Load Calculations at 208V

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flengineer

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I am redoing a single apartment with a 150A panel with a 120/208V service. The owner wants to change out the tank-type water heater with an insta-hot. Using the Optional Feeder Calculations for dwelling units, I get 151A.

My range, dryer and cooktop are rated 8KVA, 8KVA and 5KVA respectively at 240V. Is it acceptable to calculate the load they will draw at 208V and use those values instead?

I'm using this for my calcs:

V=IR --> I=V/R
P=VI --> P=V*(V/R) --> P=V^2/R

This puts the resistance, R, at:

R=V^2/P --> R=(240^2)/P (where P=rated power at 240V)

Therefore,

P (at 208V) = (208)^2/[(240^2)/P (at 240V)]
P(at 208V)=0.75 * P(at 240V)

Hope I haven't thoroughly confused anyone.
 
I would, but I live in a 3rd world country.:roll:

220.5 Calculations.
(A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.
Click to expand...
 
Yup

Yup

220.5 CALCULATIONS
220.5 (A) unless other voltages are specified for purposes of calculating branch circuit and feeder loads nominal system voltages of 120, 120/240, 208Y/120,240, 347, 480Y/277, 480, 600 volts SHALL be used.
 
current will be proportional, so 87%.
power is 75% assuming it is resistive load.

The label should indicate power consumption at 208v and 240v. For resistive appliances, it will always be 75%. The current is 87%, so unless it was just a bit past the limit, the OCPD on name plate shouldn't change.

If it was 21A @ 240v, it would recommend 30A, because next smallest is 25A and using 80% rule, 20A is the limit for continuous load.
It will draw 18.27A @ 208v, and it may say on the label you can use 25A on 208v service.

Power will be roughly the same for constant power electronic loads and motors, but will draw 115.4% current to make up for it.
 
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a 120/208Y V system is a single phase system, as defined by ANSI/IEEE C84.1. It is derived by taking 2 phases and neutral from a 3 phase system, which could very well occur in an apartment building in a downtown area.

I have to use this opportunity to stress the importance of stating and using the voltages as defined in the standards. I realize this goes against the grain of some forum members but this post shows exactly why it is important.:roll:
 
flengineer said:
I am redoing a single apartment with a 150A panel with a 120/208V service. The owner wants to change out the tank-type water heater with an insta-hot. Using the Optional Feeder Calculations for dwelling units, I get 151A.

My range, dryer and cooktop are rated 8KVA, 8KVA and 5KVA respectively at 240V. Is it acceptable to calculate the load they will draw at 208V and use those values instead?

I'm using this for my calcs:

V=IR --> I=V/R
P=VI --> P=V*(V/R) --> P=V^2/R

This puts the resistance, R, at:

R=V^2/P --> R=(240^2)/P (where P=rated power at 240V)

Therefore,

P (at 208V) = (208)^2/[(240^2)/P (at 240V)]
P(at 208V)=0.75 * P(at 240V)

Hope I haven't thoroughly confused anyone.
Click to expand...

With the appliances you have listed you already have around 101 A. Depending on your heat/AC load you might be in trouble with the tankless water heater as most of them really eat up power. One of the smaller ones requires 40 A. That would put you at 140 A not including your heat/AC and general load. Very close to your 150 A panel. Just a thought!
 
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