal with the amps coming in from my pv array via the solar charge controllers once I'll have installed them. By then, I'll have swapped the 10 AWG parallel connections with the 4/0 cables.


Thank you.


James

JamesTheCrow said:

Question:
Will my system be OK if I connect my three server batteries in parallel with only 10 AWG as long as I only draw 15.9 A per connection and only charge at 20 A?

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Yes.

Depending on wire insulation rating, you may be able to exceed this. You can also double up wires. 2X 10awg are like 7awg.

I would just patiently wait until I had all the components, but I understand the need to deal with the outages.

This should work but the #10 absolutely must be protected by fuses AT THE BATTERY terminals. Should a short occur, the batteries will source a lot of current and the wire, if not protected, will turn cherry red in a heartbeat.

I wouldn't run parallel #10s. It is a code violation, at least in the US, and there really isn't a reason to do so. The currents are small and although you have some #10 laying around, #8 isn't that expensive (if you have to go bigger) for the short lengths you're looking at.

Madco said:

I wouldn't run parallel #10s. It is a code violation, at least in the US, and there really isn't a reason to do so. The currents are small and although you have some #10 laying around, #8 isn't that expensive (if you have to go bigger) for the short lengths you're looking at.

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Please document where it's against code. Some inverter manufacturers actually allow and recommend dual DC battery connections and are UL listed.

I wouldn't hesitate to violate code in a sensible way on a temporary basis as long as I understood the implications.

"Code" doesn't mean "safe." "Code" means "compliant."

Parallel conductors are only allowed in a specific set of cases. EG some large conductor cases. Batteries are typically large conductors. #10 is not anywhere near large enough to qualify under 310.10H

There are more disallowed cases than allowed ones, so personally I would look for a positive code reference before proceeding with parallel. Same with EG conductors from same circuit in different raceways.

Guys, thanks a lot, much appreciated!

sunshine_eggo said:

Please document where it's against code. Some inverter manufacturers actually allow and recommend dual DC battery connections and are UL listed.

I wouldn't hesitate to violate code in a sensible way on a temporary basis as long as I understood the implications.

"Code" doesn't mean "safe." "Code" means "compliant."

Click to expand...

NEC 310.10H

The issue is that small wires are likely to have significant impedance differences when installed, which can lead to overload on one set. Paralleling two #10's with a 40A load you can see as bad a difference of 30A and 10A without doing something really stupid (like a 10' wire with a 1' wire).