48vdc heating element

JRHill

Wanted: a 48vdc element of 200 watts or less.

BB.

You can use 120 or 240 VAC heating elements too... Just derate them for running on 48 VDC.

P=V*I=I^2*R=V^2/R
P=V^2/R
V240^2/R=V120^2/R
R=240^2 / 48^2 = 25 ratio of power for 240 VAC element on 48 VDC source
R=120^2 / 48^2 = 6.25 ratio

P=V^2/R
R=V^2/P = 200 Watts = 48V^2 / 200 Watts = 11.52 Ohm heater or higher resistance
P=V^2 / R = 240 VAC ^2 / 11.52 Ohms = 5,000 Watt (or smaller) 240 VAC heating element

Basically, for a 240 VAC element on 48 VDC will have 1/25 the Wattage.

For a 120 VAC element, you will have 1/6.25 ratio:

200 Watt @ 48 VDC * 6.25 = 1,250 Watt rated @ 120 VAC element.

Note that AC vs DC rating does not matter for the element. HOWEVER, whatever you use to switch 48 VDC needs to be rated for (at least) 48 VDC.

DC power will sustain can arc beautifully on a switch/breaker/fuse not rated for 48 VDC (technically, 60 VDC minimum rated as the battery charging voltage--But 48 VDC rating should work too).

-Bill "I think I got the math right" B.

JRHill

I was sure I could count on you, Bill. Makes sense in front of me now - couldn't deal with it on my own. Hah, at least the part in layman's terms.

Best, Jim

BB.

You are very welcome Jim... It is scary how quickly I am forgetting stuff now that I am on the wrong side of 65. 

-Bill

Photowhit

The 'real problem' isn't with the heating elements, but with the thermostat.

I would worry the sustained arc of a 48 volt system would carbonize the contact points of a bimetal thermostat and render it useless quickly. If I was going to build a water heater, I would put the thermostat on AC with a relay designed to handle 48VDC to interrupt the DC current.

Dave Angelini

And a 1ampdc breaker rated for 125vdc.

  These are the kinds of things that home insurance might not like if they found anything but UL rated gear. It was two years back now that I saw a policy be denied.

Not a fun thing to be involved with either.

JRHill

The use case is in a 4x8 insulated pit, 60' from the house, where two 80g water well pressure tanks, controllers, battery storage, etc. reside. The well water is cold and gets much more cold in winter but has never frozen. I have pondered for years how to use the built in diversion from the controller to add some heat - but not too much heat. 48vdc is the power available. Bill has graciously answered the heat source issue for me.

Yup, the thermostat (and relay?) are of interest. If this gets too complex I might as well just run a 120vac line from the house and run the darned generator more in low light season. Also I do anticipate a possible freeze/slush potential as the storage was a bank of golf cart batteries which are shot. They added some heat. But those batteries were an overkill for the 800 watt load to the Grundfos pump. I want to go with a 48v 30Ah Helios when available and be done with lead.

I was hoping a single spot light on a thermostat would be sufficient. From experience with the chicken coop that single load really hits the whole off grid thing. That's why I was focused on the diversion use from the well system. We don't use much water in the winter. Still kicking this around. And I wanted to avoid 120VAC in the pit although that's what is running the Grundfos right now.