Voltage/Power/Branch Circuit Calculations

[Nuber]

I am trying to verify my calculations for the following installation because either I am using improper procedures, the nameplate is incorrect, or the installation manual is incorrect.

Assumptions - 2014 NEC, KW is the same as KVA (unity power factor), UL listed Electric Water Heater (tank style with resistive elements), installation instructions and wiring diagram allow for the unit to be field wired single or three phase for any of the following voltages; 208V, 240V, 277V, 480V

Name plate ratings - single phase 18KW, 75 amps, 240 volts; three phase 18KW, 44 amps, 240 volts

My thought process is that the resistance of the heating elements is the only fixed value. When the voltage is changed then current draw and power consumption will as well.

I am looking for a more detailed procedure/answer in order to determine the validity of my own process if anyone can assist.

2 questions for the nameplate ratings above

1. What is the NEC branch circuit and OCPD rating for single and three phase if a 208 volt circuit is used?
2. What is the real power consumed for single and three phase use by the appliance when the circuit is 208 volts?

 

[augie47]

Your statement is correct that resistance is the constant and the other values can be obtained by ohms law.
Looking a the single phase numbers, unless I made an error,a t 208v the actual current would be 65 amps and the actual power would be 15.52kw.

Art 422.13 might affect the branch circuit if the the tank is 120 gal or less and Art 422.11(F) will need to be taken into account resulting in more than one branch circuit on a 65 amp load.

 

[Nuber]

For the single phase I get -

3.2 ohms of resistance on the elements
65 amps of draw (90 amp OCPD)
13,520 KW

The installation manual says the recommended breaker size is 110 amps single phase 208 volts.

I smell fish, but I had a burger for lunch.

 

[Ingenieur]

Unit is rated at 240 vac
3 elements 9.6 Ohm each (18000/3 = 240^2/R)
Connected delta for 3 ph
i = 18000/(240 x 1.732) = 43.3 A

paralleled for 1 ph (9.6/3=3.2) or (18000 = 240^2/R)
i = 18000/240 = 75 A


208/1
P = (208^2)/3.2 = 13520 w
65 A
85A cb

208/3
208/9.6 = 21.67 A
line 37.5 A
Power for 1 element = P1 = 21.67 x 208 = 4507 w
P1 x 3 elements = 3 x 4507 = sqrt3 x 37.5 x 208 = 13520 w
50 A cb

 

[Nuber]

Ingenieur has the same calculation that I came up with (other than rounding up to the standard size 90A OCPD).

I can't make sense of the installation manual. Why in the world would you need a 110 amp OCPD when you connect the unit to single phase 208 volts? It is only 95 amps on single phase 240 volts.

The manual wants a 65 amp OCPD for 208 volts 3 phase and 55 amps on 240 volts 3 phase.

I am now afraid of either undersizing the circuit (violating the installation manual), or overcharging my customer by following the manual.

Rock, meet hard place.

 

[Ingenieur]

Mfg/pn/manual please
Al/Cu? What temp rating?
my cb numbers are 1.25 x i, so the conductor may be the next size up
so the cb would be larger

208/1 65 A x 1.25 = 81 A
conductor #4 85A @ 75 C
std cb 90

 

[ActionDave]

I am now afraid of either undersizing the circuit (violating the installation manual), or overcharging my customer by following the manual.

Rock, meet hard place.

Why would you consider it overcharging to install something according to the manufacture's instructions? We get paid to install electrical equipment and doing it as speced is part of the job.

 

[GoldDigger]

The only rational explanation would be if there are actually two different models for the two voltages. The one for 208V would have lower resistance elements to produce the same power at the lower voltage. That would mean more current.
Or possibly when used on the lower voltage the supply would be connected to a different tap on the heating elements.

mobile

 

[Nuber]

ActionDave - installing a circuit that is electrically unnecessary would result in overcharging. The manual's circuit sizing seems to be incorrect. Maybe overcharging isn't the best word - can't think of a better way to describe it at the moment.

GoldDigger - Same model, same box, same manual, carbon copy of one another.

I started in the field looking at the job and needed to estimate the thing. One single phase, one three phase same heater. Only had the nameplate rating at first, found the manual and chart later. When my numbers didn't match the manual I started questioning my math and sanity. Tech support says their charts are correct, but I am talking to a bubble gum chewing non-engineer who doesn't think Bill Murray was a ghostbuster.

Just have to explain to the customer the differences and let them make the call.

 

[david luchini]

ActionDave - installing a circuit that is electrically unnecessary would result in overcharging. The manual's circuit sizing seems to be incorrect. Maybe overcharging isn't the best word - can't think of a better way to describe it at the moment.

GoldDigger - Same model, same box, same manual, carbon copy of one another.

I started in the field looking at the job and needed to estimate the thing. One single phase, one three phase same heater. Only had the nameplate rating at first, found the manual and chart later. When my numbers didn't match the manual I started questioning my math and sanity. Tech support says their charts are correct, but I am talking to a bubble gum chewing non-engineer who doesn't think Bill Murray was a ghostbuster.

Just have to explain to the customer the differences and let them make the call.

So, what's the Make and Model?

 

[Dennis Alwon]

If the element is 18kw and listed as that for 240V 3 phase then the current would go down at 3 phase 208.

If you have 240v and using single phase then the 18kw element is 75 amps. You can go 150% so that is 112 amp overcurrent protective device.

75 x 1.25 = 94 amps so your conductor has to be rated for at least 94 amps

 

[Dennis Alwon]

My point above is the manual is using 110 amp overcurrent protective device for 240v not 208

 

[Dennis Alwon]

BTW, going from 240v to 208 just mutiply by 75%

18000 x .75 = 13,500

13500/208 = 65 amps single phase

13500/360 = 37.5