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Subject - Overcurrent protection
David Hyatt Pressure washer @ 59.4 FLC, what size wire and overcurrent protection should you use?
59.4 x 125% = 74.25amps Tbl 310.16 #6 THHN
59.4 x 250% = 148.5amps Tbl 430.52 inverse time CB 150amp breaker.

I believe this way of calculating is correct, but I noticed in Tbl 430.52 that this is the maximum CB size. I know for a fact this pressure washer will operate on a 70amp breaker, by code can I chose to use a smaller breaker?
Ryan_J Need to know the horsepower. You can only use the nameplate for overload protection, not overcurrent protection.
wareagle David
Table 430.52 says 250% max. You do not goto the next size breaker.
Look at 430.52C1. Note the exceptions.
David Hyatt Sorry Ryan, 20 HP.
Wareagle, What I mean is do I have to use a 150amp or 140 amp, that would be the max but, can I legally use a 70amp CB? Thats what they usually install anyway. I was thinking to go to the next higher would be OK. Atleast thats the way I read it.
Ryan_J Phase and voltage please...? ;)
David Hyatt I neglected to put those in because I had them on another post but this is a new post huh. 208volt 3 phase.
Ryan_J 150 amp is correct, as per this exception to 430.52:
quote:
Exception No. 1: Where the values for branch-circuit short-circuit and ground-fault protective devices determined by Table 430.52 do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers, thermal protective devices, or possible settings of adjustable circuit breakers, a higher size, rating, or possible setting that does not exceed the next higher standard ampere rating shall be permitted.


Next step, size the conductors:
quote:
430.22 Single Motor.
(A) General. Branch-circuit conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor’s full-load current rating as determined by 430.6(A)(1).


You're wire size is incorrect, however. I think you went off of the 90 degree column, when you should have gone off of the 60, unless listed and labelled as 75 degree, as per 110.14(C)
David Hyatt I was using 90 degree column, THHN wire, the temperature limitation for the breaker and the motor are probably 75 degree. So I could safely use #4THHN Right? How about using a smaller breaker will that be OK?
Ryan_J #4...agreed.

Smaller breaker, absolutely!

It's like I tell people when I teach...if you find that you are installing #6 wire on 200 amp breakers, and you can't sleep at night becasue of it, well...STOP DOING IT!!! :D

This is just the code minimum. The real code rules for motors are:
(1)The wires can't melt
(2)The motor has to start.

Follow these rules and I as the inspector am a happy guy.
David Hyatt Thanks, again. I will use #4THHN and put it on a 70amp breaker.
Ryan_J Any time David. I personally would put it on about a 100 amp breaker, but thats just me.
David Hyatt Last question and I will give someone else a turn, Tbl 250.122 will I size my grounding conductor using a 70 amp breaker at #8 copper?
David Hyatt Ryan, unless it would stall and hurt the motor or something, these are for Briggs equipment. They do them all over the place and this same equipment in Florida, told to me by the contractor, has been working fine on a 70, but maybe I will still take your advice. 100amp breaker #4 THHN still #8 grounding conductor.
Ryan_J Yep, you'll go right off of table 250.122, #8 will work fine :)
wareagle "Wareagle, What I mean is do I have to use a 150amp or 140 amp, that would be the max but, can I legally use a 70amp CB? Thats what they usually install anyway. I was thinking to go to the next higher would be OK."
DAvid
You can always go smaller. I usually use 2 x FLA and it works most of the time.
I've never had to have one changed.
David Hyatt Thanks for all the help guys, I now the calculations enough to pass the test but when applying to real world motor are not my strongest point.