48V relay question [RE-wrenches]

[Windy Dankoff, Dankoff Solar]

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Allan,

We have recommended a relay system for some time, and I use it on my 
own system (except my relays also transfer 48V to 24V battery).

1. Install a double-throw relay(s) to act as a transfer switch for 
the PV circuit. It switches the array to either the pump controller, 
or to the battery system's array input.

2. Connect the relay coil to battery power through the float switch 
in the water tank. The float switch can be either NO or NC, just 
configure the relay contacts accordingly.

When the tank fills, relay is actuated, and transfers the PV charge 
to battery system.


48V relay -- yes, hard to find. Alternative is to get two single-pole 
24V relays, and wire their coils in series. This is the exact 
equivalent (given same contact rating) of a 48V 2-pole relay.


About 2-pole transfer relays:

Two poles are generally used when you break both + and - (or both hot 
sides of AC). However, if the system has negative ground (like AC 
neutral), you are NOT supposed to break the grounded conductor (NEC).

Breaking both conductors, people often say, helps to reduce the 
arcing at the switch contacts, so what do you do? -- OK, it's true, 
BUT switching through two sets of contacts reduces the arcing no 
matter WHERE they are located in the circuit. So, wire the relay 
contact in series, BOTH on the positive line. This helps break the 
arc when the contacts break. (It distributes the voltage differential 
between the multiple contacts.)

You don't have to go way overboard on relay contact size or quality, 
because it is unlikely to switch more than about once per day.

BTW, if it comes up that you want to transfer array from 48V pump to 
24V battery system (this is an aside from the topic) -- Ask our tech 
support. We have a document on how to do this.

Windy

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